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General Questions

Are Performance Trends' programs compatible with Windows Vista?

What if I don't know some spec the program is asking me?

Why is the program predicting more performance than I'm seeing on the dyno (or at the track)?

How do I use a simulation program?

What is the advantage of using a simulation program?

How can I Email you a data file to show you a problem I’m having with a program?

Are these (Engine Analyzer) Program’s accurate?

Can I use your Windows programs on my Macintosh?

What is error 429?

Engine Software Questions

In the Engine Analyzer programs, what is the advantages and disadvantages of going higher than 75% on the Total Exh/Int %?

How do I simulate a restrictor on an EFI engine with a plenum and individual runner throttle plates?

Why does the spark advance value only increase by maybe 10 degs from idle to 3000 (even on stock setups) when actual distributors have 15-24 degs of advance?

Which cylinder heads have a "Compact Wedge" combustion chamber?

How much harder is Engine Analyzer Pro compared to Engine Analyzer?

Why can’t I get the Engine Analyzer to match the factory torque and HP ratings for 1960s muscle cars?

How do I enter or calculate the "Corrected Compression Ratio" for my supercharged engine?

Is the Engine Analyzer doing ECA (Engine Cycle Analysis)?

Why does the Engine Analyzer Pro show that some Thermal Barrier Coatings reduce performance?

Does the Engine Analyzer or Engine Analyzer Pro consider Port Size (volume) when estimating Performance?

My Turbo catalog only lists lb/min for airflow not cfm as Engine Analyzer uses.  Is there a formula to convert lb/min to cfm??

Can the Engine Analyzer and Engine Analyzer Pro be used for small engines, like a 5 HP Briggs?

The Spark Curve reported by the Standard Engine Analyzer starts high, then drops down, then increases again. Is this possible?

Do the Engine Analyzer’s consider Turbo A/R in their calculations? Do I alter the Turbo CFM rating to simulate different exhaust turbine sizes?

I have cam timing specs .020" lift intake and .025" lift exhaust.  Currently I am using the valve lift graphs to determine where the .050" lift point is from where I see the .020" and .025" lifts.  Is there an easier way to figure this out?

How do I enter the weight of my crankshaft, pistons or rods so I can see the effect on HP?

What is the formula for determining Intake Valve Closing angle in the Compression Ratio Calculator?

Why does the Engine Analyzer and the Engine Analyzer Pro give different results for detonation (spark knock)?

Why is there a dip in the torque and HP curve at 3500 RPM?

How do I build an engine to run a certain quarter mile ET and MPH?

How can I speed up the Calculations in Engine Analyzer Pro v3.5?

Cam Analyzer / Cam Test Stand Questions

I measured the cam with the pointer directly on the lobe and the results don't look anything like the Cam Catalog's Specs.  Why?

My Results do not repeat when I measure the same lobe again.  What could be happening?

I'm using the Virtual Follower feature, but my results do not look like what the cam should be?

What is the advantage of using the TDC Checking Bridge?

Will the software let you extrapolate from a .750 inch diameter roller lifter to a .800 inch diameter roller?

Drag Racing Software Questions

How can your Drag Racing Analyzer accurately predict Dial Ins and Throttle Stops if it doesn't let me enter several past ETs and weather conditions about my car.

I want to optimize my car to produce the quickest time to accelerate between 2 different MPHs.  How can I do this?

How do I build an engine to run a certain quarter mile ET and MPH?

Do I always have to make 2 runs to "calibrate" my Drag Racing Analyzer for predicting Throttle Stops?

Suspension Software Questions

Do you have software which deals with Rear Panhard Bar settings?

Why doesn’t Circle Track Analyzer showing a change in lap times when I change suspension settings?

When the Roll Center gets close to ground level, it moves left or right dramatically for small changes in dive and/or roll. Is this correct?

Can the Circle Track Analyzer be used for dirt track racing?

Can I use my Data Logger data (from your DataMite II or my own data logger) in your Suspension Analyzer to see what my suspension is doing on the track?

In Suspension Analyzer, why is the Ackerman Error not the same for both tires?

Dyno DataMite Data Logger Questions

When trying to achieve accurate inertia dyno results, what are the main things to consider?  By accurate, I mean as close to true horsepower as possible.

What are the main considerations when trying to achieve repeatable inertia dyno results?  By repeatable, I mean test-to-test and day-to-day consistency.

When I dyno an engine using an Inertia Dyno and your DataMite, the HP seems to peak too early, say 9,500 RPM, when I actually rev the engine to 10,500 RPM on the track.  Why?

The Torque Peak and HP peak reported on the Main Screen is slightly different than what I see on the graph which is slightly different than what I see on the report.  Why?

Can I use just engine RPM or Dyno Wheel RPM (not both) to calculate torque and HP?

How do I measure Engine RPM on a chassis dyno?

Can I use just your software for an existing dyno?

How does your software and electronics control the dyno?

How will I run a test with a water brake dyno?

Can most of the Black Box II features be added later if I don't buy them now?

How can I troubleshoot bad or "corrupt" data I'm getting from the Black Box II Dyno System?

Can you do a "coastdown" test on our chassis dyno to measure the dyno and vehicle losses, and then correct the chassis torque and HP to get flywheel torque and HP?

How can I email Performance Trends a test from my DataMite software, for Performance Trends to check out?

My Black Box II (or DataMite II) will not communicate to my computer.  What should I do?

What are the advantages and disadvantages of an inertia dyno compared to a water brake?

Are there any general tips for building an inertia dyno?

Could you elaborate on the advantages (carb tuning related) of the A/F sensor over BSFC?

I have a twin (2) roller chassis dyno.  Do I put the RPM sensor on the front roller or rear roller?

Road Racing Software Questions

I want to optimize my car to produce the quickest time to accelerate between 2 different MPHs.  How can I do this?

Can I use my Data Logger data (from your DataMite II or my own data logger) in your Suspension Analyzer to see what my suspension is doing on the track?

General Questions

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However, if you car was running in the 10s at 130 MPH or so, Frontal Area would be much more important.  Then you may want to come up with a better way to estimate Frontal Area.  Also see the "How do I use a simulation program?" question below.

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Why is the program predicting more performance than I'm seeing on the dyno (or at the track)?

Simulation programs are predicting how things should work with all systems working properly, carbs metering properly, ignition systems firing perfectly every time, combustion chambers burning efficiently every firing, etc.  They can predict a mis-match of parts, but can not predict a improper operation or failure of some component.  Therefore, programs are usually predicting "best case" performance.

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How do I use a simulation program?

We are always getting phone calls saying that these programs must be "magic" because the results exactly match the "real world" of peak HP on the dyno, or ET at the drag strip.  The important thing is not that the program matches some dyno's measurements (because another dyno can easily produce 5-10% different results), but do the programs match the dyno's trends accurately.  

