Can someone explain this to me? MAP vs RPM

[Bavafa]

I had a discussion with a friend about the relationship of manifold pressure and RPM with a CS prop. As RPM decreases, manifold pressure increases without touching the knob. In my test, it was actually by a fair amount increase but I don?t understand why. Isn?t that the throttle (Manifold) simply controls how much air is entering into the Cylinders thru the intake pipes?

Much appreciate the breakdown of the process

[Mike S]

Sounds reasonable to me.

The engine is always running at lower than ambient pressure----discounting any ram effect. Some folks would say the engine is sucking in air, others would say the higher outside pressure is pushing in the air. Dont really care, as the end result is the same.

Anyway, the faster the engine runs, the more often the manifold sees a vacuum and the slower it runs the less often for a given period of time.

Less vacuum pulses in a fixed period of time will therefore translate to a higher manifold reading.

What would the manifold pressure be if the engine were not running???

The term manifold "Pressure" is quite misleading.

[BillL]

The throttle does not "control" anything. It sets the throttle plate position which is a resistance to flow. That resistance creates a pressure drop across the plate. That pressure drop changes with flow demand. Flow demand comes from the engine, a pump. If you drop RPM with the governor control ( yes that is a control of speed) then the "pump" lowers demand and the pressure drop across the throttle plate is reduced.

Manifold pressure = Ambient pressure - pressure drop across throttle plate

Make sense now?

[rocketbob]

Bernoulli's principle in the induction system...as velocity increases pressure decreases. A manifold pressure gauge is a pressure gauge in reverse, increase of MAP is actually an increase of vacuum. Or a decrease in pressure compared to ambient.

[BobTurner]

Originally Posted by rocketbob

Bernoulli's principle in the induction system...as velocity increases pressure decreases. A manifold pressure gauge is a pressure gauge in reverse, increase of MAP is actually an increase of vacuum. Or a decrease in pressure compared to ambient.

A MP gauge IS a pressure gauge. It reads the absolute (referenced to a perfect vacuum, not ambient pressure) pressure.
With the engine off, it should read the ambient air pressure, in mm of Hg, NOT corrected to sea level.
In an ideal system (no air filter, huge diameter pipes) the MP would always read ambient air pressure at wide open throttle, regardless of RPM. In a real system there are some restrictions, and the pressure drops flowing thru them. The harder you "pump" (high RPM), the more the pressure drops. A closed throttle plate is a significant restriction, and gives a very significant drop.

[rocketbob]

oops meant to say absolute, not ambient.

[BobTurner]

Bob, I don't understand what you wrote at all. Unless you were thinking of the gauge that used to be in some cars, that read the manifold vacuum relative to ambient pressure. Not at all what is in aircraft.

[BigD]

I was getting confused, so I looked it up in the Pilot's Handbook of Aeronautical Knowledge:



but that doesn't answer the original question...
Here's one article that explains what's going on:

http://www.avweb.com/news/pelican/Pe...l?redirected=1

==dave==

[BillL]

What didn't post #3 tell you Dave?

[crabandy]

Just after takeoff and a safe altitude you pull your manifold pressure back to 24 inches, than you pull the Rpms back to 2500 and the manifold pressure has risen to 25 inches again.
With the engine not running the manifold pressure gauge should read standard pressure (29.92 inches) adjusted for nonstandard temp and pressure (roughly 1 inch loss per thousand feet Above sea level I believe, not sure about temp).

When you start the engine the throttle is closed minimizing how much air the engine can "suck" in, I think of manifold pressure as more of a suction gauge. Reads low pressure (high suction) of maybe 10 inches.

At full power ideally the manifold pressure would read the same as when the engine isn't running, but since there is a filter/airbox/bendy tubes/etc it's probably about 1/2 inch lower.

When you pull manifold pressure back after takeoff, you are setting the amount of air allowed to enter the engine at full RPM 2700ish rpm. This setting is generally 25 inches of manifold pressure, or a measure of how much air the engine is sucking. When you pull the rpm's back to 2500 with the throttle in the same position the engine is sucking less air, as in 200 rpm x engine cubic inch /4 (4 stroke). At the lower rpm setting and the same throttle setting the engine is sucking less (less vacuum) and manifold pressure increases slightly.