Manifold Pressure question

[Flyingfanatic]

Just finished re-reading a great article by John Deakins about manifold pressure. In it, he states to check the gauge before startup by obtaining the local altimeter setting (or setting field elevation in the altimeter) and then adjusting for the height above msl. The MP gauge should read roughly that figure.

Example: Baro pressure is 30.15 and airport elevation is 2000 feet. The MP gauge should read roughly 28.15.

What I can't understand is why you would need to adjust for the airport elevation above MSL? If the MP gauge is reading ambient pressure with the engine off, shouldn't it just match the altimeter setting?

I'm sure he's right, just trying to understand why.

 

[Norman]

Altimeter is corrected to sea level. MP gauge is not.

 

[James331]

Altimeter is corrected to sea level. MP gauge is not.

 

[Capt. Geoffrey Thorpe]

As mentioned, the altimeter setting is sea level pressure.

If you turn the knob until the altimeter reads 0 feet, the pressure in the Kollsman window will reflect the local pressure and will come close to you manifold pressure gauge. (Unless you run out of adjustment.)

 

[Cpt_Kirk]

That portion of the article is more "gee whiz" than it is important. It's good for a rough check of the MAP gauge. Don't try to verify the 0.15". Lol

Keep reading his articles. Great stuff.

 

[Dan Thomas]

That check sort of verifies MP gauge accuracy, but it doesn't verify that there are no leaks between it and the manifold. Any leak will throw the readings off once the engine is running.

 

[coloradobluesky]

The manifold pressure guage reads "station pressure". Your altimeter reads station pressure via its static port, but it doesnt display it.

One interesting item, if you reduce rpm by pulling the lean knob, the manifold pressure doesnt change. Reduce it by pulling the throttle back, manifold pressure decreases.

Another interesting item. If you pull the throttle back to say 18" and then turn the prop back to 2000 rpm (from 2500 rpm). The manifold pressure goes up from 18" to 20" as the rpms go down.

 

[Henning]

Just finished re-reading a great article by John Deakins about manifold pressure. In it, he states to check the gauge before startup by obtaining the local altimeter setting (or setting field elevation in the altimeter) and then adjusting for the height above msl. The MP gauge should read roughly that figure.

Example: Baro pressure is 30.15 and airport elevation is 2000 feet. The MP gauge should read roughly 28.15.

What I can't understand is why you would need to adjust for the airport elevation above MSL? If the MP gauge is reading ambient pressure with the engine off, shouldn't it just match the altimeter setting?

I'm sure he's right, just trying to understand why.

Easier way to do it, set the altimeter to zero at airfield altitude (QFE) and the number in the Kohlsman window should be what the MP reads with the engine off.

 

[coloradobluesky]

You do realize that sea level pressure is the pressure where you are lat and long, but down at sea level (as if there is a hole in the ground and someone drops a barometer down it). So changing altitude doesnt affect sea level pressure. Sea level pressure is affected by change in weather over time or change in weather from place to place. Going 1000' up in altitude above the airport weather station, the sea level pressure stays the same.

 

[BillTIZ]

Easier way to do it, set the altimeter to zero at airfield altitude (QFE) and the number in the Kohlsman window should be what the MP reads with the engine off.

Can't always adjust the altimeter that far to get to zero elev. at least not at my airports. Learn to set altimeter to field elevation and do the math.

 

[Henning]

Can't always adjust the altimeter that far to get to zero elev. at least not at my airports. Learn to set altimeter to field elevation and do the math.

True, and know the math is something you should, however for the people who live at lower altitudes, it's a useful tool for the quick check. Really though, when was the last time you gave it more than a cursory glance anyway?

 

[MAKG1]

Easier way to do it, set the altimeter to zero at airfield altitude (QFE) and the number in the Kohlsman window should be what the MP reads with the engine off.

The easier way to do it is to expect 30 inches minus 1 inch per thousand feet airport elevation. Just how precise are your MP gauges? I can't tell the difference between 29.80 and 30.05 on my MP gauge, though the altimeter has little trouble.

 

[Henning]

The easier way to do it is to expect 30 inches minus 1 inch per thousand feet airport elevation. Just how precise are your MP gauges? I can't tell the difference between 29.80 and 30.05 on my MP gauge, though the altimeter has little trouble.

That's pretty much how everybody handles it if they bother. Most people, myself included, glance at it and if the needle is in the correct portion of the gauge that's good. Until I notice a problem with that, I'm not inclined to check the calibration.