When building or modifying an engine for all-out racing, street/strip performance, trailer towing, or most any application, there are many variables to consider. One critical variable is the Compression Ratio or CR. Compression ratio is the amount of “squeeze” the piston puts on the air fuel ratio before it lights the fire. And, more importantly, it is the amount of expansion or “push” these hot, high pressure, combusted gases put on the piston. The more amount of expansion, the more amount of heat and pressure energy can be extracted from each combustion event.
CR is the ratio of the cylinder’s volume at it’s greatest volume (piston at Bottom Dead Center), compared to the volume at it’s minimum volume (piston at Top Dead Center). At BDC, the volume is the cylinder’s swept volume PLUS the combustion chamber volume. At TDC it is just the combustion chamber volume. Since the swept volume is the engine’s displacement per cylinder, and is easily calculated from bore and stroke, the more complicated part of the calculation is calculating the combustion chamber volume, which is made up of LOTS of separate small volumes. That’s why it’s nice to have the Compression Ratio Calculator to do that hard work, and keep all the math straight.
Well, there are some downsides to high CRs:
- The amount of improvement you get going from 9:1 to 10:1 (typically about 4%) is much greater than the improvement going from 16:1 to 17:1 (more like 1%). The benefits of very high CRs diminishes rapidly.
- It gets difficult to quickly and completely burn a mixture in such a thinly squeezed “pancake” of a combustion chamber.
- With such an increase in pressures, there can be more stress on internal engine components.
- It gets difficult to produce such a small combustion space and still package the spark plug, valves, valve reliefs for clearance to allow for any overlap for valve timing (needed for good, high RPM “breathing”).
- An most importantly, there is is the ever present danger of “piston destroying” Detonation (spark knock).
Detonation: The higher the CR, the higher the temperature and pressure when the spark ignites the charge. The flame travels across the chamber in all directions from the spark plug to eventually completely burn everything. This takes some time, 60-80 degrees of crank rotation, and is why you fire the spark plug at, say, 35 degrees before TDC and not at TDC. These higher temperatures and pressures can cause the unburned mixture away from the spark plug to auto-ignite, or detonate. This drastically raises temperatures and pressures and can bust out piston ring lands, blow holes in pistons, cause the spark plug to get very hot and glow and cause pre-ignition, and generally “wreck your weekend”. To avoid detonation, you use a higher octane fuel and keep the CR to a reasonable level.
So we can see that it is important for engine builders to know the engine’s compression ratio. Some race classes even limit Compression Ratio, so you want the highest CR you can get but not get disqualified in tear down. That is why Performance Trends developed Compression Ratio Calculator. It makes it much easier to keep track of all the little volumes which make up the total Combustion Chamber volume.
The Compression Ratio Calculator includes things you may not have considered, that the head gasket may be a different ID than the bore, the piston top OD is significantly smaller than the bore, providing a space above the first compression ring, etc. The program also includes various engine building features like determining the “deck height stack up” (piston compression height, rod length, etc) to fit inside the engine. We’ve also included several utility calculators (some companies call these “wizards”) to calculate piston dome or dish volume, find the needed piston dome (or some other parameter) to produce a desired CR, find engine and cylinder swept volume from bore and stroke, and much more.
We are always improving our software and that is why we are now on our third version of this program. For version 2.3 we now have two options, the Basic version and the new Plus version.
In the Basic version you can:
- Calculate compression ratio, cylinder volume and engine displacement.
- Find a desired compression ratio using chamber CC’s, deck height clearance and other inputs.
- Determine how much of a contribution various dimensions have on the overall compression ratio.
- Calculate Bore – Stroke ratios and Rod – Stroke ratios, boring and or stroking effects (if you are looking for a desired displacement you can determine the required bore and stroke)
- Calculate cranking compression pressure and dynamic compression ratio. (Dynamic compression ratio does not use the entire swept volume in the equation, but only the volume in the cylinder at the point where the intake valve has closed.)
- Determine piston dome volume or piston dish volume.
- Enter head gasket volume as Bore and Thickness or Volume CC’s directly.
- Determine any of the following: rod length, deck height, deck height clearance, piston compression height or stroke based on you specifying the other four.
- Allow for English or metric units.
- Like most other Performance Trends programs you can save your changes to compare with other versions of your inputs or print out the report to share with your workers, engine builder or your customers.
If you choose to upgrade to the Plus version (you can choose to upgrade at a later date) you will get the following inputs on top of all the inputs described above:
- Calculate Effective Compression Ratio that is used by supercharger companies to show requirements for boosted engines.
- Estimate the rod bolt loads, piston speed and G’s at a specific RPM.
- If you are looking for a better estimate of the Cranking Compression you will have more input details to do so.
- Pick the Gasket Bore Diameter from preloaded, known FelPro and other manufacturers of gaskets.
- If you are going to mill the head this program can calculate the effect of the heads CC chamber.
- Calculate the Piston dome CC’s based on simple geometry (not from CCing the piston).
Our Compression Ratio Calculator is many engine builder’s best friend.