For example, if you change header primary length 4 inches, does the program's predicted trends match the dyno's trends.  Even if 2 different dynos give 10% different HP numbers, both should show the same trends and both can be used to improve engine performance.  If the program can predict the trends accurately, then the program can also be used to improve engine performance.  Fortunately, we get just as many phone calls saying the programs are predicting the trends accurately also.

That's why we have more inputs in our programs than most others.  Its not to be more accurate at predicting performance, but to let you try more modifications to see performance trends.  What do I gain by blocking the crossover?  What happens if I can quicken my shifts?  What happens if headwind changes?  Although the programs are very good at predicting the final performance (like HP and ET), the true power lies in letting you try nearly an unlimited amount of combinations to find the best one.

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What is the advantage of using a simulation program?

Computers will never replace track or dyno testing.  However, computer simulations do have several advantages:

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How can I Email you a data file to show you a problem I’m having with a program?

This example is for our Engine Analyzer v3.0, but is very similar for other programs: The way to e-mail a file is to first save the engine file. Then go into the ENGDAT folder and attached that file to an e-mail. Send the e-mail to: feedback@performancetrends.com  Be sure to include some explanation of why you are sending the file even if you talked to use or Emailed before.  We talk to and receive many Emails from many people every day.

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Are these (Engine Analyzer) Program’s accurate?

(Background: This was asked on our website and we had several of our users provide "glowing endorsements".)

Thanks everyone for the endorsements!

Our registration cards run about 98% having positive to very positive feedback. The 1-2% with negative feedback usually are because they bought a program that was more than they wanted. Like Dave Koehler said, it may be best to start with the standard Engine Analyzer v3.0 unless you are VERY familiar with computers and engines.

Any of our computer programs are tools, like a torque wrench or camber gauge. They don't automatically make your car faster, but let you work smarter if you use the tool correctly. You working smarter is what ultimately makes your car faster.

No computer simulation replaces dyno or track testing. GM, Ford and Chrysler have invested millions of dollars on simulation programs which can still be off several %. That is why they still use dynos and test tracks. However, they are still using (and increasingly so) simulation programs because it gets them closer before they start building their first prototypes. It also helps them understand what might happen with trade offs with the actual parts and what potential problems they might encounter. For example, if the actual engine idles at 18" vacuum, but the program predicted 20", the program can show that reducing overlap 4 degrees gets them 2" more idle vacuum to get the actual engine to idle with 20" vacuum.

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Can I use your Windows programs on my Macintosh?

In the 1990s, we did know users using our products with Soft PC and Soft Windows by Insignia 408-327-6500. Some of our more powerful programs use locking codes (EA Pro, Port Flow Analyzer, DataMite, etc), which are generated from the hardware configuration. These may not work on Macs.

Today, most modern Macs can run Windows programs without any problems.

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What is error 429?

This error message can appear if a software company does not install the system files correctly.  If the error message includes the name of a file (commonly Resize32.ocx), it probably means that there are multiple copies of this file on your computer.  The correct location for system files is typically in the Windows\System or Windows\System32 folder.  (Performance Trends always installs system files to these locations to prevent this problem.)  Copies of the file in other locations are probably the problem.

For fix the problem:

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Engine Software Questions

In the Engine Analyzer programs, what is the advantages and disadvantages of going higher than 75% on the Total Exh/Int %?

The 75% number has been developed from experience over the last 60 years or so. Taylor from MIT developing aircraft engines in WWII was one the first to come up with this # that I know of.

If you have more than 75%, it does NOT means you should restrict the exhaust. It does mean that there is probably more benefit to enlarging the intake valve at the expense of the exhaust valve. Since a bore only allows an intake and exhaust valve combo so big, there usually must be a tradeoff. If you make the intake bigger, the exhaust must get smaller.

This 75% (85% for nitrous oxide or supercharged) is VERY general, and reported for info only. It is not used in the program to predict performance, as those calculations are much more detailed.

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How do I simulate a restrictor on an EFI engine with a plenum and individual runner throttle plates?

The best way is to pretend the vehicle is using a single plenum EFI Type manifold with appropriate length runners, a plenum matching your air box, and then a restrictor ahead of that To calculate the Carb/Throttle Body CFM rating, click on the Clc button for Carb/Throttle Body CFM Rating. Then just use the dimensions of the restrictor. (The restriction of the other throttle plates in the runners is negligible compared to the restrictor.) For a "perfectly designed" venturi type of restrictor, the "% Improved" spec in the Clc menu should be about 150%.

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Why does the spark advance value only increase by maybe 10 degs from idle to 3000 (even on stock setups) when actual distributors have 15-24 degs of advance?

Required spark advance is a complicated subject, especially when you the program tries to predict when detonation will occur. A/F distribution, thermal gradients (differences) across the heads, pistons, etc, combustion chamber design, are just a couple of things the program must make assumptions about. Then we have the factory, which has many different things to consider than just detonation and best power, things like: lighting off the catalysts fast, max catalyst temperatures, transient response, knock sensor calibration, cold start strategies, warranty, transmission durability, etc, etc. That the program does not match the factory calibration is not surprising.

Just a note, idle at 700 RPM is a very different condition than full throttle at 700 RPM, both requiring very different spark advance. Idle or some part throttle conditions may want up to 50 degrees of spark advance. Because of the light load and possibly EGR it may not detonate. But the same RPM at full throttle may only be able to run 10 degrees before it starts to detonate.

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Which cylinder heads have a "Compact Wedge" combustion chamber?

Compact Wedge is a chamber which has had considerable race development to improve burn rate and efficiency (power for a given amount of fuel & air intake). This includes "cutting edge" Winston Cup, Formula 1, and NHRA Pro Stock type of engines. The "chamber" is actually a combination of the chamber in the head and the piston dome design, and how closely they come together in the quench zone.

Most any combination that simply takes "this head" and puts it with "that piston", without careful dyno study, would NOT be considered a "compact wedge".

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How much harder is Engine Analyzer Pro compared to Engine Analyzer?

When people ask your question, we usually recommend the std EA3.0 which we sell for $109.95. Should you get comfortable with it and want to go to the Pro version later, you qualify for a $75 discount. Therefore, you are only out $35 for getting the EA3.0 first.

The Pro's more inputs and many more features (menus, commands, etc) can be overwhelming at first. More inputs can also mean more chances to input a wrong measurement, which will produce more inaccurate results than accurate results.

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Why can’t I get the Engine Analyzer to match the factory torque and HP ratings for 1960s muscle cars?

Unless you have the actual curves and know how the engine was set up, it is difficult to know exactly what the factory was doing. In the "old days", the factory would put on headers, block the crossover, change the spark and fuel curves, run open exhaust, etc, etc to get "factory ratings". This changed around 1972 when power ratings suddenly dropped. You've seen the "heads up" comparisons between old and new production cars in recent magazines. How else is it possible for modern 300HP Corvette to "blow the doors off" a 1968 425HP Corvette.

