quite often you will see a baseline run with conditions very different from final build run, humidity maybe 35% then would be 9% for example, with the correction factor still 1.01 or 1.02 or 1

what does that all mean? and do you get such wide variances when quite often these are controlled, well insulated specific built rooms?

This may help:

The Society of Automotive Engineers (SAE) created the SAE J1349 JUN90 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. Furthermore, the SAE J1349 JUN90 standard includes an assumed mechanical efficiency of 85% in order to provide an estimate of the true engine horsepower (without accessories).

The equation for the dyno correction factor given in SAE J1349 JUN90 (for normally aspirated gasoline engines), converted to use pressure in mb, is:

sae equation jun90

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. For more information about pressures and calculation of the vapor pressure, see Air Density and Density Altitude.

The relative horsepower is simply the mathematical reciprocal of the correction factor.

SAE J1349 Update:

In August 2004 the SAE released J1349 Revised AUG2004 which specifies that the preferred method of determining the friction power used by the motor accessories is actual measurement, and that the assumption of 85% mechanical efficiency (as formerly used in SAE J1349 Revision JUN90) should only be used when actual friction data are not available.

The equation for computing brake horsepower (for normally aspirated gasoline engines), assuming 85% mechanical efficiency, was very slightly revised (and is presented here converted to use pressure in mb) as:

sae equation aug04

**Section 5.1 of the SAE J1349 AUG2004 revision also makes it clear that this correction factor is note intended to provide accurate corrections over an extremely wide range, but rather that the intended range of air temperatures is 15 to 35 deg C, and the intended range of dry air pressures is 900 to 1050 mb. **