How does one get 30" or greater MP on a naturally aspirated engine? You're always going to have some vacuum. 25" is low at sea level but 30" would require a pretty high pressure area to be on you.
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How does one get 30" or greater MP on a naturally aspirated engine? You're always going to have some vacuum. 25" is low at sea level but 30" would require a pretty high pressure area to be on you.
flyingcheesehead
Touchdown! Greaser!
You would only get a manifold pressure equal to barometric pressure if your intake had no friction and no restriction - IE, one of those theoretical problems your high school teacher gave you ("two cars crash on a frictionless surface...").
25" does seem like it might be a bit low (maybe some other FG toga drivers can weigh in - James?), but I wouldn't ever be concerned to see as low as 27" on a normally aspirated airplane.
The gauge should show ambient barometric pressure with the engine off. Every normally aspirated engine will drop an inch or three at full throttle. Carb heat/alternate air may raise that an inch without the filter in the way.
azure
Final Approach
Not sure if you're disagreeing with me or saying the same thing in a different way. If you have a barometer that reads true, uncorrected barometric pressure, then yes it should be the same thing. I certainly don't, and when I see "barometric pressure" given in weather reports (not talking AWOS, but news outlets, TWC, etc.) it's corrected to sea level and will be in the 28-31" range even at high altitude sites. That's why I said "ambient barometric pressure at sea level, minus 1" per 1000 feet".
I don't have a commercial barometer to check at the moment, but my Dad's (long gone) was designed to be calibrated to agree with sea level corrected pressure. I assumed that all barometers in common use were that way.
Also: although the definitions are certainly different, there is usually little difference between barometric pressure corrected to sea level as given in news reports and altimeter setting. In fact, I think most of them just use the altimeter setting from an ATIS or AWOS.
Just set your altimeter to QFE (Query Field Elevation, set it to altitude 0) and read the Kollsman window reading, that is what your MP should read sitting on the ground.
tmyers
En-Route
Henning, you are correct, I am guilty of rounding up. I typically see around 29" on take off in the Arrow. It would take a pretty good day to see 30", but if I only saw 25" at my 900' Elevation airport I would probably abort. I am pretty sure the aircraft would be quite sluggish as well.
I typically glance at Oil Pressure, Fuel Pressure, MP and RPM at power up and then again at about 40kts to make sure everything looks reasonable, plus I know when the nose should start feeling light.
An engine monitor would have pointed out the clogged fuel injector at runup. Another tool in the bag.
azure
Final Approach
That would work here, or at most low altitude fields. Above about 2000 MSL, not so much unless your Kollsman scale goes below 28".
I actually never checked to see how far mine goes down, but I think they're only supposed to read between 28" and 31".
MAKG1
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You can get an inch or two extra with tuned exhaust and a large overlap cam.
azure
Final Approach
Has the OP said where he's flying out of? This whole discussion of MP with the engine off is irrelevant if he's in, say, CO.
trackersocha
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the field elevation is 2800. 7T7 south of midland TX.