Manifold Pressure Question

[gern_blanston]

We're not talking about cylinder pressure though, we're talking manifold pressure.
With a 7.5% decrease in RPM, don't you get a 7.5% decrease in the volume of air pulled through the intake manifold if the throttle plate doesn't move?
(OK, I realize that air's actually 'pushed' through the intake manifold due to the differential pressure caused by the increased displacement in all 6 cylinders on their intake strokes.)

 

[gern_blanston]

And the intake manifold doesn't know whether the engine's running or not, just that air's being transferred through it.

 

[MVSW]

This is a really funny post. I was reading this and I read about 25 different reasons why MP goes down when you decrease RPM!!LOL

 

[Donsa320]

This is a really funny post. I was reading this and I read about 25 different reasons why MP goes down when you decrease RPM!!LOL

Now, that is sad.....because MAP goes UP with a decrease in RPM, that was what all this was about. <vbg> Shows you what complicated explanations can do.

~DC

 

[Donsa320]

Lesson; Manifold Air Pressure is NOT am indicator of engine power. Lesson 2; RPM is not an indicator of engine power either. I point these facts out to light-twin engine pilots especially.
Correct Again! But only if you are only using one or the other, the two combined tell you engine power;
------------------------------------------------------------------------------

DC-8

My last word on the subject...you are WRONG again. You can have Manifold Pressure and RPM and NO power, they are both INDIRECT indicators of power, whether alone or together. Any windmilling engine will show you this. Please, get out and fly and try it out, Jeez! You do fly do you not? You claim an ATP but I guess it was not in piston power. These facts are true in both normally aspirated and geared supercharger engines. From Apaches to DC-3s, Convairs and bigger. From O-320s to R-4360s and R3350s, I have plenty of time with all of them.
When you get to Turbo Superchargers there is a difference since exhaust energy is required to get some boost. If you want to go there, do it with someone else.
Good Day Sir.

 

[Donsa320]

Can someone please tell me why manifold pressure increases when you decrease RPM'S when you are doing a run-up

Yes. You are pulling air out of the intake manifold into the cylinders at a slower rate as the RPM decreases so air coming in over the throttle plate from the atmosphere can increase the Manifold pressure closer to ambient.
Do you like it? (grin)

 

[Denizen]

Prt

Donsa, I know you bowed out of this conversation, However a quick ??

In the large engines you mentioned, I recall Power Recovery Turbines or some such from my A&P books. Only having seen them in museums, I forget how they work, Maybe you guys can explain them...they were/are pretty neat...

Thanks

 

[DC8 Flyer]

..NVM

 

[DC8 Flyer]

Yes. You are pulling air out of the intake manifold into the cylinders at a slower rate as the RPM decreases so air coming in over the throttle plate from the atmosphere can increase the Manifold pressure closer to ambient.
Do you like it? (grin)

You are still not explaining it! You said it "You are pulling air out of the intake manifold into the cylinders at a slower rate". What law jumps out at that statement?

 

[DC8 Flyer]

..NVM