Why manifold pressure?

[Skippydiesel]

Speculating:
If you have a fixed pitch prop, a MAP gauge is of very limited use.
Ground Adjustable Fixed pitch props are usually set on engine rpm.
If you don't have a factory advised initial pitch setting , tie the aircraft down & set pitch for Minimum Continuous rpm @ WOT
This should deliver acceptable TO & Climb performance
Go fly - Note TO, Climb & Cruise performance @ WOT
Land
Make pitch adjustments to improve to your performance preferences

 

[AOG]

Manifold pressure is an easily measured derivative of head pressure. It is like measuring blood pressure to determine the stress on the arterial walls.

 

[TFF]

It’s a diagnostic tool. I can tell when the points are closing up. If it’s bouncing around, cylinder to cylinder power isn’t equal, if you don’t hit the target points, of RPM and known MP, you’re making less horsepower. It’s not going to keep you from blowing it up like a constant speed prop setup. If you are a numbers person, it might annoy you when you see power deterioration. I see it as an efficiency meter. If I’m hitting the numbers I know it’s healthy.

 

[proppastie]

Certified fixed pitch...not necessary because the factory tells you what prop to use and what rpm to run....Constant Speed Prop you need it to set the power because the rpm is constant....can not set power with the rpm.... Experimental it makes sense to use it to figure out best pitch and rpm settings although what is the proper mp or rpm I have no idea how one knows....one post talked about monitor of the cht. which makes sense....good luck

 

[Bill Volcko]

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

I would be interested to see your cowling/cooling air flow setup

 

[Elviar]

I think a manifold pressure gauge and a vacuum gauge read pressure from the same manifold, just on different scales.

 

[Dana]

I think a manifold pressure gauge and a vacuum gauge read pressure from the same manifold, just on different scales.

From a different reference. A manifold pressure gauge displays the absolute pressure. A vacuum gauge reads the vacuum as negative gauge pressure below ambient. The sum of the manifold pressure and the vacuum gauge reading should equal the current atmospheric pressure (29.92" Hg at seal level on a standard day).

 

[Pops]

The late Steve Bennett of Great Plains said if you have a vacuum gauge , just use it.

 

[rv7charlie]

To flesh out what Dana said, a MAP gauge will (pretending a perfect induction system) read the same at a fixed throttle opening until you climb to a pressure altitude that has lower pressure than the initial reading. But a vacuum gauge will be continuously changing as you climb, since it's reading differential pressure between manifold and atmosphere, instead of absolute pressure.

 

[jedi]

To flesh out what Dana said, a MAP gauge will (pretending a perfect induction system) read the same at a fixed throttle opening until you climb to a pressure altitude that has lower pressure than the initial reading. But a vacuum gauge will be continuously changing as you climb, since it's reading differential pressure between manifold and atmosphere, instead of absolute pressure.

Sounds backwards to me.