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Old 06-26-2008, 04:46 PM   #1
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Default How much HP loss for EBC turbo at alititude?

I had posted this in another thread, but thought I'd create a new thread for greater visibility and input from the community:

Let's address the "how much HP loss at altitude for turbos":
Common knowledge is "not as much loss as na or S/C, but still some loss".

Would the following logic and calculations be correct:
Sea level atmospheric pressure is 14.7psi.
Atmospheric pressure at 7000' is ~11.5 psi. (21.8% loss of pressure)

Therefore, the power loss for a na engine should be ~21.8%
However, for a turbo at 15psi the loss would be:
Sea level: (atmos. pressure + boost pressure) = 14.7+15 = 29.7
7000' : (atmos. pressure + boost pressure) = 11.5+15 = 26.5
Loss at 7000' vs. sea level = 3.2 psi, which equates to ~10.8% loss.

So in this case of 15psi, the turbo engine only sees losses of about half of what a na car would lose? Is this the case for EBC or MBC turbo'd engine (assuming MAP sensor)?
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Old 06-26-2008, 05:00 PM   #2
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I don't see why it wouldn't be assuming that you are still running the same pressure. You may have to retune the boost controller however.

Altitude is the reason superchargers and turbochargers were created. Airplanes were not making enough power when they were at high altitude so they forced air into the motor.
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Old 06-26-2008, 05:13 PM   #3
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Quote:
Originally Posted by wes65 View Post
I don't see why it wouldn't be assuming that you are still running the same pressure. You may have to retune the boost controller however.

Altitude is the reason superchargers and turbochargers were created. Airplanes were not making enough power when they were at high altitude so they forced air into the motor.
It's not quite that easy. While you can probably adjust the boost controller to create the "same" absolute pressure in the manifold, you are probably moving into a different area of the compressors output map. Depending on which turbo you are using you may either gain or more likely loose efficiency with altitude.

There is some discussion about whether a supercharger has the same power loss as an NA car. I haven't done enough testing to tell you for sure, but the Seat of the Pants (tm) dyno says my SC car pulls harder at sea level and the car makes more boost there as well.
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Old 06-26-2008, 05:33 PM   #4
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Let me preface this by saying that I never studied fluid dynamics. I'm not a mechanical engineer, and this is all speculation.

If the EBC is referenced to absolute manifold pressure, as would be the case with a MegaSquirt for instance, then the power loss will be much less than what new2mud proposes. Closer to zero, in fact. Because the pressure ratio of a turbo is variable.

If you have your EBC set to make, say, 190kPa of manifold pressure, then it will hold that pressure whether you are in Death Valley or at the top of Mt. Washington. Obviously there will be some power decrease as the efficiency of the system will go down slightly (the turbo is having to work harder, and making more heat) but it will not be significant by way of comparison.

Now that I think about it, a ball-and-spring MBC should, in theory, be the same. They operate by pitting MAP against the tension of a small spring. Ambient atmospheric pressure should have no effect on this.

Given the above configuration, what you should see, ironically, is more "boost" as your elevation increases. That is to say that the ratio of the pressure in the manifold to the pressure outside will increase, not because MAP went up, but because atmo went down. Remember that boost gauges (particularly those marked in inches and pounds) generally read pressure relative to the surrounding air.

Let's say that you have your boost controller set to hold the aforementioned 190 kPa. At sea level, baro is about 100 kPa, that works out to 90 kPa above atmo, or about 13 PSI of boost. Your boost gauge will read 13 PSI.

In Devner, the ambient atmosphere is at about 83 kPa. Your boost controller is still going to make 190 kPa in the manifold, and this is now 107 kPa above atmo, or about 15.5 PSI of boost. Engine power will remain the same, because MAP remains the same. Boost, as we generally think of it, is relative.

This works because boost controllers are essentially a closed-loop system. They adjust their behavior to achieve a pre-set manifold pressure, and they shouldn't give much of a damn what's going on around them. A turbo which is operating on stock can pressure might be more affected, as the plate inside the can has atmo acting upon one side of it, but I don't think this is going to be signifigant.

Now, a supercharger is a different story. Unlike a turbo which is able to adapt its compressor speed to suit the occasion (by varying the wastegate opening) the supercharger operates at a fixed pressure ratio determined by its pulleys and lobe design.

A naturally aspirated car driven from San Diego to Denver will lose 17% of its manifold pressure by the end of the trip, and be down 17% on power. That much is easy.

Say that a supercharged car is designed with a pressure ratio of 1.5. At sea level, that's 150 kPa MAP, or about 7.35 PSI of "boost".

In Denver, atmo is 83 kPa, so the 1.5 PR makes for 124.5 kPa of MAP, 41.5 kPa above atmo, or about 6.1 PSI of "boost". And just as before, we're down 17% on total air mass.
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Old 06-30-2008, 05:12 PM   #5
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Turbos do suffer, because they have to make more boost in order to reach the same absolute manifold pressure. They'll usually drop into a less efficient range and backpressure goes up. Spool performance decreases as well. They fare much better than both supercharged and naturally aspirated cars, but they do still suffer. Around here, I can take a car from 4700' to 11,000' in less than an hour, and you'll feel the car's performance change.

However, you can't do simple math and come up with a correction factor. First, you'd have to change the shape of the curve. But even if you're only interested in peak power, you'd also have to have the turbo's compressor map to figure out how that particular model was affected by the change in pressure.

MBCs push against a spring but also against ambient pressure.

Supercharged cars are affected very much like naturally aspirated ones - in some cases, more so as the supercharger is also working in a different efficiency range. We've seen twin-screws make more power at sea level than the corrected high-altitude power as the compressor moves closer to the sweet spot.
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Old 06-30-2008, 08:19 PM   #6
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This is all very interesting information. I have wondered about all of this since i live at 7200 ft myself.

To the op: where do you live?
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Old 07-01-2008, 06:25 PM   #7
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Quote:
Originally Posted by Fireindc View Post
To the op: where do you live?
Colorado, though a bit higher in altitude than Keith and crew.
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