Hopefully I can get an answer in here! Is the conversion to get to SAE numbers from STP/STD to mulitply by .96?

For instance 364hp STP * .96 = 349hp SAE?

 

I don't think there is a linear relationship like that. Both are different correction factors, and use different baseline variables in terms of temp, relative humidity, barometric pressure etc.. Depending on the actual weather that day, the correction factors for each will vary slightly.

 

Well shoot. I have some STP numbers I'd like to see the SAE version.

 

Yeah, he's right, it changes depending on all those variables. Last time I dynoed, the SAE factor was .98

 

Is there anyway to find out what it "is" or "was" then?

 

The STD and SAE are atmospheric correction factors to a certain standard. They take the atmospheric conditions when the motor was dyno'd and correct them to that particulat standard. I don't think you can take a number such as .96 and make that correction.

 

The dyno place should be able to bring up your file and put it SAE.

 

Post the weather conditions or if you do not have them..post the date, time, and place of the dyno.

You can figure a rough estimate. I or someone else can put it through a calculator and give you a ballpark figure. Normally the difference can range from 1-5% (STD-higher..SAE-lower) depending on weather and elevation.

Best bet is the dyno shop. They can convert it quite easily with their software.

Give that a shot first. If not, we can venture an estimate.

Good luck.

..WeathermanShawn..

 

Here's all you need:
http://www.land-and-sea.com/dyno-tec...horsepower.htm

The formula for correcting to SAE is:
cf=1.18*[(990/Pd)*[(Tc+273)/298]^1/2] - 0.18
Where cf is correction factor, Pd is ambient pressure in hPa, and Tc is ambient temperature in degrees Celsius.

SAE corrects to 77°F and 29.234 in Hg and STD corrects to 60°F and 29.92 in Hg.
If we want to correct to SAE standards and the engine was tested at ambient conditions of 60°F and 29.92 in Hg, (which just happens to be the STD standards) we have:
cf=1.18*[(990/1013.2)*[(15.56+273)/298]^1/2] - 0.18
cf=.95456, which is the correction factor when going from STD to SAE.

So you have 364*.95456=347.46 ~347 RWHP

If you want to do a more complete analysis and include the friction HP of the engine and the friction HP lost to the drivetrain in the correction to SAE, the link provides the details. It might take off 3-4 HP more but 347 RWHP will be the highest SAE equivalent you'll get neglecting the effects of friction.

The difference when going from STD to SAE will always be ~4.5%...no maybes or probablys to it. Here's another link with less detail about the calculation and a "rounded" number:
http://www.woodcarbs.com/hpcalc.pdf

 

Mmm..Interesting.

I was not aware that the SAE to STD HP conversion had an absolute linear relationship. Especially since the formula involves an exponential function. Normally that changes the results as each parameter changes. Otherwise it could be just a simple A+B-C.. It would be interesting to hold a couple of those functions constant and then change elevation or pressure and verify that.

I'll let the engineers debate that, but ZR1Racer summarized the math and technique quite well. Seems to normally fall in that percentage range..just seemed like I have done a few conversions out here at altitude that were within 2-3%.

Any math 'gurus' or engineers that can validate that absolute percentage? It is always good to learn something new..just my experience with formulas including exponents are that they are never quite that easy.

Thanks. To the OP, hopefully the discussion will help you.

..WeathermanShawn..

 

Actually it's not even linear, it's a constant. I'm not sure what you mean by different elevation/pressure because the pressures for the SAE and STD are 29.234 in Hg and 29.92 in Hg respectively...they don't change. It would be the same thing as if you were to dyno a car at the same temp/pressure everytime and correcting to SAE standards, the correction factor will be the same every time. I think you're getting confused with changes in ambient conditions when a car is run on the dyno and trying to correlate it to the correction factor for STD to SAE. The formula to correct to STD is the same as SAE but with different numbers in the numerator to match the temp/pressure for STD conditions. At that point, the conversion to SAE is a constant.

A few things worth noting, the pressures used are absolute pressures and not barometric pressures you see given to you by your local weatherman. Barometric pressure is "normalized" to neglect the effects of elevation while absolute pressure reflects the elevation where you're at. Since you're screen name has "Weatherman" in it, I'm sure you already knew this but others may not. As I alluded to in my previous post, the correction does not take into account the effects of friction in the engine and drivetrain. BHP=IHP-FHP where BHP is brake HP, IHP is indicated HP, and FHP is friction HP. (It can also be stated BMEP=IMEP-FMEP where MEP is mean effective pressure if you really like to torture yourself with calculations.) The air going into the engine affects IHP but the correction factor is only applied to BHP. For RWHP numbers, the HP lost to drivetrain friction escapes the correction factor also. The friction in the engine and drivetrain are a constant relative to the temperature/pressure of the air the engine dyno or chassis dyno is performed in so we don't "capture" the effects of ambient air conditions on that part of the IHP. Chassis and engine dynos have a portion of the program to capture the FHP portion while the formula from the link above does not. But like I stated above, it's only a few HP and typically within the percentage of error of the dyno so no biggie.

PS I love math and am a mechanical engineer...plus my minor at Georgia Tech was in Internal Combustion engines where I was the TA my senior year and got to run all sorts of dynos plus a CFR engine that could vary the compression ratio while it was running. A long time ago and neat stuff but I never used it in a real job so I've forgotten a lot.

 

Yea, I think I say what your saying. The conversion from ambient weather to STD or SAE involves the correction factor as the formula(s) indicate.

It is probably the use of absolute pressure that throws me. SAE uses 29.234 and STD 29.92 in Hg. At elevation (5000') if I compute an air density (HP reduction) and reduce it to each of those pressure levels, are you saying that no matter whether it is 0 C or 50 C, only the density difference of the absolute pressure matters?

Sounds like you have a good background in all this. I use Density Altitude and corrected pressure so much at Denver..and the relationships are not as constant as with the STD/SAE formula. But we are reducing pressure using a different methodology. I guess I just learned something new today.

It is interesting that you do not see that .96 conversion factor bandied about that much. When I started to do conversions a few years ago, I strictly went with just air density calculator and resulting HP loss.

Thanks for the heads up. Anybody disagree?

..WeathermanShawn..