The "SAE Air Correction Factor" which is used by all the dyno guys is:

SAE CF = 1.18[Ps/(Pm-Pv)][sqrt(Tm/Ts)]-0.18

(sqrt = square root)

This formula is right out of SAE Paper #J1349 (revision June 1990), and is the latest form of the air correction formula. SAE assumes 15% frictional loss within the engine, which is probably close for a piston engine. NOTE: this is NOT the losses from the crankshaft to the rear wheels, that is a different loss.

Ps = standard pressure

Pm = measured pressure (barometric pressure)

Pv = vapor pressure of water (relates to RH%)

Ts = standard temperature

Tm = measured temperature

The SAE baseline values are:

------------------------------------

Ps = 29.235 inHg

Ts = 77.0 deg F (534 deg R)

Pv = 0 inHg (dry air, no humidity)

Pv will not be 0 inHg during a dyno run, but will equate to the vapor pressure of water vapor in the air, which is a function of humidity.

P units = inHg

T units = deg R (deg F + 460). If not in deg R, you will get a wrong answer!

So, the CF will depend on where the actual air temperature, pressure and humidity is with respect the standard values listed above.

The CF can be less than, or more than 1.00. If you tested your Z06 on a dyno on a day when T = 77 deg F, P = 29.325 and the RH = 0% in the shop where the dyno was, then the CF would be exactly 1.00. As you can see, there are MANY combinations of T, P and RH that will give you the same CF.

Note, the Pv (vapor pressure of water) defines the RH% and is subtracted from the atmospheric pressure because the water vapor "displaces" that much pressure, and essentially makes it equivalent dry air (RH = 0%) at the resulting pressure of Pm - Pv.

---------- Post added at 07:04 AM ---------- Previous post was at 07:03 AM ----------

They are probably referred to as STD (standard) because the temp and pressure are for standard sea-level air conditions.

59 deg F (STD sea-level air temperature)

29.92 inHg barometric pressure (STD sea-level pressure)

I guess in 1990, SAE dropped the STD values and went with the ones I listed in my first post. Don't know the reason why.

If you plug in the STD temp and baro pressure values into the SAE formula as the measured values (like if a car was dyoed at those Atm conditions), then the CF comes out as:

SAE CF = 1.18[Ps/(Pm-Pv)][sqrt(Tm/Ts)]-0.18

CF = 1.18[29.235/(29.92-0)][sqrt(519/534)]-0.18

CF = 1.18[0.9771][0.9859] - 0.18

CF = 0.9567

Note that:

59 deg F = 519 deg R (deg F + 460 = deg R)

77 deg F = 534 deg R

Pv = 0 inHg = 0% humidity

So, this means that if a car was dynoed at STD conditions (59 F, 29.92 inHg, 0% RH) it would be making 1.0000 - 0.9567 = 4.33% more HP than if it was dynoed at the current SAE baseline Atm values of 77 deg F, 29.235 inHg and 0% RH (which would give a CF of 1.000).

---------- Post added at 07:06 AM ---------- Previous post was at 07:04 AM ----------

Plain and simple SAE are used to compare cars on different dyno's. Works great unless its a mustang dyno vs mustang dyno.

STD is like perfect weather etc etc it will make XXX amount. Most people prefer to know SAE.

---------- Post added at 07:12 AM ---------- Previous post was at 07:06 AM ----------

STD usually seems to a little bit higher from what I have seen.