When you are dyno testing engines, you want to be sure that each time you test the same engine your dyno comes back with the same power curve. If your dyno can repeat exactly for each test, then you can trust what when in does show a change from a modification, it is a true change. This is called repeatability, and is the goal of all dyno operators.
A lot of things affect dyno repeatability, like engine temperatures, fuel type, weather conditions, how the test was run, engine condition and build (like air cleaner type, spark timing) etc. For those things which you CAN control, you want to do things exactly the same each test. For example, engine water and oil temperature can have a significant effect on engine power, and are things you have some control over. You can install coolers and have them hooked up to temperature controllers. Or you can make sure you start each test with the same temperatures.
However, weather is something very difficult to control, and it has a large effect on the results. For example, a drop in barometric pressure from 30 inches to 29 inches would reduce power output by 3-4%. That’s more effect than many modifications you want to test for. Fortunately, there is an answer to “controlling” the weather for dyno testing.
First, here’s a little background info. An engine makes power from the fuel it burns, the more fuel, the more power. However, we all know that you can’t just keep richening up the carb and expect to keep making more power. That’s because there would not be enough oxygen to burn all the fuel. What actually limits the power an engine can make is the amount of oxygen you can pump through the engine. Most race modifications are designed to get more air through the cylinder, like reducing flow restrictions, more cubic inches, supercharging, etc.
Well, weather changes are much like supercharging. Superchargers make power by raising the density (pressurizing) of the air entering the engine. The same thing happens when the barometric pressure goes up, the air density (and therefore oxygen density) goes up. As the air temperature comes down, the air density goes up (like what an intercooler does). As the humidity goes up, the water vapor displaces the oxygen in the air and oxygen density comes down, reducing the engine’s power.
This all may seem quite complicated, but fortunately a lot of work has been done to understand these effects. Formulas have been developed to “correct” the power output measured on a dyno with some particular weather to what it would have produced on some standard day.
The first thing to decide is “what is a standard day?” For most race dynos, a standard day was 29.92” barometer with 60 degrees air temp and completely dry air (0% humidity). These conditions are much better for producing power than what most people would see driving their car. So the Society of Automotive Engineers (SAE) came up with a more typical day of 29.60” barometer, 77 degrees and 36% humidity. Dyno power curves corrected to the SAE conditions will show about 4% less power than curves corrected to the Race Dyno conditions.
Say you measured 182 HP in some actual weather that had weather conditions much worse than the standard day. The Dyno Correction Factor formulas would increase the 182 HP to, say, 192 HP if you use the SAE correction factor, or 200 HP using the Race Dyno correction. Theoretically, if you ran this engine on a day with the Race Dyno conditions (29.92”, 60 deg F, 0% Humidity), you would measure 200 HP on the dyno.
If you ran this same engine on a day with weather BETTER than the standard conditions, you could measure 212 HP. Again, but applying the Race Dyno correction factor, the “Corrected HP” should be brought down to 200 HP, the exact same as when you ran it in the bad weather. See the table below.
Weather conditions Bad Air 29.92” * Good Air
Measured HP ** 182 200 212
Corrected HP 200 200 200
* Standard Race Dyno weather conditions.
** Measured HP is many times called “Observed HP”
As you can see, the HP you measure (Observed HP) can vary significantly with weather conditions. However, the engine repeats exactly (200 HP) if you only look at the Corrected HP.
If we had chosen the SAE correction factor, the trends would have been the same, just that the 200 HP number would have been 192 HP instead.
The example data shows the dyno correction factor working perfectly. In the real world, it does not work perfectly. However, the corrected data is almost always more repeatable than observed data, providing the weather sensors are repeatable, and care is taken to eliminate all other testing variables.
The effect shown on Observed HP is very much the same as seen with Observed Torque.
This shows the basic function of Dyno Weather Correction Factors. Because weather affects power, we factor the measured torque and HP up or down the proper amount to make it more repeatable from day to day. Because the Corrected data is more repeatable, when you DO see a change in Corrected Power, you can have more confidence the change is real.