Positive Displacement Blowers vs NA at Altitude...

[Mr. Wyndham]

What I'm talking about here is a recent magazine article pitting the Boss LS Mustang against the ZL1 at Inde Motorsports Ranch...approximately 4000 ft above sea level.

This post is less about the cars, and more about the effects altitude has on different form of induction.

It's always been my understanding that Positive Displacement supercharged engines have no more advantage at a higher altitude than a Naturally-Aspirated engine does. Whereas Turbocharged engines are virtually immune to altitude changes (to a point).

I've long thought that the blowers rotors, being belt-driven, cannot spin faster to compensate for reduced ambient pressure, so the engine would experience the same percentage of power loss as a NA motor.

Have I understood wrong this whole time?

[SUX2BU]

Originally Posted by Dragoneye

Have I understood wrong this whole time?

Yes.

[Jessika]

I don't know the answers, but a PD blower will lose approx 1lb of boost for every 3,000 ft of altitude.

[PQ]

2 words.......................

Pulley

Down

[RSBAD454]

Ask all the air planes that ran superchargers at much higher altitudes for many years.

[izaks]

Originally Posted by Jessika

I don't know the answers, but a PD blower will lose approx 1lb of boost for every 3,000 ft of altitude.

Correct.

If your "key on" map is 100kpa at sea level, and 85kpa at 6000ft (like we are) than you have "lost" .15kpa (2.175lb) in boost.
We have to run our Mp1900's (on the L76/L98) with the 2.8" pulley to compensate for the loss of boost due to altitude

[old motorhead]

Both S/C and N/A will lose power as altitude increases. The difference is that you can spin the blower faster to make up for the loss. N/A, you don't have any options to get the power back. As long as you have plenty of turbo along with some type of boost control, the turbo will compensate for altitude increases without much adjustment.

[bigsquirrel]

Bingo, as someone that lives at high altitude the difference between here and sea level is pretty ridiculous. I could pulley down/up to compensate. A turbo wouldn't need a physical change just turn it up.

[SUX2BU]

You guys aren't understanding his question.

What I think he means is let's say you have two cars, one NA and one with a PD blower, run the EXACT same time in the 1/4 mile and make the exact same rwhp at sea level. The OP thinks that at 4,000 ft DA the two exact performing cars will still perform almost exactly alike. Both will run about the same slower 1/4 mile time and both make about the same amount less rwhp.

I say that the while both cars will obviously be affected by the DA, the NA car will be much more affected by the change.

[asrautox]

Originally Posted by SUX2BU

You guys aren't understanding his question.

What I think he means is let's say you have two cars, one NA and one with a PD blower, run the EXACT same time in the 1/4 mile and make the exact same rwhp at sea level. The OP thinks that at 4,000 ft DA the two exact performing cars will still perform almost exactly alike. Both will run about the same slower 1/4 mile time and both make about the same amount less rwhp.

I say that the while both cars will obviously be affected by the DA, the NA car will be much more affected by the change.

Yes, it's vacuum versus boost, and it's pretty much all about manifold pressure.

NA MAP goes from say 14psi to say 12psi (-14%)
SC MAP goes from say 22psi to say 20psi (-9%)

NA loses 5% more power.

[JamesNoBrakes]

Turbocharged engines are NOT immune to altitude, that much I know for sure.

I don't know of any CAR manufacturers that tune their turbos like AIRPLANE manufacturers do. In the turbo airplanes that I've flown, the general idea is that you can take an engine that makes 235hp at sea level N/A, and make that 235hp up to something like 14,000' and then slowly drop off in power at even higher altitudes, but generally not as much as if it was N/A.

This means the turbo is essentially doing nothing at sea level (just making up for slight losses in tubing and induction bends, leaks, spinning turbines, etc, at something only marginally above ambient pressure).

These engines are designed to operate continuously high altitude and at full power with the turbo running to produce it's "max rated horsepower", that is what it's designed for. So at high altitude, the turbo is spinning and so on. It's also real cold up there by the way.

A car's turbo is usually designed to be working hardest during maximum engine output, at sea level. If this weren't the case, you'd see turbos having a much shorter lifespan in places like Denver and others as they'd be working harder, but this is not the case. My turbo made maximum pressure at sea level (guage) and as I got higher and higher, it bled off gradually. I live at high altitude, so I saw the effects every day. The other issue is that it takes longer to "build up" the boost at higher altitude, so even if you can tune it for higher pressure, it lags more in getting there, but as far as I know, no car manufacturer actually designs and tunes their engines like aircraft manufacturers do. If anything, the turbo engine doesn't "drop off" as rapidly as the N/A engine, but the turbo's efficiency max is reached rather quickly if you are just going up in altitude and it may be spinning super fast, but not making all that much pressure anymore.

In any case, a simple way to think of it is turbo aircraft engines use turbos to maintain displacement as altitude increases. Car manufactuers use it to increase displacement at SL. Lot of people think that in a car it works like an airplane. It doesn't.