I see some Cessna 172/Piper CHerokee with Manifold Pressure gage and some without.

Can someone tell me why some would have MAP guage and some wouldn't?

thanks

I'd bet you that if you look at an airplane with a MP gage, it has a prop control knob between the throttle and the mixture (unless it's a Baron...hehe). MP gage's give an engine power output indication on airplanes with controllable pitch props.

Most of your basic single trainers will just have a fixed-pitch propeller...where throttle controls RPM.

Constant-speed propellers are found on higher performance engines where the throttle controls manifold pressure and prop controls the pitch of the blades which translates into engine RPM.

I never flew a Piper Cherokee but I would assume it does not have controllable pitch props. When I was cecking out the cherokee to rent, it had a MAP gage.

Of course I can ask the club owner for the answer, but I would like to get some info here.

Easy to tell - if the aircraft has throttle, prop, and mixture levers - constant-speed.

Throttle and mixture only - fixed-pitch

Cherokee is Fixed pitch. So what would be the significance of MAP Gage? if it is significant, why? again, I ask because I'm not use to seeing this on 172's that I usually fly.

Why Manifold Pressure for Fixed Pitch Props?

Many would think that the manifold pressure sensor is most suited for constant speed prop equipped airplanes. While this may be feasible with production airplanes which include power setting charts, kitplanes typically do not have this information, if for no other reason than the vast combinations of engines, props, and factors affecting drag. This leaves the fixed pitch propeller user with the need to determine engine power when cruising at altitudes below about 8-9000 feet. Above this altitude, wide open throttle produces less than 75% power, making leaning safe. Below this altitude, the manifold pressure is necessary for accurate determination of the power setting to allow for safe leaning.

[PHP][/PHP]I really appreciate your help. So it's great tool to determine the power setting/leaning amount.
I read on FAR part 91.205 that each altitude engine requires MP gage. Isn't Cessna 152 equipped with altitude engine? However I never saw a MP gage on 152.

Read the FARs it defines an altitude engine as:

"Altitude engine means a reciprocating aircraft engine having a rated takeoff power that is producible from sea level to an established higher altitude."

In other words it would be a turbocharged reciprocating engine. Last time I checked a 152 isn't turbocharged. A majority of those definitions are found in Part 1 of the FARs. Some are found in the begining of different parts such as 61 and 91.

So as an example, the Cherokee Arrow has a constant speed propeller. With this it also has a manifold pressure gauge. The propeller control sets the RPM of the propeller as the pilot desires and the throttle will control the manifold pressure (power setting). If we go out to the airplane and find that the manifold pressure gauge is inop we can still go flying provided it is not listed in the type certificate data sheet, airwothiness directives, and the kinds of operations list for the aircraft. It is not listed in any one of these documents and our engine is turbocharged so it is not an altitude engine. All we must do then is determine if we can make the flight safely and then placard and deactivate the MP gauge and we're good to go. Now some Arrows do have turbocharged engines so just watch out and know the systems associated with the specific aircraft you are flying!

Kind of a long explanation but hope it clears some things up!