Also, if the factory marketing department wanted to "push" low RPM torque, they would simply pick a low RPM point off the torque curve, even if it was not the peak. In addition, dual plane intake manifold tuning is difficult to simulate accurately. Some of these engines could have made more low end torque than the Engine Analyzer program estimates.

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How do I enter or calculate the "Corrected Compression Ratio" for my supercharged engine?

The Compression Ratio stays the same, whether supercharged or not. Compression Ratio is a volume ratio and does not change with pressure. To try to explain that supercharging increases cylinder pressures and the tendency to knock (detonate or ping), people have come up with this idea of an "effective" or "corrected" compression ratio based on boost pressure and static compression ratio. However, if everything that effected cylinder pressures and the tendency to knock would have an "effective compression ratio", then we would have one for cam timing, carb size, port restrictions, type of fuel, nitrous oxide, RPM, etc., etc. This term is rather "old fashioned".

The Engine Analyzer program is considering all these aspects of supercharging "behind the scenes". You will notice that torque (an indication of peak cylinder pressures) goes up and the tendency to spark knock goes up when you start increasing boost. (The Pro version would actually graph you cylinder pressures with and without boost for comparison.)

At present, we don't feel a need to display a "Corrected Compression Ratio" in the Engine Analyzer Results. That is not to say we won't do it in a future version if we get other requests.

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Is the Engine Analyzer doing ECA (Engine Cycle Analysis)?

In general, ECA means to analyze the cylinder pressure over the 720 deg, 4 stroke cycle. Usually this is by measuring cylinder pressure and crank degree very precisely and quickly. Much can be learned about performance by doing this. EA30 does NOT display this type of simulated data, however the EA Pro does. EA Pro also displays many other "cycle" details like port pressures and velocities, piston acceleration or thrust on the cyl. wall, pushrod force, valve opening, etc. If this is the type of detail you want, I'd recommend the EA Pro. Download the demo to see yourself. If you already own EA30, you get a discount upgrading to EA Pro.

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Why does the Engine Analyzer Pro show that some Thermal Barrier Coatings reduce performance?

Coatings which reduce friction should always help performance. Coatings which reduce temps on the intake port/runner should always help performance by keeping the intake charge cool and dense. However, coatings which reduce thermal transfer to the piston or cylinder head do 2 things:

1) They reduce heat transfer (loss) which should help performance.
2) They usually increase the surface temp of the piston which should hurt performance.

The total effect of both things determines if there is a net gain or loss. Engine Analyzer Pro v2.1 "C" or later has an alternate coating choice which reduces heat loss but doesn't increase the surface temp as much. Coating manufacturers. say this is more typical of "modern coatings". Unfortunately, we don't know which description of "modern" or "typical" is more accurate.

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Does the Engine Analyzer or Engine Analyzer Pro consider Port Size (volume) when estimating Performance?

Yes, the EA Pro (and even the standard EA v3.0) will show the effects of large ports at high and low RPM. Both programs ask for port volume (or average port diameter) for both the head and the intake manifold runner. Most other programs don't ask for these details, but they are required for reasonably accurate predictions. You will definitely see low RPM performance suffer when ports get too big.

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My Turbo catalog only lists lb/min for airflow not cfm as Engine Analyzer uses.  Is there a formula to convert lb/min to cfm??

Multiply lb/min by 13 for an approximate CFM.

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Can the Engine Analyzer and Engine Analyzer Pro be used for small engines, like a 5 HP Briggs?

Yes, we have checked both programs against dyno runs from 5 HP Briggs engines, and other small 4 stroke engines. The agreement is quite good, but we haven’t checked it as much as we have with typical V-8 dyno runs. Its just that we’ve got a lot more V-8 dyno experience.

When HP and torque numbers are reported at very low numbers, they are reported to the nearest .01 ft lbs and HP. Inputs like Bore and Stroke can be specified to .001 ". The displacement is calculated with no rounding for performance calculations. When displacement is displayed, it is rounded to the nearest 0.1 CC.

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The Spark Curve reported by the Standard Engine Analyzer starts high, then drops down, then increases again. Is this possible?

The standard Engine Analyzer tries to make the spark curve somewhat automatic. It gives the engine as much spark as the program thinks the engine wants, but not so much as to let the engine detonate (spark knock) for the given conditions (intake air temp, humidity, fuel octane, CR, etc.) The EA Pro is not so automatic, but does give you much more freedom to try different things with spark advance.

In the actual engine, spark advance curves have been developed from what a simple centrifugal advance could do in an "old fashioned" distributor: start at the "initial advance", then at some RPM start advancing to a higher "full advance". This approximately follows what most engines "want".

Most spark requirements increase gradually with RPM. However, if your engine has a big cam, for example, it may produce poor low RPM torque (poor combustion due to poor mixture) and may benefit from more advance until the engine gets "up on tune", where spare requirements then fall, then pick up gradually as RPM increases. Most of the time you really aren't interested in max performance until the engine does get "up on tune", anyway, so don't be too worried about spark advance at low RPM. Just be sure not to let the engine detonate.

Another possibility is if the engine is supercharged or turbocharged, at low RPM there is little boost so the engine can run a normal amount of spark. Then, when boost starts to come on, the program "pulls out" spark advance to avoid detonation. Then as RPM increase, spark can be put back in as detonation is not as likely at higher RPM.

Please note that retarding the spark advance a few degrees from optimum timing will not show a large loss in performance, but will give you a better safety margin for avoiding detonation.

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Do the Engine Analyzer’s consider Turbo A/R in their calculations? Do I alter the Turbo CFM rating to simulate different exhaust turbine sizes?

Our Engine Analyzer v3.0 and Pro v2.1 both have specs that describe the turbine (exh) side of the turbo separately from the compressor side. A flow map describes the compressor, not the turbine at all. Turbo A/R does NOT affect the flow map, but the turbine size, and how quickly (at what RPM) the turbo starts to develop boost.

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I have cam timing specs .020" lift intake and .025" lift exhaust.  Currently I am using the valve lift graphs to determine where the .050" lift point is from where I see the .020" and .025" lifts.  Is there an easier way to figure this out?

Cam lift changes very gradually at the very beginning or end of the cam lobe where seat timing is measured.  For example duration at .001" could be 320 degrees.  Duration at .002" for a cam with mild ramps could be 305 degrees, but for a cam with aggressive lobes could be 316 degrees. This allows minor differences in cam lift aggressiveness to produce very different timing figures. Using the same seat timing events for 2 cams, minor differences in design can produce totally different durations at .050", .100", .200", etc. That is why the cam grinders adopted the .050" cam lift point for comparing cams. It is much more accurate than seat timing.

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How do I enter the weight of my crankshaft, pistons or rods so I can see the effect on HP?

These weights do affect the rotating inertia of the engine.  Rotating inertia is an input in the Short Block specs screen, however currently there is no way in the program to estimate the change in rotating inertia for rod or piston weight changes.  This is addressed in our Rotating Inertia Calculator program, and may be included in Engine Analyzer Pro updates.

