MAP behavior question

On Aug 24, 6:22 am, Thomas Borchert
wrote:
Bob,

Same indication as with engine running.

The important thing to always remember is that "Manifold pressure
sucks", which is also the title of John Deakin's brilliant Avweb column
on the topic - absolutely required reading.

--
Thomas Borchert (EDDH)

I absolutely second that recommendation. I learned a lot from reading
John Deakin's columns. But you have to read past his writing style. He
tends to get wordy, and after a long introduction, he quickly glosses
over the final point. Its like a novel that ends rather abruptly. I
had to read them several times to get all the subtleties. But it is an
absolutely required reading for anyone wanting a deeper understanding
of aircraft piston engines.

"Dave S" wrote in message
...
Capt. Geoffrey Thorpe wrote:
As a result the rate at which air is pumped out of the
intake manifold is reduced, which allows the air flow through the
throttle body to fill the manifold up to a somewhat higher pressure.

Careful.. there is very little "filling" going on. Unless you are in a
Mooney, at wide open throttle, with the ram air door open.

Of course there is filling. Where do you think the air that mixes with
gasoline in the engine comes from? Open the throttle wide, lots comes in,
manifold pressure goes up. Close the throttle, and only a little comes in,
manifold pressure goes down (manifold "emptying").


Manifold pressure is almost solely the effect of the engine sucking air
in by piston action past the air filter, and ram air difference is
negligible in the typical trainer.

Ram air and filter pressure drops are pretty small compared to the pressure
drop across the throttle body except at wide open throttle. It's the
throttle that pretty much controls manifold pressure at any given engine
speed. (ignoring boost from turbo's, and effects of altitude for the
moment).


Respectfully, you guys are trying to measure something with a micrometer
(ram air contribution), that was cut with an axe (throttle) (to quote Jim
Weir)..


Who said anything about ram air?

--
Geoff
The Sea Hawk at Wow Way d0t Com
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When immigration is outlawed, only outlaws will immigrate.

Nomen Nescio wrote:


Quite simply, a piston on the down stroke (suck) has no idea why it's
on the downstroke. Is it being droven by a windmilling prop, or the other
cylinders firing? It doesn't know............it doesn't care.

For a given RPM and throttle setting, MAP will be a constant.



No, cause engines don't have 100% volumetric efficiency. On the exhaust
stroke, with combustion happening, there will be a higher volume of
exhaust, which is discharged less completely, resulting in less
intake... and a higher MAP when the engine is running compared to not
running (firing); RPM and throttle remaining the same.