manifold pressure question

dan carley · #1 05-09-2015, 07:29 AM

a while back i had a post about a manifold gauge reading low. i replaced the gauge with a new one and it's reading much better. the question i have, is at 2500 rpms i'm only reading 22 in. at full throttle i am getting 28 in. if i remember right on my last rv4 with the same 0320 i could get 24 square. the cht readings are from 290 to 340. according to my fuel computer i'm using more gas. should i be looking for a possible air leak? or could this be normal? the engine is running fine and the performance is about where it should be. if there is a leak where should i look?

thank you
dan carley

AX-O · #2 05-09-2015, 08:36 AM

Dan, are your old numbers compared to the new numbers at the same altitude? ie are you in the same airport? What altitude were you when you read the numbers you are quoting?

edit, more questions

Did you keep all the same lines to your MP gauge when you changed the gauge? Also, are you assuming the other aircraft is a source of truth data? What if there were something wrong with the other set up and this one is working correctly?

dan carley · #3 05-09-2015, 08:41 AM

test were at 3000 ft. same airport. these are new numbers from new gauge

dan

DanH · #4 05-09-2015, 09:28 AM

A leak would make MP read higher. A throttle is a pilot-controlled leak.

Assuming a constant speed prop, pulling 2500 RPM down to 2400 will cause MP to rise.

Loss of MP taken in flight may not have anything to do with the gauge or the engine itself. For example, a flap of sealing rubber may be in the wrong place at a vertical induction airbox inlet...outright blockage, or a loss of dynamic pressure because sealing is poor.

ccsmith51 · #5 05-09-2015, 09:42 AM

We did the condition inspection on my -6 this week. One thing I noticed was that the AWOS said that the pressure was 30.04. My MP gauge read 29.5, about 0.5 in Hg less.

I'm not going to worry about it, but am curious how accurate MP gauges, and AWOS broadcasts, are. Could the AWOS be accurate and my gauge off, my gauge accurate and the AWOS off, or a combination of both?

BobTurner · #6 05-09-2015, 10:53 AM

In FL I assume you are close to sea level, so altimeter setting and MP should agree. I'm at 400 ' MSL so MP with engine off is 0.4 " less than altimeter setting. At higher elevations the corrections are non trivial.

esco · #7 05-09-2015, 03:32 PM

Bob Turner provided a way to validate the MP gauge on the ground: compare ATIS, MP, and the barometric pressure displayed in the altimeter Kollsman window (set to known altitude.)

(I'm in SoCal, and the big one is coming; some day field elevation will not be what's on the chart

. Until then, before cold engine start, MP should equal barometric pressure.)

At my home field, ATIS typically changes hourly; a difference of .01 or .02 between MP/baro and the recorded broadcast isn't an issue on the ground.

Before takeoff, I reset my altimeter to ATIS so I'm consistent with others in the pattern, arriving, departing...

BobTurner · #8 05-09-2015, 03:38 PM

Quote:

Originally Posted by esco

Bob Turner provided a way to validate the MP gauge on the ground: compare ATIS, MP, and the barometric pressure displayed in the altimeter Kollsman window (set to known altitude.)

ing...

This only works at sea level.
For modest altitudes, subtracting 0.1" per 100' MSL from the altimeter setting is pretty close.
At high altitudes, say Lake Tahoe airport at 6000', it's more complex. You need to get the absolute local pressure.

Jesse · #9 05-09-2015, 05:32 PM

Quote:

Originally Posted by ccsmith51

We did the condition inspection on my -6 this week. One thing I noticed was that the AWOS said that the pressure was 30.04. My MP gauge read 29.5, about 0.5 in Hg less.

I'm not going to worry about it, but am curious how accurate MP gauges, and AWOS broadcasts, are. Could the AWOS be accurate and my gauge off, my gauge accurate and the AWOS off, or a combination of both?

AWOS should be accurate, but you can check another local airport to verify.

dan carley · #10 05-10-2015, 05:53 AM

the gauge agrees with the awos and altimeter. i'm questing in flight at 2500 rpms the gauge reads 22 in i'm asking if that a low reading for that throttle setting. i'm a little confused when the engine is idle it reads low around 10 is that correct? full throttle 28"

dan carley