[Skip Fix]

So I know to compare from Houston to Denver to LA dyno results we need a standard correction factor. But the actual non corrected numbers are often lower-not "perfect air" -but that is actually probably closer to what the track conditions the car is going to be in running down the track so probably more real world numbers from my thinking.

[Stan Weiss]

Skip,
Yes, if the dyno was on the starting line the uncorrected numbers is what the engine would have to make the run.

But correction factor is for more than just different locations. If you dyno you engine and then 6 months later make some changes and go back to the same dyno but the weather is much different (like winter and summer) the uncorrected numbers may not tell the same story as the corrected numbers do.

Stan

[Skip Fix]

Stan I understand that, but if I live in Houston as I do there are only brief windows or "good air" /DA . So when I take my car to the track I may have 2 shots a year to actually be making "the numbers" if I use corrected. Track numbers are changing depending on weather, as well as tuning at the track.

If you expect your car to run corrected numbers in 90-100 degree 70% humidity normal Texas days(it was in the 90s here a week ago!) you are going to be awfully disappointed.

My IA/500 using the Wallace calculator for my best run in"average air" is 55hp down from corrected-closer to uncorrected. I don't think the alternator or 3.5" exhaust is hitting it that much.

[Stan Weiss]

Skip,
I am used to here where in the summer DA is a couple of thousand feet and in the fall can be zero.

Stan

[Steve C.]

About 25 years ago we dyno tested a engine at USA Racing in Houston. Their dyno cell is actually located below seal level. It was during February on a cold day with low humidity and it corrected backwards ! The raw numbers were actually higher than the corrected numbers. The dyno operator had a hard time keeping the cell water temperature high enough.


.

[Steve C.]

From the internet....

"The essence is in the collected data channels, with the least important being the correction factor. Correction factors provide only an estimate which means very little to most engine builders. The bigger your dyno correction factor, the more suspect the power numbers are. Uncorrected numbers are what the engine delivers for any prevailing conditions."


.

[wolf]

Correction factors aren't perfect (they rely on assumptions of engine mechanical efficiency for example), but they have their utility.

For well financed race teams, in hyper competitive classes, the correction factors aren't accurate enough for the small power gains they're developing. So for example, when I was at the Richard Childress campus almost 20 years ago, they had a dedicated building where the engine dynos were, and the HVAC system maintained sea level air density in the dyno cells, so no correction factors were needed.

[Stan Weiss]

Originally Posted by wolf

Correction factors aren't perfect (they rely on assumptions of engine mechanical efficiency for example), but they have their utility.

For well financed race teams, in hyper competitive classes, the correction factors aren't accurate enough for the small power gains they're developing. So for example, when I was at the Richard Childress campus almost 20 years ago, they had a dedicated building where the engine dynos were, and the HVAC system maintained sea level air density in the dyno cells, so no correction factors were needed.

While the Super Flow software does this, the SAE calculations do not.

Stan

[wolf]

Originally Posted by Stan Weiss

While the Super Flow software does this, the SAE calculations do not.

The J1349 standard does. It assumes a constant 85% mechanical efficiency in the calculations, and because it separates the atmospheric effects on indicated HP from frictional HP, I think it's a more accurate correction standard.

[Stan Weiss]

Originally Posted by wolf

The J1349 standard does. It assumes a constant 85% mechanical efficiency in the calculations, and because it separates the atmospheric effects on indicated HP from frictional HP, I think it's a more accurate correction standard.

Do you happen to have a copy of the SAE J1349?

This what I have that Ford uses.

SAE J1349
This formula utilizes the observed inlet air temperature and wet bulb readings to calculate saturated, current and corrected vapor pressure. The corrected vapor pressure is subtracted from the observed barometric pressure. It is subtracted because this pressure is due to water vapor in the air. This yields corrected barometric pressure.

The conditions for correction are 77° F and barometric pressure of 29.31 inches of mercury. Once the corrected barometric pressure is calculated and the observed inlet air temperature is known, those values are plugged into the following formula. The correction factor formula is:

C.F. = 1.18 #[(29.31/Corrected Barometric Pressure) * {(Observed Inlet Air Temp+460)/ (537} ^.5]-.18

Stan

[Tom Vaught]

TRUTH

Tom V.

[wolf]

Originally Posted by Stan Weiss

Do you happen to have a copy of the SAE J1349?

This what I have that Ford uses.

SAE J1349
This formula utilizes the observed inlet air temperature and wet bulb readings to calculate saturated, current and corrected vapor pressure. The corrected vapor pressure is subtracted from the observed barometric pressure. It is subtracted because this pressure is due to water vapor in the air. This yields corrected barometric pressure.

The conditions for correction are 77° F and barometric pressure of 29.31 inches of mercury. Once the corrected barometric pressure is calculated and the observed inlet air temperature is known, those values are plugged into the following formula. The correction factor formula is:

C.F. = 1.18 #[(29.31/Corrected Barometric Pressure) * {(Observed Inlet Air Temp+460)/ (537} ^.5]-.18

The J1349 standard is actually in metric units only, with reference T=25 C and (dry) p = 99 kPa. When using imperial units, it creates some confusion on the reference barometric pressure (since Hg density changes with temperature) so you'll see 29.31 in Hg, as you've referenced, but also sometimes 29.23 in Hg (for colder reference temperatures). As quoted, I believe your formula is correct for imperial units (and assumed mechanical efficiency).

The J1349 standard is pretty lengthy, and provides for measurement of frictional HP losses too, instead of having to make the 85% mechanical efficiency assumption (the 1.18 number comes from 1/.85 if you weren't aware).

Another interesting thing I think worth mentioning, is the standard specifies: "The magnitude of the power correction for tests run at non-standard conditions should not exceed 3% for inlet air or 3% for inlet fuel corrections. If the correction factor exceeds these values, it shall be noted as a nonstandard test in accordance with 8.1". So trying to correlate dyno numbers between Denver and Death Valley would be a challenge.

SAE is pretty strict about their copyrights, so unfortunately I can't share my copy of the standard.

Land and Sea still has a page up that does a good job of explaining the correction factors (although the illustrations seem to be missing now): https://dynomitedyno.com/tech-corner...ed-horsepower/

[Stan Weiss]

I have seen the 29.23 as well.

I have also seen 990 and 25 with both 1.18 and 1.176

I don't know if it is true. But I heard dyno jet uses 29.23 and 77 with no ME%.

Stan

[wolf]

Originally Posted by Stan Weiss

I don't know if it is true. But I heard dyno jet uses 29.23 and 77 with no ME%.

Stan

My understanding is that for most dynos "STP HP" is J607 correction (no consideration for mechanical efficiency), and "SAE HP" is J1349.