Rotating inertia only affects power when the engine is accelerating, like in a vehicle when it is accelerating or on a dyno's accelerating ("sweep") testing, say at 300 RPM/sec.  Then some of the engine's power must be used (lost) to accelerating the engine's own internal inertia.  The lower the inertia, then the less power lost.

People often think that because a piston is lighter, it takes less of the engine's power to accelerate it down from TDC.  That is true, but what they forget is that energy is recovered when the piston is decelerated when it again approaches TDC.  Therefore there is no net loss when running steady state dyno tests.  (Note that there could always be some secondary effects on performance that are very difficult to model.  These could be things like lighter pistons don't stretch the rod as much, therefore things like compression ratio or ring seal is maintained, or lighter pistons "rock" in the bore differently as they approach and accelerate from TDC, thus sealing or friction could be slightly different, etc.)

Then you may ask "Why do engine builders strive to use lighter pistons and rods?"  That's because lighter pistons and rods put lower stresses on the rods and rod bolts.  Lower stresses mean longer durability and/or you can rev the engine higher.  With proper choice of cam, head flow, etc., most anything that lets rev the engine higher will produce more HP.

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What is the formula for determining Intake Valve Closing angle in the Compression Ratio Calculator?

The "Formula" is quite complex (many formulas actually) and has to do with our Cam Profile Designer routines.  At .050 tappet lift, the valve may still be .100 " off the seat (not closed).  For Dynamic Compression ratio you want to know when the valve is ON the seat so pressure will start to build in the chamber, which changes with profile type, Hyd vs Solid lifters, Lash, etc. for a particular intake closing at .050" tappet lift.  If you know the intake closing at .000 valve lift (seated timing), you should enter it directly.  (The .006" lash point is used for hydraulic lifters because they are assumed to have about .006" effective lash.)

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Why does the Engine Analyzer and the Engine Analyzer Pro give different results for detonation (spark knock)?

The 2 programs work differently:

EA30 retards spark to not allow detonation to occur.  EA30 does not report detonation level, just spark advance.

EA Pro runs the spark curve you give it or one it believes will produce best power and just reports the detonation level.

Behind the scenes, the 2 program's calculations are quite different, especially how they determine detonation levels.  As we say for both, you can not use the absolute levels of detonation (or spark curve in EA30) to tune your engine, just use them to see trends.  This cam or CR will produce about this much more or less detonation.

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Why is there a dip in the torque and HP curve at 3500 RPM?

All highly tuned engines with high overlap cams will encounter good and bad tuning peaks through their RPM range.  See Appendix 3 in user's manual "Intake and Exhaust Tuning".  For header primaries from 30-40 inches long, a "bad peak" for exhaust tuning comes around 3000-5000 RPM, where you will see a "dip" in the power curves.  This is NOT an indication of bad performance, just a "necessary evil" to get good tuning and performance at other RPMs.  If you try to eliminate this dip (close up overlap), you will hurt performance at other RPMs.  You can move where this dip occurs by changing header primary length.  For example, going to longer primaries will move the dip to lower RPMs.

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How do I build an engine to run a certain quarter mile ET and MPH?

You would need both an Engine Analyzer (std, Plus or Pro) and Drag Race Analyzer (std or Pro). The Engine Analyzer builds the engine and shows the torque and HP curves. The Drag Race Analyzer takes the power curves and couples it with vehicle specs to come up with ET, MPH, etc. You can Auto-link the 2 programs so that each time you make an engine mod in the Engine Analyzer, the ET and MPH (and 60 ft, finish RPM, etc) from the Drag Race Analyzer are also displayed in the Engine Analyzer results.

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How can I speed up the Calculations in Engine Analyzer Pro v3.5?

The Engine Analyzer Pro's calculations are VERY detailed and do require a significant amount of time to perform accurately.  Some things which will speed up the calculations include:

Once you find some promising combinations, you can go back to smaller RPM increments, turning ON valve train dynamics, etc. and do more refinement.

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Cam Analyzer / Cam Test Stand Questions

I measured the cam with the pointer directly on the lobe and the results don't look anything like the Cam Catalog's Specs.  Why?

Measuring a cam profile with a pointer (not the actual follower) directly on the cam lobe DOES NOT give the true follower lift as it would in the engine. You have to either measured the cam profile with the actual follower, OR use the Virtual Follower feature, part of the Plus version of Cam Analyzer.

Click here to see what the Plus version adds, paying close attention to the Virtual Follower feature.

Check the graph below of typical test results "as measured" (pink line is lift, yellow line is accel) and after simulating a roller follower using Virtual Follower (blue line is lift, green is accel). You can see they are totally different, which explains why your measurements do not match the catalog's cam specs. You can see the Virtual results, simulating a follower will give more lift at TDC, larger duration numbers, much lower acceleration over the nose of the cam.

My Results do not repeat when I measure the same lobe again.  What could be happening?

Repeatability should easily be within .0005 for lift and within .25 deg for duration and events.  If not, check some of these testing tips:

Click here for the PDF we send with the Cam Test Stand with these and possibly more Accuracy Tips

I'm using the Virtual Follower feature, but my results do not look like what the cam should be?

Here's some things to check for when using the Virtual Follower feature:

Be sure your measurements are accurate.  When you put the very small radius pointer from the linear encoder directly on the lobe, errors which are minor when using the actual follower on the lobe now are very critical.

Be sure your calculations are accurate.  This means you have accurately filled in all the Virtual Follower specs correctly.  

What is the advantage of using the TDC Checking Bridge?

Without bridge, you must tell program where #1 Intake Lobe is timed (like 108 centerline, .045" lift at TDC, etc). Then the rotary encoder is timed from this for all other lobes on the cam.

With the TDC checking bridge, you find dowel pin or keyway location. Then with info Performance Trends has on where these are suppose to be located (timed) for about 100 common engines, we can say how the cam was designed to be timed in the engine. This assumes you will use standard timing gears, timing marks, etc.  However, if Performance Trends does NOT have information on your particular engine, and you can not obtain it, you will have to measure a few cams to obtain your own number.

Will the software let you extrapolate from a .750 inch diameter roller lifter to a .800 inch diameter roller?

The "Virtual follower" feature in the Plus Version means you must measure cam with encoder's pointer, not a follower. Then you can simulate most any roller diameter, .750, .751, .752, etc.

If you measure the cam with a follower, then no simulation of different lifter types can be done.

Drag Racing Software Questions

How can your Drag Racing Analyzer accurately predict Dial Ins and Throttle Stops if it doesn't let me enter several past ETs and weather conditions about my car.

Good question.  It is correct that you can build up a data base of how your car performs in different weather conditions.  Then the data base program will predict how your particular car will respond to different weather conditions.  However, this is much easier to do on paper than it is in reality for these 4 critical reasons:

To summarize, a data basing program can work if the data you input is perfectly accurate, your car is extremely consistent, and you never modify your car or driving style.

