MP on the ground

[455 Bravo Uniform]

Was going up for an evening flight.

I've never paid attention to MP on the ground. Is this typical/normal:

Cold startup idle 11"
Warm lean idle 14"
Hot idle after runup 16"

MP read 29 engine off which is normal.

I'm usually at 15" at 70 kts at pattern altitude, then drop to 12" to get my 500 fpm descent.

I almost said screw it, I'll just listen to the engine or use the tach, but then decided to abort the flight due to the unknowns.

When I built hot rods and drag cars, my engines always made better vacuum (low MP) at hot idle than when cold.

Normal or not?

 

[Clark1961]

It's funny which ones get you on the ground. I once canceled a flight because the tach was bouncing "funny". Departures are optional...

I dunno 'bout the hot/cold question. Might pay more 'tention because you brought it up. did have a hangar mate who had a vacuum leak in the intake manifold see some weird manifold pressure problems.

 

[coloradobluesky]

I dunno, but I can tell you one tidbit. When your turn the prop back (constant speed prop), leaving the throttle where it is, the manifold pressure goes up. So manifold pressure isn't just a measure of throttle position. Nor is it a measure of power. To prove this, when I pull the mixture out, enough so it reduces power (I know its reducing power because my airspeed decreases), the manifold pressure does not change. Manifold pressure is a measure of the air pressure in the intake manifold.

 

[455 Bravo Uniform]

I dunno, but I can tell you one tidbit. When your turn the prop back (constant speed prop), leaving the throttle where it is, the manifold pressure goes up. So manifold pressure isn't just a measure of throttle position. Nor is it a measure of power. To prove this, when I pull the mixture out, enough so it reduces power (I know its reducing power because my airspeed decreases), the manifold pressure does not change. Manifold pressure is a measure of the air pressure in the intake manifold.

I assume the MP goes up with prop pulled back because you're reducing RPM which means MP will rise (less vacuum).

When you pull mixture out enough, RPM should rise slightly and thus MP will drop (vac increases).

I'm not really concerned with power, but rather the abnormality (at least to me) of having MP at 16" idling on the ground when I'm used to seeing it at 11-12" at idle in the pattern. Plus the fact that vac drops/MP rises when engine is warmed up does not seem normal to me, but I'm a 40 year car guy and just a 2 year plane guy.

With a car I'd be looking for a vac leak. Induction system leak here perhaps?

 

[455 Bravo Uniform]

I flew it last night and today. No issues. It pulls 11" at speed and idling. Must be the prop moving through the air at speed that allows less resistance to the engine.