Low MP near idle?

[Jeff R]

What should the manifold pressure of a IO-320 engine be at around 1000 RPM? Today, while doing a brief engine test, it was down around 16-19" for RPMs around 750 to 1000. For an earlier engine run at 1700 RPM it was around 23", which seems normal, but I didn't keep a record closer to idle RPM back then. What I was getting today seems a little low, but I have never flown an airplane with a MP guage so I have little frame of reference here.

Thanks.

[Kyle Boatright]

15" plus or minus is what I see at idle.

[mannanj]

Might help to think "manifold vacuum" instead of "manifold pressure". When the throttle is wide open, the "pressure" inside the manifold is close to atmospheric, which is around 30 in. hg. at sea level. When the throttle is closed at idle, the pistons sucking in on the maniflod will cause the "pressure" to read about 15 in. hg. or so, maybe even less.

It looks like it is reading backward if you are used to a car vacuum guage needle moving clockwise.

Of course you knew that, but others reading this may not be familiar with the way vacuum is measured in the aeronautical world. It is reading pressure, it's just a little different.

[RV10inOz]

Read this several times until you know it off by heart!

http://www.avweb.com/news/pelican/182081-1.html