Ok, so I just looked at my car's dyno charts ... both the charts generated in Colorado and now the new ones we just did in Chicago. Some questions come to mind:

1. Does the Gear Ratio setting affect how HP/Torque are calculated?

2.
a. On the CO dyno, changing from uncorrected to SAE corrected boosted the numbers by ~25%

b. On the Chicago dyno, changing from uncorrected to SAE corrected changed the numbers by <5%

...any idea why this is? Is this (SAE ratio) something that the dyno operator programs into the system?

3. Does it make a difference if the dyno pull is performed in 3rd gear or 4th gear? I.e. will one read higher than the other?

Any clarification on this would be greatly appreciated.

l8r)

 

The SAE correction is for a naturally aspirated vehicle and takes into account for atmospheric conditions, humidity, etc.

When you dyno a car at altitude and apply these correction factors to a turbo car, you are going to get inflated numbers simply due to the fact that turbo cars do not lose as much power at altitude compared to N/A cars.

 

I agree.. Altitude corrections will inflate HP values on a turbo car.. Ambient air temp and humidity at sea level to calculate corrections are usually how I do it just so I can get a consistent bit of information on different days.

Air density is calculated from barometric pressure, Temp, and humidity.. All of which will have an impact on the power an engine produces. ITs just nowhere near as dramatic on a turbocharged car if you have the ability to keep the same boost levels. The only issue I can see is the amount of heat in the compressed air due to intercooler not functioning as efficiently. So I suppose altitude in that respect could have an impact.

 

It is very different up here. we make peak power at ~6200rpms with a car with full exhaust cams etc. that is about a thousand rpms lower than at sea level. As you stated above we just run out of turbo, if we did not run out of turbo the numbers would indeed be inflated but because of this the corrected numbers are pretty close. I made 338 corrected on the dynojet that is close to what my car would do at sea level on the same dyno.

If the turbo did not run out of puff and the power peak kept climbing too 7200rpms then the numbers would be inflated (and higher) . So the correction is pretty accurate for stock turbo cars but inflated for upgraded turbo cars (at least on EVO's) that have enough flow to compensate for the thin air.

 

Ok, I think I've come to the conclusion that ANY kind of correction ratio is asinine. I mean let's face it, multiplying what the dyno actually read by some arbitrary number is kind of silly, or no? Personally I'd much rather just know how much power my car put down onto the dyno rollers that day ... so what if my car wasn't able to exert as much force on the dyno in Colorado as it was closer to sea level? Afterall, it was measured in Colorado, and any other car that would be strapped to the same dyno would suffer from the same penalties of running at altitude. From now on, I'm gonna insist on uncorrected numbers anytime I dyno my car...

Any clues on the gear ratio and 3rd gear vs 4th gear pulls?

l8r)

 

As posted earlier, I think its the extremes that end up needing correction.. But the biggest point is equalizing the power curves for different environments, and it does that pretty well if your in an environment where you are at a disadvantage. A turbo running out of steam is a good example of when correction would be used effectively.

 

4th gear would be a better choice simply because of the gearing.. the closer the gearing is to 1:1 drivetrain losses should be less of a factor.. From what I know, 3rd gear is the one most commonly used on dyno pulls.

My experience is you do get slightly different readings (on the street, remember, I use data sampling from my UTEC) also in my case 4th gear would result in very high vehicle speeds, but it would result in better data aquisition. I can only think that this would also apply on a chassis dyno. However the torque #'s would likely be skewed a bit since you no longer have any multiplication in the drivetrain (the software I use does compensate for that, as would a rolling road dyno should too) This could be good or bad..

Now.. with all of that said.. I think Third gear pulls are the most common and therefore that is what I generally use (besides safety in my case)

You could make 2 pulls in third, two pulls in 4th, and a pull in 2nd, to determine the drivetrain loss though.. since all three will be slightly different..