If I get the truck dyno'd one day and then do it again a different day, how much variation can I expect to see based on things like weather, humidity, dyno, operator, etc??

How does the correction factor play into it?

 

Dynojet's SAE explanation is here.

Starts near the bottom of the middle column.

Dan

 

The Society of Automotive Engineers (SAE) has created a standard method for correcting horsepower and torque readings so that they will seem as if the readings had all been taken at the same "standard" test cell where the air pressure, humidity and air temperature are held constant.

The equation for the dyno correction factor given in SAE J1349 JUN90, converted to pressure in mb, is:






where: cf = the dyno correction factor
Pd = the pressure of the dry air, mb
Tc = ambient temperature, deg C

The pressure of the dry air Pd, is found by subtracting the vapor pressure Pv from the actual air pressure. The relative horsepower is simply the mathematical reciprocal of the correction factor.

 

Temperature, Humidity, Bar Pressure play a BIG role in the power of these trucks. Ive seen people dyno one day, add a part, which is KNOWN to give gains, and dyno 2-3 days later and show a loss. The heat kills the power of these trucks, so to find gains of certain parts, you have to almost do it back to back within hours, and even the weather can change in hours.

-Mike

 

Personally, in the past I've picked up over a half second in the cool fall weather compared to the summer heat.

 

So a variance of about 5-10 hp may be normal, which is what I expected.

 

I've been told (by an automotive engineer, well versed in turbos) that the SAE corrections don't apply to supercharged engines very well, for whatever that's worth.

I think the worse thing to do is make comparisons on two different dynos.