The Drag Racing Analyzer is much easier and foolproof. It uses the same sophisticated techniques used by automotive engineers (GM, Ford, etc) to predict the effects of weather conditions.  These techniques are based on the physics and aerodynamics of a broad range of cars, fine tuned over decades of experience.  

When you first get the Drag Racing Analyzer, you enter your car's specs into the program and save them.  You can check these inputs by seeing if the program can predict your ETs, RPM range, 60 foot times, etc.  This fine tunes the program's predictions for your car. For example, a pickup truck will be affected by wind conditions differently than a Corvette, and the Drag Racing Analyzer knows how differently.  

To predict a Dial In, you make your first pass (time trial or practice run) and tell the program how you ran and the weather conditions.  This could be with a different cam, tire pressure, converter, etc. than last week.  (Many of these modifications you can enter into the program so it will be close after you make these modifications.)  Entering the exact ET and weather for a pass for how your car is performing right now makes the upcoming predictions extremely precise.

Now the program is ready to predict your ET for the next pass.  Just enter the weather conditions for the next pass and the program predicts the ET based only on the change in weather.  As long as you don't modify the vehicle between passes, the prediction will be as precise as is mathematically possible.  (Throttle Stop prediction is a little more involved and requires 2 practice runs.)

In addition to Dial In and Throttle Stop prediction, the Drag Racing Analyzer lets you try all sorts of "what if's" with vehicle, engine, transmission, converter/clutch, weather and driving style modifications.  It is a complete drag racing analysis package.  Say, that's a good name for it:  Drag Racing Analyzer!

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Do I always have to make 2 runs to "calibrate" my Drag Racing Analyzer for predicting Throttle Stops?

If you read Example 4.4 in the Drag Racing Analyzer v3.0's user's manual on page 138, it explains how to let the Drag Racing Analyzer know how your particular throttle stop, engine, vehicle, etc responds to a certain change in throttle stop setting.

A new feature added in v3.2 is the ability to predict throttle stops based the "Adjustment Factor" obtained from these 2 runs.  This means you do only have to run the 2 "calibration runs" once for your car.  Once you obtain the "Adjustment Factor", you simply supply it for the next series of runs at various tracks and events.  Click here for the Drag Racing Analyzer v3.2's Supplement of changes which describes how this is done.  Also check pictures below.

If you have the time, it is a good idea to check this "Adjustment Factor" if you have made a big change to something, different gearing, engine, major altitude change, etc.  This feature just gives you an additional option.

Throttle Stop Screen from 2 "calibration runs" to obtain the "Adj. Factor
 

Throttle Stop Screen using this "Adj. Factor" for predicting a Throttle Stop Setting in "new air"

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Suspension Software Questions

Do you have software which deals with Rear Panhard Bar settings?

Our Circle Track Analyzer v3.5 will automatically pick a panhard bar height (actually rear roll center height) to produce certain changes in handling to produce a "balanced" condition.  This is done by knowing other vehicle characteristics like front roll center height, front roll bar specs and front and rear suspension geometry.  From this it calculates the Front Lateral Load Distribution or FLLD.  This number along with front to rear weight distribution is useful to determining how tight or loose the car will feel at the apex of the turn.

These features are found by clicking on the Details and Find buttons on the main screen in the " 'Transition' Handling Rating " section.

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Why doesn’t Circle Track Analyzer showing a change in lap times when I change suspension settings?

Which suspension settings are optimum and how they affect lap times depend on many things, many of which we just don’t know. These include the track surface, driver preference, tire performance curves, etc. We are not able to predict changes from suspension settings ACCURATELY enough at this time. Therefore, we believe it is better not to try and to show NO effect at all. You will see the demo or program make this VERY apparent in the Suspension Specs menus. We would like to be able to predict all effects accurately for anything you can change in the program, we're just not there yet.

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When the Roll Center gets close to ground level, it moves left or right dramatically for small changes in dive and/or roll. Is this correct?

The program IS calculating the Roll Center position correctly MATHEMATICALLY. When both instant centers are at approx. ground level (0.0 height), minor changes will move the Roll Center left or right considerably, but not change the height much. Most circle track cars are set up so the instant centers and Roll Center stay well above ground, so this does not become an issue.

This brings up a debate about Roll Centers. How critical is the position left or right. Some "authorities" only use the Roll Center HEIGHT to determine handling characteristics. Most experts agree that asymmetries in spring and wheel rates left and right have a large effect on how the body rolls. This contradicts the simple Roll Center theory that says the body rolls about the Roll Center, and spring rates have no effect. Performance Trends is simply showing you the mathematical roll center and letting the user decide how to use it.

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Can the Circle Track Analyzer be used for dirt track racing?

Many aspects of the Circle Track Analyzer are equally applicable to asphalt or dirt. For example, Roll Center, Camber Gain, etc only depend on the front suspension layout, not the track surface. However, what Roll Center or Camber Gain you decide to run is your decision.

The program’s lap times are based on the basic theory that you get your best traction (fastest lap times) right before the tires break loose and start spinning. This is definitely true on asphalt, and on many types of dirt tracks also. However, the program does not simulate any conditions where you are breaking the tires loose and "steering with the throttle". There are also some recommendations (springs, stagger, etc) the program can calculate and these ARE for asphalt.

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Dyno DatateMite Data Logger Questions

When trying to achieve accurate inertia dyno results, what are the main things to consider?  By accurate, I mean as close to true horsepower as possible.

What are the main considerations when trying to achieve repeatable inertia dyno results?  By repeatable, we mean test-to-test and day-to-day consistency

When I dyno an engine using an Inertia Dyno and your DataMite, the HP seems to peak too early, say 9,500 RPM, when I actually rev the engine to 10,500 RPM on the track.  Why?

There are several reasons:

For best performance you do want to rev the engine past the HP peak.  For best performance, you want to keep the engine in the highest HP range available.  This usually means revving past the peak, so the lowest RPM on the track is still near the HP peak.

The dyno can only show performance for the RPM range you have tested.  In simplest terms, it can't show you torque and HP at 10,000 RPM if you don't rev it to at least 10,000 RPM.  

Most all dyno data looses accuracy near the point where you open or close the throttle.  This is usually the start and the end of the run.  One reason can be due to not knowing exactly where the throttle closes.  Another is that in the process of filtering (smoothing) data, the data happening a small time before and after some event gets averaged in with the event.  This means that data at 9800 and 10,200 RPM  gets averaged in with data at 10,000 RPM.  Since 10,200 RPM may be very low power (closed throttle), it will bring down the average data at 10,000 RPM.  This filtering effect is even more pronounced with inertia dynos, which must also diferentiate the data.  This means that data at 9,500 RPM and 10,500 RPM gets averaged in to produce the result at 10,000 RPM.  (The RPMs above are not exact, but are just examples to illustrate the point.)  The point is, to accurately see what is happening at, say, 10,000 RPM, you should rev the engine well past 10,000 RPM during your dyno test.  Note, however, you must consider the safe RPM limit for both your engine and dyno !

