Manifold Pressure Question

[stewartb]

With guys installing CGRs and similar instruments, what are you seeing for manifold pressure at full power with typical fixed pitch STOL props, like Borers, Cattos, etc? What about when you pull back to 2400 or 2500 RPM?

 

[dgapilot]

Full throttle should always be close (within 1/2" hg considering induction inefficiencies) to the current barometric pressure unless supercharged or turbocharged.

 

[BC12D-4-85]

What are you seeing with yours?

Gary

 

[stewartb]

What are you seeing with yours?

Gary

I have a constant speed. I've never had an MP gauge on a fixed pitch airplane. With the common notion that fine pitched props don't create enough cylinder pressure for proper break-in, I wondered what the MP was in those engines.

 

[BC12D-4-85]

Good point re flat props vs MP during break-in. I recall borer 82/42 being a bit less than square in cruise 2400 and 1/2 to 1" less WFO. Eventually MP exceeds square at higher rpm not sure when. Can look up WFO rpm in Cub. I had MP for carb ice and % power-fuel flow.

Gary

 

[Dave Barras]

Just finished with breakin. Whirlwind ground adjustable.

 

[Dave Barras]

Now

 

[Dave Barras]

Current prop setting WOT will produce 2710 rpm and I think 107kn.
The whirlwind is great

 

[stewartb]

Interesting. If using Pmags the max advance is happening in the low-mid 2400 rpm range. That’s what I was curious about. Thanks.

 

[KJC]

I thought all the rumors about breaking in a motor with Borer were b.s. until I tried it. I ran that motor hard and was very unhappy with the way the rings set. Ended up pulling the jugs, honing and new rings. Borrowed a cruise prop and tried again with no issues. That was with an 82/42 on a 150hp O-320 running never less than 2600 rpm.

Kevin at Buldoc Aviation told me afterwards that on low compression Lycomings, you need at least 25” of manifold pressure to seat the rings properly. With a flat Borer and the throttle WFO, I bet it’s still marginal at best. I think post #5 above is spot on.

Bottom line, I don’t recommend it. YMMV

 

[BC12D-4-85]

My experience with 82/42 was engine never broke in a OH 150 good. Mineral oil out the breather with 6 quarts and comp mid-70's. Didn't have MP gauge then. A&P said use this stock prop for 25 hrs at 2500 minimum and let's check. That helped but should have had MP gauge to find out what changed. It took more throttle throw to get 2500 is what my old logs said yesterday.

Gary

 

[hotrod180]

Buddy of mine used to have a 150hp Pacer with a MP gauge that I flew a bit.
Don't remember what pitch on the prop but it was a 74" Sensenich.
MP pressure was well below square in cruise- about 22"/2400 as I recall.
Don't recall what it was at TO but well below ambient altimeter.
I can only imagine how low the MP would be with a borer.

 

[Gordon Misch]

Mine runs about 21 or 22 inches with 8241 between 2400 and 2450 rpm.

 

[supercrow]

My Sensenich at about 40" pitch ( middle range pin) is between 21 and 22 inches at 2450RPM