Can someone explain manifold pressure?

[daepp]

The Super Corsair story reminded me of this. I know it was important in WWII aircraft - and maybe still. Was it the intake or exhaust manifold? Why is this measurement important in aviation but not in autos/trucks/boats etc?

[red-beard]

They are talking about intake manifold pressure, most likely.

As you go up in altitude, the air pressure drops off. You need to adjust the mixture to take into account the change.

In a modern car, the MAP sensor does this for you. Although until recently for emission reasons, no one cared too much for cars, except those that live at higher altitudes.

[crustychief]

Intake manifold pressure is used to set / manage the power ( throttle ) on a constant speed / variable pitch propeller engine. the tach is used to set the propeller pitch. Basically it is there so the pilot can help the engine maintain efficiency without putting to much stress on it.

[fingpilot]

Also, once the RPM/engine speed (propellor) is constant, manifold pressure (usually boosted by turbo or supercharger) is primary indicator of power output, up to mechanical and design limitations. Normally aspirated engines also use MAP pressure to indicate power, but that pressure, and the power output, decrease with altitude and increasing air temp. The standard lapse rate of 2C cooler per thousand feet increase of altitude does not offset the loss of MAP.

And yes, MAP can be a negative number. Usually not good for continued flight.

[Porsche-O-Phile]

Yes to all of the above. Seen on variable-pitch propeller, piston-engine aircraft. Particularly useful on turbo/supercharged engines to precisely set power. It's intake manifold pressure (instantaneous, average) typically. The gauge will read ambient (barometric) pressure just sitting there with the engine off. When you start the engine, it will drop to near-zero. As you go to WOT on a normally aspirated engine you'll get close (typically within 2-3" Hg) of manifold pressure, but never quite there due to frictional losses inside the engine upstream of the sensor and manifold. On a forced-induction engine, you can most definitely go above barometric pressure although the rationale behind it is more to retain sea-level performance up to some higher altitude, rather than to just "boost the heck out of it".

Typical power settings are "square" - e.g. 25" Hg at 2500 RPM or 26" Hg at 2600 RPM, etc. Obviously this depends on the airplane but it's a somewhat useful rule-of-thumb for most. Setting power "oversquare" (e.g. 30" Hg and 2200 RPM) means the engine is working really hard and is efficient, but puts a heck of a lot of strain on the internal components. Conversely setting power "undersquare" (e.g. 23" Hg and 2600 RPM) is gentler on the engine but less efficient and noisier. It also arguably puts stress on the engine due to a higher number of rotations (cycles) to go the same distance.

Think of it in terms of trying to drive a railroad spike into the ground. Oversquare is the big burly guy with a sledgehammer trying to drive it in with only a few mighty whacks. Undersquare is a skinny guy with a mallet trying to tap-tap-tap it in. Both will work, both have their plusses and minuses.

[sammyg2]

There is a column of air above all of us that extends upward all the way to space. That air has mass, and is effected by gravity to the tune of about 14.7 psia at sea level, or 1 bar. The taller the column, the higher the pressure.

Go higher up in altitude and there isn't as much air above you so that means less pressure.

In a piston engine driven aero-type plane, going real high means there is less atmospheric pressure to fill the intake manifold and combustion chamber so the volumetric efficiency falls off pretty quickly and the fuel mixture ternds to go rich (which is not a good thing when you are up in the air).
Less power, less trim on the prop, less of everything including fuel required.

[on2wheels52]

I would add in automotive use manifold vacuum it varies all over the guage, especially with a manual transmission.
My first car was a '61 Ford with vacuum wipers, going uphill they were frozen in place; downhill they almost flew off their mounts.
Jim

[Dantilla]

To oversimplify, as sea level, the atmosphere is around 30 in/Hg. Anything over 30 means the engine has forced-induction.

At 60"/Hg, that's 2 bar. Take THAT, all you 930 guys with the turbo spooled up to 1 or 1.2 bar.

It is not uncommon for Reno racers to run at 75". That's gotta put some serious strain on the engine.

[Dantilla]

Originally Posted by on2wheels52

My first car was a '61 Ford with vacuum wipers, going uphill they were frozen in place; downhill they almost flew off their mounts.
Jim

My first truck was a '48 Chevy. I remember having to lift off the throttle quickly when climbing hills to get one sweep of the wipers. Fun truck. 216ci 6 cylinder and a Three-on-the-tree. Drove nicely up to 50 mph or so. Above 50 was more exciting than anything Disneyland can dish out.

[rattlsnak]

Originally Posted by daepp

Why is this measurement important in aviation but not in autos/trucks/boats etc?

Because theres not many cars/boats at 35,000 ft.!

As said earlier, the higher you go, the less air, (pressure) and the less performance a piston engine will have. Non boosted engines make the most power at sea level. If turbo or supercharged, you are forcing air (pressure) into the engine to make more power when the outside pressure is low (up high). Turbonormalized engines do this to match sea level pressure, other models are regulated or adjustable whereas you can 'overboost' the engine which if not done correctly, something will grenade itself, no different than say, a top fuel car.

[rick-l]

Originally Posted by fingpilot

And yes, MAP can be a negative number. Usually not good for continued flight.

Whoa
Manifold Absolute Pressure can be below zero?

Absolute zero pressure is the pressure that would occur at absolute vacuum. How do you get more nothing than a vacuum?

[fingpilot]

Actually, MAP in an aircraft reads zero when at rest, shut down. The term 'absolute' is not really correct in aviation.