Fixed Pitch Manifold Pressure

[celstar]

What should the MP be on a fixed pitch, carb. runing at 2200 rpm 2800 feet msl O360?

[Norman CYYJ]

That is an impossible question to answer. Each engine would be different according to the engine load, altitude, mixture setting, throttle setting. The MP at 2800ft would be the altimeter setting from the closest airport minus about 2.8 inches.

[gasman]

Originally Posted by Norman CYYJ

That is an impossible question to answer. Each engine would be different according to the engine load, altitude, mixture setting, throttle setting. The MP at 2800ft would be the altimeter setting from the closest airport minus about 2.8 inches.

AT WOT AND.... "IF ".... NO INTAKE RESISTANCE can't happen.... GMCJETPILOT knows what I mean.

[gmcjetpilot]

Originally Posted by celstar

What should the MP be on a fixed pitch, carb. runing at 2200 rpm 2800 feet msl O360?

Seriously, my pet peeve, get a MAP gauge. : Sorry I couldn't resist. I agree with "Norman CYYJ" answer but that would be ideal. It's about 1" loss of MAP per 1,000ft altitude. I respectfully disagree with "gasman", there is resistance even with wide open throttle, WOT, but I know what he means. However if the throttle is partially closed, forget it. There is no telling or only guessing what the MAP and how much power you're making,

RV's are close to ambient air pressure at wide open throttle but not quite. As Norman CYYJ said there are many variables. The same engine installed in different airframes can difference produce several inches (in-Hg) difference in MAP at WOT! Look at factory plane AFM's with 180hp/fix, eg. Grumman Tiger or Cherokee 180, you'll see RPM only power charts and a difference. I am guessing but I bet you will also see way less than ambient pressure and the minus 1" rule. This is why you need a MAP gauge even with a fixed pitch prop.

The aircraft itself and engine installation affects MAP: air-box, exhaust, prop and airspeed, not to mention non-standard pressure/temp changes aloft. If you want to guess, that is fine, but it's not much real use. You don't know your power (MAP x RPM) with out a MAP gauge. On factory planes with out a MAP (eg C172), they publish power verses RPM only charts after flight-test WITH a MAP gauge; than they reduce flight test data to charts. It still is a guess even than with out a MAP gauge.

Of course all planes with constant speed props have their power charts based on MAP + RPM. To do other wise is guessing. Fixed pitch prop planes should have a MAP gauge as well. Why did Piper and Cessna not install MAP gauges in all planes? Well cost and honestly you do have less control over MAP than you do with constant speed props, but it does not make MAP less important for the FIX Pitch prop.

"Norman CYYJ" is right, albeit very ideal. With out a MAP gauge that's all you can do, is guess.

The MAP gauge is cheap, easy to install w/ no electical connections (using a mechanical gauge). Van's MAP electical MAP gauge w/ transducer is only $73.40. Vans MAP

All RV's (even fixed pitch prop RV's) should have a MAP gauge in my opinion.

Here is what Lycoming says. You notice they don't have a formula. It says see airframe manufacture data.


-Operating the Direct-drive Fixed-pitch Lycoming Engine
Some pilots have demonstrated a lack of understanding with regard to the operation of the direct-drive, fixed-pitch, normally aspirated power plants. When a power chart is provided, it will indicate that as the airplane is flown at different altitudes above sea level, it is necessary to use a higher RPM for adequate cruise performance with an increase in altitude. A typical example might be the 0-360, 180 HP Lycoming power plant. The power chart by the airframe manufacturer for this fixed-landing gear aircraft lists 75% power at 7,500 feet at 2675 RPM (no manifold pressure gage in their airplane). The pilot throttle at takeoff, the pilot may want to reduce power 100 or 200 RPM and still not see performance suffer.

Those direct-drive normally aspirated engines with a prop governor are also rated indefinitely at full power, and the manuals all recommend full power for takeoff, but specify a small reduction in power, generally to 85% power climb. Study the specific airplane Pilot?s Operating Handbook for detailed power settings.

[jclark]

Originally Posted by gmcjetpilot

Seriously, my pet peeve, get a MAP gauge. : Sorry I couldn't resist. I agree with "Norman CYYJ" answer but that would be ideal. It's about 1" loss of MAP per 1,000ft altitude. I respectfully disagree with "gasman", there is resistance even with wide open throttle, WOT, but I know what he means. However if the throttle is partially closed, forget it. There is no telling or only guessing what the MAP and how much power you're making,

RV's are close to ambient air pressure at wide open throttle but not quite. <<<SNIP>>>

The MAP gauge is cheap, easy to install w/ no electical connections (using a mechanical gauge). Van's MAP electical MAP gauge w/ transducer is only $73.40. Vans MAP

All RV's (even fixed pitch prop RV's) should have a MAP gauge in my opinion.
<<<SNIP>>>

On our RV6 fixed pitch O-320, we have a manifold pressure gauge (module added to EI engine monitor).

I have become a bit hooked for several reasons includinh ...
1. I fly formation a bit and when I am flying lead I can easily set a known power setting adjusted for the other engine combinations in the flight.
2. Our tachometer became intemittent and sometimes goes to ZERO (still haven't figured out exactly why, even after replacing the unit)
3. I have a better "feel" for how hard I am "pushing" the engine <I use the 1" per 1000' rule of thumb applied to my sea level full throttle take off.>

Now the problem with answering the original question is (as mentioned elsewhere) there are SO MANY variables.

Here is one example. We have used an Ed Sterba wood prop that was the recommended pitch (and a good one I think) as well as an MT fixed pitch composite prop (made for RVs) but with a "custom pitch". Everything else on the plane was the same. The weights of the two props came out about the same. The MP from the engine ends up being the same. BUT the RPM's are different (blade aerodynamis etc.) and the speeds are, well, somewhat different based on where in the RPM curve you measure. <The top speed on the MT was a little bit higher and their appropriate pitch is probably quicte a bit higher.>

But again, for a few $$, you get to know quite a bit more about how your engine is running.

James

[Sam Buchanan]

Seriously, my pet peeve, get a MAP gauge.

I sounds like the original poster has a MAP gauge but is wondering if it is functioning properly.