Manifold Pressure Question

[flyifrvfr]

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

 

[jspilot]

you are reducing the engine power which in turn reduces the air flow through the intake which increases manifold pressure....slower velocity - higher pressure......well at least it sounds good!!

 

[Yellow Snow]

you are reducing the engine power which in turn reduces the air flow through the intake which increases manifold pressure....slower velocity - higher pressure......well at least it sounds good!!

Sounds backwards to me.

 

[UnAnswerd]

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

It doesn't actually increase, technically. You see, anyitme the engine is running, the pressure within the manifold will be lower than atmospheric pressure. We tend to think of it as a "vacuum". But "vacuum" isn't really the best term, because the pressure inside the manifold is not negative, it's just lower than atmospheric pressure.

When you close the throttle, the downward moving pistons are pulling this "vacuum" against a restriction. The actual pressure in the manifold decreases, but the pressure differential or so-called "vacuum" within the manifold increases.

 

[Yellow Snow]

It doesn't actually increase, technically. You see, anyitme the engine is running, the pressure within the manifold will be lower than atmospheric pressure. We tend to think of it as a "vacuum". But "vacuum" isn't really the best term, because the pressure inside the manifold is not negative, it's just lower than atmospheric pressure.

When you close the throttle, the downward moving pistons are pulling this "vacuum" against a restriction. The actual pressure in the manifold decreases, but the pressure differential or so-called "vacuum" within the manifold increases.

Keep trying you're closing in on it. Think about turbo charged engine though might have to rethink this.

 

[UnAnswerd]

Keep trying you're closing in on it. Think about turbo charged engine though might have to rethink this.

I still think he has it backwards. He claimed the manifold pressure increased with lower RPMS. We know that's not true with a naturally aspirated engine.

A turbo??? Well lets see. The turbo uses exhaust gasses to spin a turbine, which in turn forces more air into the manifold than the vacuum created by the pistons could otherwise provide. This means that when you increase RPM, you will also increase the manifold pressure as the turbine spins faster. When you let off, the turbine spins slower, and the manifold pressure decreases.

I say that it doesn't matter whether an engine is turbo-charged or not. The manifold pressure, in the exact meaning of the word, shouldn't be increasing with a decrease in RPM. Of course, I'm dealing with the normal situation, wherein closing the throttle decreases the RPM. Since RPM can in fact decrease with a constant throttle setting, it could be possible to have a higher manifold pressure with a slower RPM. Let's see:

You are climbing at full-throttle in a "cruise climb" with the props forward. You decide to switch to Vy, so you pitch up the nose and climb at a higher rate. The RPM decreases, but the throttle is still wide open. Since the throttle is wide open, the pressure within the manifold is probably close to atmospheric pressure. As the RPM decreases, less of a "vacuum" is created within the manifold because the pistons are moving slower. Therefore, the actual pressure within the manifold does indeed increase even though the throttle is wide open. Though I have little exposure to constant-speed props, I believe it would be possibe to have an increase in manifold pressure if the decrease in RPM was caused by pulling the propeller control back.

 

[UnstableAviator]

When you reduce the engine RPM, you reduce the engine's demand of air. Since the throttle position hasn't changed, the air will in effect "back up" in the intake tract thereby causing a pressure increase. It is just like Bernoulli's principle in reverse to the way we commonly think of it; Slower moving air exerts a higher pressure.

 

[five-alive]

Think about it, turn the engine off, you get maximum manifold pressure (local pressure). Start the engine, and the pressure will decrease for a given throttle position. Changing the rpm lies between these points and gives you a sliding scale.

This is why in order to set say 23" and 2300 RPM you first set the MP to 22.5" and then lower the RPMs, the pressure should rise to 23" or so.


sounds backwards but isnt

 

[UnAnswerd]

When you reduce the engine RPM, you reduce the engine's demand of air. Since the throttle position hasn't changed, the air will in effect "back up" in the intake tract thereby causing a pressure increase. It is just like Bernoulli's principle in reverse to the way we commonly think of it; Slower moving air exerts a higher pressure.

This is true.

In conclusion, I'd say it depends on what is exactly causing the reduction in RPM. If it is caused by closing the throttle, the manifold pressure will decrease with a fixed prop. If it is caused by other factors such as engine loading, the manifold pressure will increase so long as the throttle is held constant, fixed prop or not.

 

[Yellow Snow]

But wait...

If you have a given RPM and increase the throttle this lets more air into the engine right. But the manifold pressure also rises. There is no back-up of air, the engine is actually increasing its power output and taking in more air. This doesn't jive with your theory of less air higher man pressure.

Speak amongst yourselves.