Lower manifold pressure than field altimeter?

[FullDeflection]

I'm sure this is a trivial, stupid question, but I can't seem to find anything in the PHAK or elsewhere.

When a piston engine is not running, we can all agree that the MAP will read what the prevailing altimeter setting is at the field (or, God forbid, at whatever altitude you might find yourself.)

How is it that when reducing power on a running engine, it causes a drop in manifold pressure below that of the prevailing relative pressure?

I can't seem to wrap my head around this scenario. Does it have something to do with vacuum? Thank you in advance.

 

[Swen]

Good question. Here's an article I saw linked from here about 2 years ago. Now I realize I need to read it again.

AvWeb

 

[Blackhawk]

http://www.advancedpilot.com/downloads/prep.pdf

Edit- Oops. Swen beat me to it.

 

[FullDeflection]

What an enlightening article. Thanks.

 

[MidlifeFlyer]

Good question. Here's an article I saw linked from here about 2 years ago. Now I realize I need to read it again.

AvWeb

One of the all-time great articles on the subject. And with a title that's hard to forget!

 

[Capt Obvious]

What I got from that article it the "standard" 25" and 2500 RPM is a useless procedure? We should just climb out at full power? What I got out of this is just leave the throttle full open, except when operating at low RPM's.

 

[jdlilfan]

What I got from that article it the "standard" 25" and 2500 RPM is a useless procedure? We should just climb out at full power?

yup oversquare is an old wives tail...

 

[Guy]

yup oversquare is an old wives tail...

^^^^^ OLD WIVES TAIL ^^^^^^^

....... as opposed to an Old Wives' Tale.

 

[CoffeeIcePapers]

What I got from that article it the "standard" 25" and 2500 RPM is a useless procedure? We should just climb out at full power? What I got out of this is just leave the throttle full open, except when operating at low RPM's.

It depends on what the POH says. For example, the Seminole POH I have suggests that you climb with reduced power for better cooling and extending engine life, although I am skeptical of it. Either way, I would rather just stick with the recommendation from the manufacturer.

 

[crazyjaydawg]

Good article, but in reality there is no such thing as suction; just a difference in pressures, but still for this article it works.

 

[LV-ARG]

I'm sure this is a trivial, stupid question, but I can't seem to find anything in the PHAK or elsewhere.

When a piston engine is not running, we can all agree that the MAP will read what the prevailing altimeter setting is at the field (or, God forbid, at whatever altitude you might find yourself.)

How is it that when reducing power on a running engine, it causes a drop in manifold pressure below that of the prevailing relative pressure?

I can't seem to wrap my head around this scenario. Does it have something to do with vacuum? Thank you in advance.

I think you have got the right idea but your wording leaves room for interpretation. If you are sitting on an airport at 10,000ft, the altimeter setting can be 29.92 but the manifold pressure indicator willl show 19.92 (roughly 20"). So Manifold pressure indicator will indicate QFE (station pressure) not QNH.

 

[FullDeflection]

I think you have got the right idea but your wording leaves room for interpretation. If you are sitting on an airport at 10,000ft, the altimeter setting can be 29.92 but the manifold pressure indicator willl show 19.92 (roughly 20"). So Manifold pressure indicator will indicate QFE (station pressure) not QNH.

Absolutely. Thank you.