I'm missing something...

[Greg Hermann]

At 12:03 AM 11/12/01, Bob wrote:
>Isn't the reason for the MAP is to calculate the % Oxygen in
>the air at different elevations?
>
>MAF would still measure just the amount of Air entering the engine
> , even if it's Oxygen content is much lower at higher elevations...
>
>or do I have it all wrong?

MAP (together with MAT) will tell you the density of the air entering the
engine, allright. BUT--the backpressure on the engine will vary with
altitude, and MAP tells you nothing about this.

Generally, an engine will need more fuel (richer jet or longer injector pw)
to get the same  AF ratio as backpressure increases--so you STILL need to
know barometric pressure as well as MAP in order to compute the correct
mixture.

This is also the reason why it is a good idea to measure exhaust back
pressure (upstream of the turbo) (and input it to the ecu as a mixture
correction factor) on a turbocharged engine.

Greg


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