oddwraith

iTrader: (1)

I just keep hearing about this "sae" and simply wonder how much I would put down before and after just calculation?

Bitemark46

**cliff notes*** SAE usually reads about 2-3% lower than STD

In the United States this is usually Society of Automotive Engineers (SAF) standard J1349 Rev JUN90. Correcting to SAE J1349 alters the data to make it seem as if it was taken when the atmospheric pressure was 29.23 in/hg., the temperature 77°F and the humidity zero.

The degree to which temperature, pressure and humidity affect power output is constant. If we accurately measure those parameters at the dyno location, we'll know the difference between the atmospheric conditions at the time and location of our test and those of SAE J1349. That difference is applied to the raw data and the result is corrected power and torque which we can compare to other data taken anywhere in the world that is also corrected to J1349.

The DynoWare EX+ measures and inputs all variables relating to the correction process. Older Dynojet hardware (an expansion board in a 286 or better PC running MS-DOS) requires the operator to measure humidity with the other variables inputted automatically. The software does the corrections based on the atmospheric data.

The older PEP software (v4.x) corrects only to SAE J1349. The newest WinPEP (v6.03) corrects to not only J1349, but to other standards, including "standard corrected," popular with aftermarket manufacturers (no doubt because it uses 29.92 in/hg and 68°F' to get bigger numbers), "EEC," used in Asia and parts of Europe, and "DIN," used in Germany.

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.

BTW, the pressure values in the formula has more effect on HP than the temperature values (square root of a ratio).



And this about STD



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).

4.33% more RWHP would equate to about 15 RWHP on a stock Z06.

You can also substitute the STD values of Ts = 519 R and Ps = 29.92 inHg into the SAE formula (instead of the current SAE Ts and Ps values) and run the numbers each way to see the differences in each dyno case. Tm, Pm and Pv will always be the measured values during the actual dyno test run regaudless if you use the old STD values for Pm and Tm or the current SAE values

94Mustang302ci

^^^ owwwwww my brain ^^^^^^^^^^

oddwraith

iTrader: (1)

Wow! That should be made a sticky or something lol. Nice work man, well beyond me but thanks nonetheless. So in summary *( lol__), what am I looking at in terms of

rwhp std-

and then rwhp sae?

Bitemark46

Well without getting technical when you see numbers on dyno thats in either "STD" or "actual" (they're usually almost the same) at that time based on the current weather conditions your car makes say.... 350rwhp STD. Now the computer software can be switch edit to SAE so your 350rwhp STD would now be around 340-343rwhp SAE.

In theory if you dynoed your car in 30* weather it made 350rwhp it "should" make the same power in 100* weather due to the calculation factor. However it does take into account weather conditions but there is an estimate factor. 1-3rwhp IMO. -Mark

ponygt65

ROFLMAO...FUggin mark...going all ******** on him.


Great explenation though.