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The Torque Peak and HP peak reported on the Main Screen is slightly different than what I see on the graph which is slightly different than what I see on the report.  Why?

This is still good information, but Version 3.2 (released around October 2002) provides a Preference to ensure the torque and HP filtering and RPM step sizes for torque and HP are always the same, thus ensuring that you always get the same numbers on the Main Screen, in the Graphs or in Reports.

The torque and HP peaks are the highest values the program finds in the data.  However, the user must understand that the data has been arrived at by doing a "bunch of math".  For example, from an inertia dyno, the data is dyno wheel RPM and engine RPM measured at various periods of time, not torque and HP measured at various RPMs.  For absorber dynos, the data is volts measured at various periods of time.  In addition, all measured data has a certain amount of error associated with it.

In the process of doing the math, the program "smooths" or filters the data to help reduce the effects of measurement errors and so the data looks more like what the user expects.  The Dyno DataMite program gives the user a lot of freedom to choose the amount of filtering, and also the RPM increments for reporting data.  Both of these settings will affect the numbers and therefore the highest number (peak number) you will see.  The Figure below shows a torque curve from an absorber dyno.  You will see that the top curve with the least filtering is also the "jumpiest" curve.  It has more highs and lows and therefore produces the highest "Peak Torque" of 856 ft lbs at 5600 RPM.  Applying some "Light" filtering reduces the peaks and fills in the dips.  Now the Peak Tq is only 849 at 5650 RPM.  Applying "Heavy" filtering greatly reduces the peaks and fills in the dips, to produce a Peak Tq of only 844 at 5450 RPM.  This is all with the exact same recorded data, just doing the math on the data differently.

You will now ask "Which number is right?" and the answer is "They all are, if you include the math which produced the results."  If we keep getting this question, Performance Trends may need to take away the user's freedom to set various amounts of filtering and the RPM increment size for reporting data.  This would be to avoid confusion on the part of the user.  Currently, these are your options:

If you make the RPM increment size and filtering the same for the Main Screen and the reports, you will obtain the same peak numbers.  Graphs are done at 50 RPM increments (smaller than available with reports or at the Main Screen, and therefore will be slightly different).

When you make comparisons from run to run, you must use the same filtering and RPM increments to do so.  That is why the best comparisons are done with graphs, putting both or several runs on the same graph.  Then you know the RPM increment and filtering for all runs are exactly the same.  Comparing Peak numbers between runs is not a good way to compare runs.

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Can I use just engine RPM or Dyno Wheel RPM (not both) to calculate torque and HP?

Yes, there are several options.  These options are set in the Dyno and DataMite screens.  The Dyno screen is available by clicking on Dyno at the top of the Main Screen.  The DataMite screen is available by clicking on DataMite at the top of the Main Screen.

To use just Engine RPM for an Engine Dyno, select Dyno Type of "Engine, No Clutch" and the Gear Ratio to the correct ratio (at lower left in the Dyno specs screen).  Then in DataMite specs screen, be sure to change the Sensor & Calibration of the dyno wheel RPM to something other than Dyno Wheel RPM, or set it to Not Being Used.  Not finding Dyno RPM, the program will use engine RPM and the gear ratio set in the Dyno specs screen to calculate the Dyno Wheel RPM, torque and HP, assuming NO CLUTCH SLIP.  If you graph or report the Dyno RPM channel, it will show the actual recorded Dyno RPM, not the calculated RPM.

To use just Dyno RPM for an Engine Dyno, select Dyno Type of "Engine, No Clutch" and the Gear Ratio to the correct ratio (at lower left in the Dyno specs screen).  Then in DataMite specs screen, be sure to set the Sensor & Calibration of the dyno wheel RPM to Dyno Wheel RPM.  If necessary, you can set Channel 1 (typically Engine RPM) to Dyno Shaft RPM in the Pro version of the software.  Finding Dyno RPM, the program will use it and the gear ratio set in the Dyno specs screen to calculate the Engine RPM, torque and HP, assuming NO CLUTCH SLIP.  If you graph or report the Engine RPM channel, it will show the actual recorded Engine RPM, not the calculated RPM.

To use just Dyno RPM for a Chassis Dyno, select Dyno Type of "Chassis, No Engine RPM" (at lower left in the Dyno specs screen).  Gear Ratio in the Dyno Specs screen now becomes disabled (grayed out).  Then in DataMite specs screen, be sure to set the Sensor & Calibration of the dyno wheel RPM to Dyno Wheel RPM.  In Test Conds, you must set the correct gear ratios and tire size so the program can correctly calculate the Engine RPM, torque and HP, assuming NO CLUTCH SLIP.  If you graph or report the Engine RPM channel, it will show the actual recorded Engine RPM, not the calculated RPM.

If you have the Pro version of the software, you can get Calculated Engine RPM to show up on the Current Readings screen on the gauges in Preferences.  If you have chosen one of the options above and are using Dyno RPM, you can do the following:  Click on Preferences at top of main screen, then the Calculations Tab.  Set "Engine RPM is Calculated RPM" to Yes, then click on OK to keep this change.  Now you can see this Calculated Engine RPM on the Gauges if you select to display Engine RPM.  BE CAREFUL, because if you have entered too low a gear ratio in the program, the Calculated Engine RPM will be too low and you will over-rev the engine.  Also, be sure to turn this off when you start to MEASURE Engine RPM again.

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How do I measure Engine RPM on a chassis dyno?

Because there are so many different engine ignition systems, we have developed several methods to measure engine RPM.  

DTM-IPU Inductive Pickup.  This method involves tie wrapping a wire from our harness to a plug wire or secondary wire from the ignition coil.  We have also had good luck placing this wire near the coil on a "coil on plug" type engine.

BB2-IPULV Low Voltage Inductive Pickup  This sensor clips to a low voltage wire, like the coil primary wire or fuel injector wire.

DT2-RPMR  Reflective Optical RPM Sensor  This sensor "watches" a piece of reflective tape you will place on something spinning at engine RPM, like the crank pulley or dampener.  This method is VERY reliable and universal, but requires with you coming up with a bracket.  The sensor has a red "pointer" beam which greatly aids the setup process.

Calculate Engine RPM from Dyno RPM  The software allows you to CALCULATE engine RPM from dyno wheel RPM from simple vehicle measurements or from a simple calibration test (Pro version of software only).  This method assumes the clutch and tire slippage is 0, so it does not work well with automatic transmissions.

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Can I use just your software with an existing dyno?

Our software only works with our electronics. You will need to install our electronics and sensors and then you can use our software. We do NO control of the dyno at this time, which is very important with absorber (water brake, hydraulic pump, eddy current, etc) dynos, either a chassis or engine only dyno.  By control, I mean the system which holds RPM constant as you make changes to the engine or throttle, or the system which lets the engine accelerate at a slow, continuous rate as you are making a power run.  Being inconsistent at how you let the engine accelerate during a run can lead to not-repeatable tests.

