A MP gauge is nothing but an absolute pressure gauge. If you
disconnected it from the induction manifold and took it aloft,
it would read the same as a manometer; absolute air pressure
decreases with increasing altitude.

When connected to the induction, all it shows is the air pressure
on the downstream side of the throttle plate; it is NOT a direct
measure of engine power! The tables are just attempting to
explain how ****ty a MP is at measuring engine power...

MikeM
Skylane '1MM




Journeyman wrote:
So, I'm going through the performance charts for my new ride (in
case anyone missed my long postings), and there's a graph that
lists fuel flow for a given power setting (eg. 9.16 GPH @ 65%).
Another charts lists MP at altitude for each power setting.

The weird thing is the MP drops with altitude. ISTM, at constant
power, MP should increase to compensate for reduced air density.
I.e. at a given fuel flow, which translate to a given power, you
need a bigger volume of air to maintain the proper mixture.

We're talking small changes in MP, few 1/10th of inches per 1000',
but they're definitely decreasing with altitude.

Am I wrong or are the tables bogus?


Morris (so why didn't Jim Fisher comment on my transition to low wings?)