What is the difference between Dyno #'s of SAE vs STD. I just got my cobra with my 2.3 whipple tuned today and the #'s were 592HP & 549TQ @ 17lbs, and the tuner changed from I believe SAE to STD and the same runs #'s went to 604HP & 560TQ. Is it just a different spot were a sensor reads? I forgot to ask while is was there and thought someone here could enlighten me.
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I was enlightened on this a couple of years ago. I can't remember if it was on this site, somewhere else online or a magazine. Since the the enlightenment has burned out and I'm in the dark again. I have the memory of a goldfish. What was I talking about again....? Oh, I believe the SAE is more of a real world dyno number and the STD is more inflated than actual when matched with actual performance and real world parameters. I could be wrong but according to your post the STD numbers are higher, so it makes a little sense. I know for whatever reason I preferred the SAE number, because they were more realistic maybe. Anybody else wanna educate both of us?
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Dyno Numbers: XXX RWHP/XXX RWTQ SAE
DD: 2003 Carbon Black BMW M3 Coupe 6MT (Stock)
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.
BTW, the pressure values in the formula has more effect on HP than the temperature values (square root of a ratio)
Last edited by 03Snake04; 04-27-2010 at 01:15 AM.
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This is from a magazine article on "dynojet" dynos.
SAE j1349 correction:
77 degree temp
0 percent humidity
68 degree temp
0 percent humidity
What this all means is that "Standard" will give you 2.6 percent better numbers (very popular for the import crowd). The example the magazine gave was a Vette making 412 (SAE corrected hp). With the Standard correction it made 423. All that you will have to do is multiply your "Standard rwhp" by .974 This should equal SAE (the one everyone uses). Up here in the great white North (Washington) the temps are in the 40-50 degree area, along with 29.9-30 in/hg. So, our numbers are actually corrected down from what it made at the rear. Usually, 96-97 percent of actual posted numbers. If you dyno when it's a hotter temp or higher altitudes the numbers will usually correct up to equal the desired temp, humidity and pressure of the correction factor.
^ Good info
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Well, I feel like a dumbass now. But an enlightened dumbass. That explains why I didn't remember exactly what the difference was. It's just a little complex to remember from a few years ago after reading once. Somehow I thought the difference was greater than just 2.6% though.
Last edited by YGOSLO; 04-27-2010 at 07:28 AM.