Dyno correction, turbos and altitude

[JCX]

So I've been thinking. Most of the time when you throw a car on the chassis dyno you correct for altitude no? Of course there are several different correction factors and such. Anyways, a turbo or supercharger will somewhat compensate for the thin air at higher altitudes becuase it's compressing the air. On a turbo car, it should make (for example) 12 psi no matter what altitude, though it might take a bit longer to compress the thinner air. Is it fair to say that if we apply the same correction factor to a FI car as an NA car (especially at high altitudes) it is somewhat misleading? Should they apply different corrections for FI and NA cars?

Agree? No? Comments? Should I quit drinking?

[WGR4Pussies]

very compeling argument, i don't understand altitudes affect on air when pressurized enough so i can't really comment, but yeah it sounds possible.

but i think 12 psi at different altitudes will have a different mass.

[legendboy]

yes they do. fi cars have alot less actual correction (supose to) for altitiude

[Phuqu]

Originally posted by WGR4Pussies


but i think 12 psi at different altitudes will have a different mass.

Wouldn't 12 psi be 12 psi no matter what the altitude?


JDM - Stop drinking so much.

As for correction factors I think it should be consistant whether or not the car is NA or turbo. The correction factor is for altitude only is it not?

[bart]

no, in calgary, if your boost gauge shows 16psi you are really pumping 17. because calgary to sea level is ~950 mbars, so about a psi, so add 1. but i may be wrong...

[googe]

JCX, thats how it is. When you enable the SAE correction on a dyno with a forced induction car, you end up actually over-correcting. Do a dyno run on a cold day, then on a hot day. Most people will assume that youll get better numbers on a cold day, but if you do it on a hot day, youll see they are higher. Did you make more power on the hot day? No, but the SAE J1349 correction factors dont take forced induction into account, so you get overcorrected.

While it is true that 12 psig = 12 psig, the dyno is acting as though your car has atomospheric pressure. The formula used for the correction takes pressure into account to perform its math, but no fields ask for or check boost pressure, only ambient dry air pressure. This causes the extra boost you are running to be disregarded. It would be a pretty easy fix to implement in software if the dyno had the boost curve available, but for some reason they dont bother.

So if you want to pull big numbers without manipulating the dyno directly, do the following

1.Take your car to a dynojet
2. Do it on the hottest day of summer
3. Dyno it in calgary rather than edmonton, or somewhere even higher if you can find it

Another reason why dyno numbers can be sorta silly :)

[googe]

Originally posted by bart
no, in calgary, if your boost gauge shows 16psi you are really pumping 17. because calgary to sea level is ~950 mbars, so about a psi, so add 1. but i may be wrong...

I dont think thats quite right. At sea level with say 10 psi of boost you are running 14.7psia + 10psig = 24.7psia. Assuming calgary is roughly 1psi off in terms of absolute pressure, (i think its like 1.5psi but we will say 1) it would be

13.7psia + 10psig = 23.7psia

So in a sense, youd want to subtract a psi from your gauge. Its measuring the difference in pressure between your absolute pressure and the atmospheric pressure (your boost). Your boost gauge will always read accurate for what its meant to be measuring, but if you want to look at it and say your car is running 1 less psi of what it reads, it makes sense in a way, but altitude wont have any effect on the accuracy because its only a measure of relative pressure above and beyond the atmospheric pressure, which is where the loss is.

[rage2]

Most boost controllers and gauges will reference ambient pressure to reference boost. Which means...

At sea level, 14.7psi, if you run 10psi of boost on the gauge, your car is running 24.7psi absolute pressure. An NA car would be 14.7psi.

At 3700ft in calgary, 12.9psi ambient pressure, if you run 10psi of boost on the gauge, your car is running 22.9psi absolute pressure. An NA car would be 12.9psi.

Which means an NA car in Calgary would make 87.8% of the power as opposed to Sea Level. The turbo car would be making 92.7% of sea level power.

This is why stock FI cars generally lose less power as it goes up in altitude.

Now, let's say the car's not stock and you can adjust the boost, or the boost controller references absolute pressure. I believe the late model porsche/vw/audi's are like this...

The ECU asks for 25psi absolute pressure. At sea level, it gets 25psi absolute pressure. At 2700ft in Edmonton, the turbo is big enough to be able to provide 25psi absolute pressure. In Calgary, 3700ft, the turbo is still big enough to be able to provide 25psi absolute pressure, but off the efficiency range to affect density at 25psi absolute pressure. Finally, in colorado at 6000ft, the turbo can only provide 23psi of absolute pressure.

At sea level, the car makes sea level power.

In Edmonton, the car makes the same power, but it takes longer to spool up (JCX is correct in that it's laggier... or takes longer to compress thinner air).

In Calgary, the car would make a little less power because it's off the turbo's efficiency range even with same boost level. Even more laggier.

In colorado, the power would drop off a lot more, because first it can only make 23psi absolute pressure. Second, at 23psi, the turbo is off the efficiency range. Whenever a turbo is maxxed out (can't make more boost), it's at the edge of the compressor map on the worse efficiency island.

Finally, the dyno correction... looking at my examples, you can see that there are tons of factors that can affect turbo/supercharged cars, so there is no general rule for correction. Just use SAE correction ;).