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mp question

dan carley

Well Known Member
I have a rv 4 with 0-320 160 hp fixed pitched prop.
when the engine is not running the mp gauge is reads ok. at idle it ok and full throttle reads 28"

in cruise at 2450 rpms the mp reads low 21" is this a sign of a leak or problem? if so where would I start looking

thanks
danny
 
There could be a leak, but nothing there sounds wrong. Where is your throttle when you are at 2450rpm and what is your altitude?
 
May be normal. As altitude increases, MP will decrease. A leak in the intake system may cause the MP guage to fluctuate rapidly.

Roberta
 
Mp

I remember something vans wrote
When mp is24 and rpm is 2400 that would be about 75 present power I can't get them to a square up together
 
On my plane with a CS prop, manifold pressure is an indication of throttle position. In that context your situation sounds perfectly normal. I assume that 2,450 rpm is partial throttle for that altitude.

MP also is affected by altitude, of course. But at 2k, partial throttle, sounds about right.

Dave
 
I remember something vans wrote
When mp is24 and rpm is 2400 that would be about 75 present power I can't get them to a square up together
Of course you can’t, you have a fixed pitch prop, you only have one lever that is controlling both MP and RPM together your throttle lever, if you had a constant speed prop you would have two levers, one to set RPM and one to set MP, then you can set them independently to get any combination you like. With a fixed prop you get what you get.
 
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I have a rv 4 with 0-320 160 hp fixed pitched prop.
when the engine is not running the mp gauge is reads ok. at idle it ok and full throttle reads 28"

in cruise at 2450 rpms the mp reads low 21" is this a sign of a leak or problem? if so where would I start looking

thanks
danny

I have an O-320 / FP RV-6A. The other day I was at 5500' doing 2500/22 for a little less than 75% power. For a cruise pitched prop, I would expect a higher MP at that configuration/altitude. At any given RPM/air speed/altitude, a courser pitch gives a higher MP and vice-versa. Your prop may be pitched a bit fine for climb. Have you run WOT at 8000' DA? Let us know what RPM you get in that configuration.

Larry
 
I remember something vans wrote
When mp is24 and rpm is 2400 that would be about 75 present power I can't get them to a square up together

That is only true at sea level. further, with a FP, your RPM/MP combinations are based on the pitch of your prop. I made a table from the Lyc manual that shows % power for each RPM/MP combo for every 2000' of altitude. I never completed the list, as I found the D-100 EMS to accurately calculate this for me and I now trust it. You will always be getting different combinations of RPM/MP based upon MANY factors (altitude, speed, temp, mixture, etc.)

FP props are a fixed compromise of Climb/Cruise performance. You pick your most important quality and have it pitched accordingly. The generally accepted test is to run WOT at 8000 DA (this should be appx 75% power) and see what RPM you get. The RPM will tell you if you are pitched for climb or cruise. a Prop guy can help you. After this, you get what you get regarding power levels at different configurations.

Larry
 
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