Dyno and altitude question
 

Trying to get my head around correction factors and altitude change as it relates to superchargers.

As I understand it, the correction factor takes into account the changes in density altitude to adjust the output numbers.

Where I get confused ( okay so it's just one of the places I get confused ) is this...

At 6200 feet, my blower will only make 215 kPa at 1219 feet it will make 246 kPa. Is the correction factor taking that into account, or is the corrected number what I would make at 215 kPa ?

This chart at 6200 feet correctedhttp://racing.systemstrategies.com/NatsMotor_18PSI.jpg

What the correction factor is supposed to do is give you a reading that would equal what you would have at sea level, but it can be very messy. What you should take from the dyno is that you made X whp *uncorrected* at Y manifold pressure and Z rpm.

If you went down the mountain, put on a lower boost pulley, but were still able to make the same manifold pressure at the same rpm, then your uncorrected whp numbers should be equal. This is a simplistic sample and would have to assume all conditions are the same--dyno calibration, dyno operator, oil temp, air temp, and so on.

What the correction factor does not take into account are things like compressor efficiency, fuel headroom, ignition ability. You might legitimately make your corrected number if ran at sea level. Or you might break a piston into chunks because you ran out of fuel and charge cooling.

Chris, I can't answer your question, and forgetting about the super/turbo thing, I have more questions about your car than I can even start to itemize.

Everyone should look at Chris's build, that is one very badass autocross car.

i can't see how the dyno could correct for a different amount of boost on a different dyno at a different altitude.

The "different amount of boost" is kind of what the dyno correction factor is. Atmospheric pressure at 6200 ft is roughly 80 kPa and at sea level it's 100 kPa.

So to phrase my question another way does the correction factor take into account the higher boost value as well as higher atmospheric value?

I know we can talk about compressor efficiency islands and dyno calibration etc etc etc I am just trying to get a rough feel for gross comparisions. I can tell you the supercharged car feels much healthier at 1200 feet:giggle:

Bah. I just answered this.

At your altitude, your barometric pressure is approximately 20 kPa lower than sea level, or about 3psi lower.

Assume you would make the same amount of "boost" at sea level or at 6000' ft elevation. This is not necessarily a fair assumption as the s/c will be more efficient at lower altitude/higher atmospheric pressure, but we can ignore this truth to uncomplicate things.

The boost is difference between manifold absolute pressure and atmospheric.
However at 6000' elevation, your manifold absolute pressure at peak boost is 20kPa lower because atmospheric pressure is 20kPa lower.

The correction factor is intended to "add in" the missing 20kPa/3psi so that you can compare to what the car would pull at sea level.

Let's go the other way; up the mountain instead of down. At sea level, you pull 290 whp. You go up the mountain to 6000' and pull 260. Everything else being equal (which it never is), the 260 figure could be corrected up 11% to 290. I just pulled those numbers outta my ass, so use them only as abstract and not absolute.

But this whole thing is again very messy. As atmo pressure increases, your blower is able to work more efficiently. So a correction factor based on atmo pressure alone might not tell the whole story. Truth is at sea level, you would see several psi greater manifold absolute pressure than just the difference in atmospheric pressure alone.

Let's go down the mountain again. Assume you make 290 whp uncorrected at 6000'. We correct it up 11% to 320 at sea level. You have plenty of fuel and are well below knock threshold. You go to sea level and blow up your motor because you didn't have fuel and/or charge cooling sufficient for the higher atmospheric pressure, thus the higher absolute manifold pressure. You never make 320 hp to the wheels. Instead, you dine on metal salad while awaiting AAA to tow your busted ass automobile.
For this reason, IMO, best thing to do is ignore correction factors and always look at absolute manifold pressure on dyno runs.

Supercharged cars are affected by altitude in much the same way naturally aspirated cars are. The correction factors are close, the only oddness comes from the change in compressor efficiency. We once dynoed a supercharged car at 4700' then it was shipped right down to sea level. It actually made a bit more power at sea level than the corrected altitude number because the supercharger got more efficient. The car was easily able to support the higher uncorrected horsepower.

Basically, the correction factor tells you that your car is equivalent to a car with X horsepower dyned under the standard temperature and pressure, given the current conditions. It doesn't tell you that you'll be able to make that power at sea level, as you certainly can run into limitations of your current setup. We see this with the Voodoo superchargers, for example, where they can run out of injector when run at higher ambient pressures. But as long as you keep that in mind, you're good.

Ignoring correction factors isn't a great idea. It prevents you from comparing runs from session to session. Heck, from morning to afternoon. If you're looking to tune your fuel and make sure you're safe, then certainly look at at uncorrected. But if you want to know if a certain change you made since the last time the car was on the dyno was beneficial, use the correction factors.