If you already have control with your dyno, you should keep that existing dyno system to do the control. Then add our sensors and electronics to do the data gathering, graphs, etc.

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How does your software and electronics control the dyno?

At this time, we do NO automatic/electronic control of the dyno, which is very important with absorber (water brake, hydraulic pump, eddy current, etc) dynos, either a chassis or engine only dyno.  By control, we mean the system which holds RPM constant as you make changes to the engine or throttle, or the system which lets the engine accelerate at a slow, continuous rate as you are making a power run.  Being inconsistent at how you let the engine accelerate during a run can lead to not-repeatable tests. 

If you already have control with your dyno, you should keep that existing dyno system to do the control. Then add our sensors and electronics to do the data gathering, graphs, etc.

If you do not have automatic control now, you will continue to control your dyno manually with the throttle and water control valve.  You will continue opening the throttle and applying load with the water control valve until the engine is at WOT (wide open throttle) and at the starting RPM you desire.  This requires a little experience and practice to get good at it, especially with high powered, turbo'd or "peaky" engines.  You will adjust the control valve to get the engine to either accelerate or decelerate at a slow (100 to 300 RPM change per second), continuous rate through the entire RPM range.

A big advantage of the inertia dyno is the "control" is near perfect.  You start at a low RPM, hit the gas (go to WOT or wide open throttle) and the engine accelerates smoothly and consistently because of the large amount of inertia in the system.

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How will I run a test with a water brake dyno?

All absorber or brake dynos (water brake, hydraulic pump, eddy current, either chassis or engine only) tests are done as a "sweep" test, where the engine goes through its full RPM range and the DataMite or Black Box system records the data at a very high rate.  When the test is done, then the software will report or graph the data at standard RPM increments, like every 100 or 250 RPM, whatever you select.  This is much more accurate and efficient than trying to stabilize at various RPM steps and then record data, then go to the next RPM step.

Check the other FAQs here about controlling the dyno, and section 4.3 in the user manual at this link:  PDFs/DTMMANL7.pdf

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Can most of the Black Box II features be added later if I don't buy them now?

There are 2 options to consider, which are difficult to add at a later time: 

$100 Optical Isolator, DTM-OI. Although this can be easily added, it may then be too late. The optical isolator helps prevent a high voltage "jolt" from the engine, traveling back through the data logger to damage your computer. About a third of our customers get one, about 2 thirds do not. We rarely hear about this happening, but it CAN happen, especially if you pull a plug wire to kill the engine. The ignition's high voltage travels back through the inductive pickup through the data logger to your computer's com port. On desk top computers, only the com port board (or USB board on computers using USB adapters) may be damaged, which typically can be easily replaced by a computer tech. On a laptop computer, the com port is part of the motherboard, which means the whole computer may be damaged. On older, desktop computers, I'd say you probably do not need one. On a new, expensive lap top computer, I'd recommend getting one. 

$200 Internal Weather Station Sensors BB2-IWS: The weather station allows the DataMite software to automatically do weather corrections to the data exactly the same for each test. This helps produce more repeatable data. These sensors and fan must be installed and checked by Performance Trends, which means you must ship the unit back and we then ship it back to you. If these shipping costs are expensive (shipping outside the USA), you may want to get this option right away and save all the shipping expense. 

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How can I troubleshoot bad or "corrupt" data I'm getting from the Black Box II Dyno System?

Take the top off the box, but leave the wires attached to the lid plugged into the board.  Make sure the ribbon cable connector from the computer connector is firmly plugged into the board connector.

If that was OK, then:

What happens if you just run a test without the engine running, just record data for 20 seconds and download it. Does this show up as corrupted?

If Yes, if you have an optical isolator, what happens if you repeat the step above without the optical isolator in the system?

If not, then unplug all sensors from the box and run a test with engine running and download. Does this show up as corrupted?

Plug in optical isolator for tests with sensors hooked up.

If not, then start plugging in sensors 1 at a time (starting with dyno RPM) and run a test and download. Does this show up as corrupted?

What sensor being plugged in shows up as corrupted?  

Let us know what happens when you try these tests.

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Can you do a "coastdown" test on our chassis dyno to measure the dyno and vehicle losses, and then correct the chassis torque and HP to get flywheel torque and HP?

Doing a coastdown test of the vehicle you just tested to estimate the losses in the driveline, to then come up with flywheel HP is VERY MISLEADING. Reason is because the losses with power ON are much higher than when coasting, especially for an automatic trans.

For example, if the losses from coastdown at 4000 RPM show 32 HP, the losses with full power will be much higher (perhaps 100 HP for a 500 HP engine) when accelerating. Tire slippage alone will be 3-10%, which equates to a direct loss of that percentage in power. (During coastdown, tire slippage is 0%.) The difference between full power and coasting gear box losses are the difference between power-On efficiency and simple "spin losses".  Torque converter slip during power-On can be 30% or more, again a huge loss that is 0 when doing a coastdown.

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How can I email Performance Trends a test from my DataMite software, for Performance Trends to check out?

This is done by putting a floppy disk, memory stick, or writeable CD into the computer. Then click on File at upper left of main screen, then select Save to CD/Floppy Disk.  (If the drive letter does not match what drive letter you want to use, you can change this in Preferences at the top of the main screen.  Click on the Filing System tab, then change the Default Floppy Disk drive letter.  Then click on OK button to keep this change.)

This will copy the 3 files which make up the test to the floppy or CD. Take this disk to an email computer and send all 3 files to a Performance Trends email address to evaluate.  The Performance Trends tech will give you and email address to send it to.

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My Black Box II (or DataMite II) will not communicate to my computer.  What should I do?

Remove the optical isolator from the system if you have one.  If this fixes the problem, be sure to set the Preference to Enable Optical Isolator power.  If this is set correctly, then you may need a powered optical isolator from Performance Trends.

Be sure you are using the cable supplied with the system, called a Null Modem cable.  Not all 9 pin cables are the same.  You need one where pins 2 and 3 swap through the cable, pin 2 is connected to pin 3 and vice versa.

With the Black Box II, remove the lid (keeping all wires connected) and be sure the ribbon cable from the computer connector is firmly  plugged into the connector on the board.

Click on DataMite at top of main screen, then Troubleshoot, then Check Com Ports.  Does your COM port pass the "paper clip" test?  Can you find a COM port which does pass this test.  Note that COM 3 many times may pass this test, but will still pass the test with the paper clip removed, and is not a valid port.  If no ports pass this test, you need to take your computer to a shop and have the Com Port activated correctly.

If you find a Com Port that does work, then note which Com Port Number. Click on Preferences at top of main screen, then General Operation tab, then set Automatically Find Com Ports to No. Then click on OK at top right of this Preferences screen.  Click on DataMite at top of Main Screen, then set the Com Port to the correct number you've just found.  Also be sure to have set the correct Type of DataMite to the correct type for you system.  Click on Back at the upper left corner, and say Yes to Saving these changes and Yes to Saving these changes as the Master DataMite Specs.

