MAP and Fixed Pitch Prop

Hang 4

Hang 4

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Hang 4
I have a snazzy engine monitor that has MAP displayed prominently. I have a fixed pitch prop. Does MAP tell me anything?
 
MAP is a component in percent power. See if you can't change the display to percent power. That's what's in your POH for calculating endurance and speed. Also, I remember reading that at about 60% power no mater what level of leaning you do, you won't hurt your engine.
 
With fixed pitch prop aircraft the POH states percent power based on rpm, since in new configuaration the plane didnt have a manifold pressure gauge. So its not very useful.

Constant speed prop aircraft have percent power charts based on manifold pressure and rpm.
 
You could take it up and compare altitude to MP at various altitudes. Then you could have a 'rough' altimeter in case your altimeter fails. Never heard of that though. You'd have to duplicate the throttle setting you had when you made the comparisons.
 
Thanks - pretty much what I thought, interesting, but not all that useful. Think the engine monitor (EDM 930) has a bit more capability than my engine does. Fixed pitch, so MAP doesn't help and carb, so Lean of Peak is a bit of a stretch as well.
 
Hang 4 said:
Thanks - pretty much what I thought, interesting, but not all that useful. Think the engine monitor (EDM 930) has a bit more capability than my engine does. Fixed pitch, so MAP doesn't help and carb, so Lean of Peak is a bit of a stretch as well.
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Yeah, EDM 930 is serious overkill for a fixed pitch prop carb engine.
 
It will tell you if the engine develops an induction system leak.
 
Salty said:
Does it use that same annoying voice that GPS driving apps use?
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Naw, it does it subtly by giving an invoice for instrument repair. The "repaired" instrument is installed and the anomalous behavior remains so then the mechanic looks for the intake manifold leak. At least that's what happened to the guy with the 210 hp Franklin on his 172. It's sorta tough to find some Franklin parts so they welded the corrosion hole on one of the intake tubes. Sometimes ya gotta do what ya gotta do.
 
My 172N with 180 hp O-360-A4M and fixed-pitch prop came to me with a manifold pressure gauge installed. I like it and use it.

The power charts in the Lycoming manual use manifold pressure in the calculation of horsepower. Yeah, it's far more complex and precise than necessary for routine operation, but you can't use the chart at all without manifold pressure data.

Screen Shot 2018-03-02 at 11.21.48 AM.png

I use the manifold pressure gauge for initially setting cruise power when I level off. The rpm will fluctuate with changes in airspeed and in up- and downdrafts, so setting power with the manifold pressure is quicker and more reliable.

Just a couple of days ago I was cruising at low power and noticed the rpm was dropping slightly. A slight downdraft? Carb ice? (conditions were right for it). MP was holding steady; had it also been dropping that would have suggested carb ice.

Not sure where it came from, but my airplane has a little placard on the panel with a "rule of thumb" power setting chart (photo below). You take the MP in inches and add it to the RPM in hundreds. The sum corresponds to a percentage of power, example: 22" and 24(00) rpm = 46, which according to the table is about 68% power. The beauty of this is it works -- reasonably accurately -- at any altitude.

Screen Shot 2018-03-02 at 11.30.42 AM.png

Also, as shown above, on every low-altitude takeoff I can scare myself that I'm ruining the engine by running "oversquare" ... :eek::p:D
 
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Pilawt said:
My 172N with 180 hp O-360-A4M and fixed-pitch prop came to me with a manifold pressure gauge installed. I like it and use it.

The power charts in the Lycoming manual use manifold pressure in the calculation of horsepower. Yeah, it's far more complex and precise than necessary for routine operation, but you can't use the chart at all without manifold pressure data.

View attachment 60617

I use the manifold pressure gauge for initially setting cruise power when I level off. The rpm will fluctuate with changes in airspeed and in up- and downdrafts, so setting power with the manifold pressure is quicker and more reliable.

Not sure where it came from, but my airplane has a little placard on the panel with a "rule of thumb" power setting chart (photo below). You take the MP in inches and add it to the RPM in hundreds. The sum corresponds to a percentage of power, example: 22" and 24(00) rpm = 46, which according to the table is about 68% power. The beauty of this is it works -- reasonably accurately -- at any altitude.

View attachment 60618

Also, as shown above, on every low-altitude takeoff I can scare myself that I'm ruining the engine by running "oversquare" ... :eek::p:D
Click to expand...
Very cool. Not everyday that you see a 172 with a Manifold gauge.
 
I decided not to bother with MAP on my build (with fuel injection & FP prop) in the interest of simplicity. One less thing to fix or troubleshoot.

I don't really care how about the percentage of power I'm making, and only go LOP when I'm above 8000' DA where I can set the mixture anywhere and not harm the engine.
Lower than that and I'm leaning to about 1250 degrees EGT, which is well ROP to keep out of the detonation zone.
 
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