Why does the manifold pressure rise when you exercise the prop on the ground?

Thanks.

 

Not sure, a few suggestions could be the fact that the engine is working harder and not as hard as the prop moves from a feathered position to a low pitch. Another could be the amount of oil that is moving in and out of the prop hub in such a short amount of time and it is affecting the MP in a way you are seeing.

Just my two cents...Sounds like a good question though..maybe an AP can help......or just someone more intelligent than I

 

I think you're on the right track with the change in prop pitch creating more work for the crank shaft and adding to manifold pressure. If there's another reason, I'd like to know as well.

Manifold pressure rises any time the prop is set to a higher pitch/lower speed. In our DA40s, I have my students reduce MP an inch lower than desired then reduce the prop. Works almost every time.

 

http://www.avweb.com/news/pelican/182544-1.html

View "Manifold Pressure Sucks!" and "Those Marvelous Props".

Free registration priceless info.

 

Something like that.

Key Concept: MP is the air pressure in the manifold and is determined by how much air goes into AND out of the manifold...change either one and MP changes.
-Throttle controls the plate which allows air INTO the manifold.
- The engine is an air pump...it sucks air out of the manifold with each intake stroke. The faster the engine runs, the more air is sucked out of the manifold.


What actually happens when you cycle the prop...

1) Prop control causes the governor to drive the prop to a deeper pitch.
2) The prop takes a deeper bite out of the air, moving more air and doing more work.
3) Since the throttle has not changed, the engine power did not increase. Since the prop is now doing more work, but the engine is making the original power, the extra load on the prop drags down the engine.
4) As the engine slows, it sucks less air out of the manifold.
5) But the throttle setting didn't change...

So we are allowing the same air into the manifold (throttle), but sucking less out (slower engine), so the MP rises slightly. This rise will also cause a slight increase in engine power, but not enough to overcome the high-pitch prop's drag.

 

thanks guys, that article was great

"Now a real-world example: Assume you're cruising at some low altitude (say 4,000 feet), throttled well back to about 20 inches MP and 2,000 RPM. (Remember, this means the throttle plate is somewhat cocked, restricting induction airflow.) Now reduce the RPM to 1,200 without changing anything else, and you'll see the MP rise sharply. Why? Simple: The ambient pressure hasn't changed; the throttle plate hasn't changed; the only thing that has changed is the speed at which the pistons are pumping the air. Since they are moving much more slowly at the lower RPM, they are not sucking nearly as hard — not creating as much of a vacuum — so the MP goes up, towards ambient pressure. The natural extension of this experiment is to reduce the RPM to zero, when the MP will rise all the way to outside ambient pressure (about 25 inches at 4,000 feet). "

That basically answered my question..same thing rickair said
!