Click on DataMite at top of main screen, then Troubleshoot, then Check Boot Message and follow the instructions.  Do you get a message from the Black Box II or DataMite II?  If yes, it means that you can at least receive info from the box, but may not be able to send commands to the box.  Contact Performance Trends tech.

If you are using a USB adapter, check its status:
-Right click on My Computer, then click on Properties. The "System Properties" window appears.
-Click on the "Hardware" tab then "Device Manager". This gives you a list of devices on your machine.
-Look down the list for "Ports (COM and LPT) and click the "+" next to it. You will now see a list of ports on the machine. You should see a "Communications Port (COMx)" (probably with "USB" mentioned also) where the "x" is the port #.  This is the port # you put in the DataMite specs screen. 

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What are the advantages and disadvantages of an inertia dyno compared to a water brake?

An inertia dyno uses the inertia of a large flywheel or drum to absorb the power from the engine and slowly accelerate through the RPM range.  By knowing the inertia and measuring the acceleration rate, torque and HP can be calculated.  An absorber dyno (water brake, hydraulic pump, eddy current, etc) absorbs the power of the engine in some type of "pump" which raises the pressure of the fluid, eventually turning the power into heat.  The amount of torque required to keep the housing of the "pump" from rotating is also the amount of torque the engine is producing.  Both methods produce accurate results, but each has advantages and disadvantages.

Advantages of Inertia Dyno

1. Very simple to build.  All that is required is a shaft and a sufficiently large flywheel to provide a slow enough acceleration rate for accurate measurements.  The power run should be 6-10 seconds ideally.
2. You only need to measure dyno RPM and do not need a torque sensor.
3. "Perfect" speed control.  You start at a low RPM, hit the gas (go to WOT or wide open throttle) and the engine accelerates smoothly and consistently because of the large amount of inertia in the system.
4. Because of the simplicity of the system and the "perfect" speed control, the inertia dyno system can be extremely repeatable.
5. The "bang for the buck" of the inertia dyno can NOT be matched by an absorber dyno, especially for small engines.

Disadvantages of Inertia Dyno

1. Can only measure performance while the engine is accelerating.  You can NOT hold speed at a particular RPM.
2. Very large amounts of inertia are required for large engines.  Flywheels or drums weighing 1000 lbs or more may be required.
3. Can be unsafe.  Due to the large amounts of inertia and speeds involved (especially for large engines), that is a lot of energy waiting to be released should something break, or you slow down (brake) the system too quickly.
4. Can NOT measure the torque and HP at the exact beginning and end of the run.  In order to accurately measure the acceleration rate, we need to know the engine RPM both some time before the current RPM and AFTER the current RPM.  At the end of the run, we don't know the RPM some time AFTER the current RPM, because we never get to that RPM.  What this means is if you need to accurately know the torque and HP at 10,000 RPM, you have to run your test out to 10,500 RPM or possibly 11,000 RPM.

Advantages of Absorber Dyno

1. This system CAN measure performance while the engine is setting at a steady RPM.  This can be very handy when making tuning changes while the engine is running.
2. The system can be quite small and compact.  All power is converted to heat in the fluid, which is typically quite safe.
3. You can bring the engine RPM down very quickly because there is so little inertia in the system (very safe).
4. It CAN measure the torque and HP at the exact beginning and end of the run.

Disadvantages of Absorber Dyno

1. More complicated and expensive to build.  Large dynos require a lot of water to absorb all the power from the engine.
2. Needs a good torque (force) measurement system.
3. Need good speed control to produce repeatable results.
4. May require a lot of cool down time between runs once the dyno water system gets warm.
5. These systems can be very repeatable, but generally only if you spend a lot of money, especially the control system.

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Are there any general tips for building an inertia dyno?

The following is a sample warning report which can be generated by the Dyno DataMite program.  It shows some specific RPMs and MPHs based on certain sizes of components describing a particular inertia dyno system.  These limits will change for various dyno systems as you change critical dimensions.

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The following are general tips to explain that if you are not careful, your dyno could turn into a 'bomb', injuring or killing yourself and bystanders. As with any relatively high RPM rotating machinery, you should have the design checked by a qualified engineer.
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The largest diameter in your dyno system is 30. inches.

It is recommended that you do not exceed 1333 RPM

There is no way the program knows exactly how your system is designed.  Poorly designed systems should not be used at all, but should definitely NOT exceed 1000 RPM, while well designed systems can operate safely to RPMs of 2000 RPM or higher.

Because Chassis dynos typically have relatively thin wall rollers (drums) and because of the speed limitations of the vehicle and tires, do NOT exceed 92 MPH on this 23.0 inch diameter roller. That means do not test a vehicle to more than 92 MPH on its speedometer.

More Chassis Dyno Tips:

- Chassis dyno rollers are more complicated to build and to analyze for safety than simple flywheels. 
- Always note the tire's speed ratings and keep vehicle speeds well below the tires limits.
- Keep tires properly inflated for safety and consistent results. 
- The smaller the diameter of the dyno roller, the more tire deformation, the more tire heat buildup and the more HP losses.
- Change transmission gear for testing to a lower one to keep the MPH low. However, if the run is over too quickly (accelerates too quickly), you can move to a higher gear ratio (but keep the top speed below all speed limits discussed above).

The important thing to note is that you want to keep the RPM low to keep your dyno system safe. A flywheel or chassis dyno roller can literally burst if you over-rev it, or cause the entire system to start tumbling violently if something jams or brings the flywheel to a stop (brake) too quickly.

More tips include:

- Karting inertia dynos should NOT use hollow 1.25'' shafts. (Solid 1.25'' shafts are commonly used with 24'' diameter x 1.25'' thick flywheels, but even this is only marginally safe.
- Do not weld flywheels to shafts, unless the shaft is a large diameter.
- Use collars, hubs or flanges to avoid creating a stress riser right where the edge of the flywheel meets the shaft.
- Avoid very large, but thin flywheels. Do not use flywheels with a diameter more than 20 times the thickness.
- Put some type of 'cage' or containment around the flywheel to 'catch' a magnet which could fly off or to allow the flywheel to 'coast' down slowly (rubbing on the inside of the cage) should a shaft or bearing fail.
- Many inertia engine dyno operators like to use a one-way clutch, so the engine can come to a stop while the dyno wheel coasts down more slowly.

There are many more things to consider to make a dynamometer which is safe to operate. Be sure to consult other references like the Machinists Handbook.

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Could you elaborate on the advantages (carb tuning related) of the A/F sensor over BSFC?

BSFC stands for Brake Specific Fuel Consumption and is reported in pounds of fuel consumed per hour, per HP.  Very efficient engines at full power at their torque and HP peak typically run about .40 to .45.  This means the engine requires .40 lbs/hr of fuel flow for every HP it makes.  If it was making 1000 HP, the fuel flow required would be 400 lb/hr (or approximately 1 gallon of fuel/minute, GPM).