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Why manifold pressure?

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cpd

cpd

Well-Known Member
Joined
Jul 24, 2008
Messages
160
Hopefully not a dumb question, but reading through years of posts here and the newsletter archive it seems like most sonerai owners set power using manifold pressure rather than rpm. I was just curious what the reasoning behind it is as it seems to be unique to the Sonerai/ KR2 guys. I've got 600 or so hours behind VWs on lower performance planes and never once questioned not having a manifold pressure gauge.

Chris
 
Manifold pressure measures how hard it’s working. It also gives an indication of efficiency. Max horsepower your engine can make is measured at standard day. 59 degF and sea level; 29.92 in of mercury. You can see how close you are.

It will give a window to your combination. A climb prop will have low manifold pressure and will overspeed before it could reach max manifold pressure. A cruise prop will be lower manifold pressure and not get to max RPM.

It’s nice for consistency. Set RPM at your favorite altitude and it should tell you xyz pressure for your combo. If it’s off, from normal, it might be time to do some maintenance like adjusting valves or timing.

They can be annoying if you want a rock steady gauge. It’s going to follow the engine. Rough air where the plane is bouncing around, it’s going to see every change. A constant speed prop keeps the engine RPM in check so they read solid manifold pressure.
 
Power comes from a combination of manifold pressure and rpm. If you are power limited by steady state cooling (VW type1 is) then once you determine your max steady state power for adequate cooling. Then instead of setting power and seeing if it gets too hot, you can just stay within power with manifold pressure and rpm....
 
is there any kind of a baseline graph or expected numbers out there for a 2100 VW or is it just a gather data on your specific engine as you go kinda thing?

Chris
 
cpd said:
is there any kind of a baseline graph or expected numbers out there for a 2100 VW or is it just a gather data on your specific engine as you go kinda thing?

Chris
Click to expand...
20 is a good pressure as I've heard for VW. I don't have a manifold gauge bit been thinking about it.
 
Chopndrag said:
20 is a good pressure as I've heard for VW. I don't have a manifold gauge bit been thinking about it.
Click to expand...
Same here, I'm restoring a sonerai II that hasnt flown in 30 years and doesnt have a manifold pressure gauge and im debating adding one.

Chris
 
I have a manifold pressure gauge on the 1835 cc, 60 hp, VW engine in the SSSC. The late Steve Bennett of Great Plains said to cruise at 22/23 " of MP and what ever RPM's that gives on the prop you are using. The day I tested my 1835 cc , VW engine I was getting about 26.5" at WOT. 22" gives me about 2650/2700 rpm at 3 gph fuel burn. Believe that is about 55/60% power.
I thought I was wrong one time, but found out I wasn't . :)
 
Pops said:
I have a manifold pressure gauge on the 1835 cc, 60 hp, VW engine in the SSSC. The late Steve Bennett of Great Plains said to cruise at 22/23 " of MP and what ever RPM's that gives on the prop you are using. The day I tested my 1835 cc , VW engine I was getting about 26.5" at WOT. 22" gives me about 2650/2700 rpm at 3 gph fuel burn. Believe that is about 55/60% power.
I thought I was wrong one time, but found out I wasn't . :)
Click to expand...
What kind of sensor and gauge did you use? I have tons of irons in the fire but this I can do when I do my annual and in between trying to try the Howland planes going if that deal happens.
 
Chopndrag said:
What kind of sensor and gauge did you use? I have tons of irons in the fire but this I can do when I do my annual and in between trying to try the Howland planes going if that deal happens.
Click to expand...
Just an aircraft MPG, they come in two flavors, 2 1/4" and 3 1/8" diameter. I have one left over that I had bought on ebay a long time ago for $15. Ones for turbo charge engines will read higher .

https://www.aircraftspruce.com/search/search.php?s=mechanical manifold pressure
 
Don’t need a guage. In climb, just push the throttle full in, that is maximum manifold pressure. Then pull the throttle back about 100 rpm if you want less than full. Same for cruise. Or if you want 200 rpm below full then do that.
 
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Some gauges are mechanical (air line manifold to gauge); others are electronic and do need a sender.

20" max sounds like flying from a high altitude airport, or a really bad intake system. max MAP should be within about 1.5-2" of local barometric pressure which is 29.92" of mercury on a 'standard day'.

You might not *need* a MAP gauge, especially with something like a VW, but it can have it's uses when tweaking for max efficiency, or even determining whether the engine is performing up to its potential.
 
Or an over compressed engine for maximizing fuel economy. (Taken to the extreme for an example only to clarify the concept.. a 14/1 static compression engine with a manifold pressure limit to prevent detonation?)
 
Need for a fixed pitch, no. It sure is interesting. My friend has one in his 320 powered AA1. The STC uses the prop as a speed limiter for the airframe. It’s pretty much a climb prop. Still faster than stock by about 15 kts, but it could go as fast as a RV6 with more pitch. Certified rules to follow. The numbers are interesting. Climb, 2550 RPM 25-26 inches of manifold pressure, just about perfect climb numbers. About a thousand feet per minute more climb than stock. Level out, if you let it will go 3000+ RPM. Throttling to not go over 2700 RPM closer to 2650, 19 inches. If it was 2500 RPM and better than 22 inches, it would be cruising 150 kts. Instead of 135.

Even if you don’t keep one on the panel, it would be good to gather the info during test flying. My plane project had one in the panel and never hooked up until I did. I think I will want more pitch, and it will help tell.
 
Never paid much attention. My efis has manifold pressure. I don't think it ever reads atmospheric. Always a bit low 1-2". My last x-country 4500msl, 80kt ias, 2650 rpm, 17.7" hg mp, 30.53 qnh
For what its worth. Always willing to learn.
 
have a 2275 VW that dyno at 83hp, run a 52/50 sterba prop, normally run between 24-26"mp do not worry about rpm it will not over rev[apx3350rpm] gauge not very accurate,. this gives me a fuel burn of 4.1 gph, speed of 150mph with good cooling
 
A MP gauge will help you know more about the engine power and the prop being used. My 1956 C-172 came from the factory with a MP gauge with the fixed pitch prop and I think Piper factory included a MP gauge in some of the early Tripacers. My 1956 C-172 was the 226 one made in May of 1955.
 
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