Hi there,

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.

basically, MP should not change with altitude at all, at least as long
as we are talking absolute pressure. To maintain a certain power at a
given engine speed, you need to have the same amount of air mass going
into the engine. As the volume of air the engine sucks in is determined
by the displacement and does not change, the density needs to remain
the same also, hence the pressure in the manifold stays the same.

(Of course, relative pressure will increase, as ambient pressure
reduces! However MP is always absolute, as relative pressure doesn't
tell you anything really. On a non-turbocharged engine relative
manifold pressure is always negative.)

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

that said there is an influence by temperature also. As at altitude you
have lower temperature in general, for a given density you need
slightly less pressure.

Also, with lower ambient pressure there is less backpressure on the
engine exhaust, leading to a slight power increase as the work to push
the exhaust gas out reduces. The volumetric efficiency of the engine
increases slightly (more air can be sucked in for given MP), again
increasing power for given MP.

- slightly increased power for given MP at altitude, resp. slightly
reduced MP for given power.

Am I wrong or are the tables bogus?

The tables are correct.

regards,
Friedrich

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