Power Loss at Higher Elevation -Turbo ICE

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morris39

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I am doing some logging and wanted to establish a reference point for peak power (MY S3 Audi) The question is how much loss occurs at higher elevation (1100 ft in this case).Google was not useful at all. Many obviously wrong ideas and a few true statements which did not address the question, this mostly from car forums.
My take is that since the air density (so also O2) declines with elevation, the delivered air mass will decline in proportion IF the turbo is limited to to the same work output power power will decline. But if the control point is air mass set point then the turbo will engine will produce the same power as at sea level (ignoring efficiency effects due to temp. rise and material safety limits).

If my reasoning is correct the question is what controls engine air mass intake? MAF? If so can it compensate for lower air density over a small range? Can someone with this knowledge educate me?
 
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Uwe

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On most turbocharged engines, the turbo has more capacity than it needs at sea level. Tuners take advantage of this to get 20-25% more horsepower. The OEM uses this excess capacity to maintain power at higher elevation. In the aircraft world, this is called "flat rating" an engine, where it will make rated power anywhere between sea level and some specified altitude.

The exact scheme that the ECU uses is not entirely clear, but it's probably based on a combination of inputs from the MAF and the MAP sensors. Some engines may also limit power based on EGT sensors (e.g. the 2.7T from the early 2000s).

A rule of thumb for naturally aspirated engines is that power will be reduced 3.5% for every 1000 ft above sea level, but it's more complicated than that, since air temperature and humidity also have a substantial influence. In the aircraft world, take-off runs and climbing ability are calculated based on density altitude, which is not a very straightforward calculation at all.

However, on a modern turbocharged engine, I would think that 1100 ft above sea level is of no consequence at all; the turbo is not working that much harder than it would at sea level to maintain power.

-Uwe-
 
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morris39

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@Uwe
Thanks. Clear and brief. Another reason to like this blog.
 
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Jack@European_Parts

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Don't forget intake air temp or IAT and comparison of BARO to MAP to MAF & ECT or EGT.

In aviation when the turbo fails to make up for the needed OXS in higher altitudes it's a boot strap and usually due to high humidity levels and temp and or a wastegate being stuck or improperly set.

Turbo props with CIS are known for such issues & where output is fine on ground during a ramp up, however, in air, indeed boot strap showing air inadequacy to burn fuel injected.


The big thing often forgotten by tuners or modifiers is the spring of WG to hold boost from being blown off by boost itself by default.
 
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