View Full Version : Manifold pressure

03-14-2010, 07:07 PM
Hi! Could someone make the concept of Manifold Pressure clear to me? I figured out that MP is usually (or always) proportional to Throttle, but still they are not the same. I also learned from these threads that observing Manifold Pressure is more important than Throttle. But I'd like to know what MP actually is.

03-14-2010, 08:29 PM
Have a look here mate.

link (http://en.wikipedia.org/wiki/Manifold_vacuum)

All you need to know and more, enjoy.

03-14-2010, 10:18 PM
For what it's worth, in my RL experience, manifold pressure is an indicator of how much power the engine is generating. The more fuel going into the engine, the more the engine wants to suck in air to create full power. The correct balance of air to fuel gives the full power at a given altitude. MP indicates the amount of air going into the engine, and thus indicates power output. Limit fuel (with the throttle) or limit the air (with the mixture or by climbing to thinner air) and the engine creates less than full power.

In real life, you have a handful of settings that get repeated over and over. For take offs in a Beechcraft Sierra or Piper Arrow (both fuel injected 200 hp single engine complex aircraft with Lycoming IO-360 engines), it's 100% throttle (which generates about 30 psi MP), at 100% prop pitch (which is around 2600 rpm). Raise gear after a positive rate of climb. At 500 feet AGL, raise flaps, pull back the throttle to 24 inches MP and pull back prop pitch to 2400 rpm (called "squaring the engine" - 24/2400) for a power climb. At cruise altitude, you set the throttle and pich per the pilot operating handbook ("POH") (iirc, something like 22 inches MP at 2400 rpm at 7,000 feet at 7 degrees C) to get an economy cruise power setting at 75% power. (There are other settings for more economy or different altitudes and temperatures.) Lean until you get the correct fuel consumption or EGT, and fly to your destination.

(You might be able to find a POH for a given warbird to find the RL power settings for your favorite rides, but I don't know how well they correspond to the flight models for IL2.)

For in game, I do something along those lines - climb at full throttle (you don't have to pay for fuel or repair burnt valves and fouled plugs in the sim) and go to 70% prop pitch/70% throttle (or 80/80) for economy cruise (in order to loiter for 45-60 minutes at altitude without a drop tank). I know a lot of folks have all sorts of different engine/prop settings in game and think they get some benefit. For my experience, a couple of settings are enough.

Good luck.

03-14-2010, 11:06 PM
MP is related to throttle but it can change with altitude. The measurement is based on the difference of the outside air pressure and the pressure created by the engine as it draws air through the intake.

Generally, manifold pressure reduces as the plane climbs but throttle/rpm settings are used to hold or steady the desired manifold pressure.

You can over boost an engine by using too much throttle and generally max power is attained in combination with the appropriate RPM setting.

For example:
Max power for some engines is at 2800 rpm and 70", however to obtain 70" of MP may only require a portion of throttle in which case too much throttle can cause knocking or undesirable effects. As the plane climbs more throttle can be added to hold the 70" of pressure, and when the throttle reaches a max power position that it considered "full throttle height" and is usually where the plane reaches its peak performance in speed.

I'm not sure if this is entirely accurate because i've heard pilots dog fighting at lower altitudes using the words "everything forward" which usually implies max throttle, rpm and supercharger but no mention of MP.

03-15-2010, 04:47 AM
"Everything forward" or WOT (Wide Open Throttle) is going to result in a manifold pressure of current atmospheric*compressor ratio. IE: If the charger is a 3:1 ratio, You're going to end up with ~44-45psi of manifold pressure (on the deck). If "too much" throttle is possible, it means rather that the pressure ratio is too high for the current atm pressure. Most engines avoid this with bypass/pressure relief valves, or by building the engine assembly such that at max atmospheric pressure, the pressure ratio isn't enough to blow the engine apart.

03-15-2010, 03:48 PM
As it says..
Pressure inside the piston chambers ( which relates to power output, dependent on altitude, mixture and rpm).

An engine is designed to certain specifications (max MP is one of these).. beyond these specs and it starts popping gaskets, and making other wierd noises.

In the game you want to maximise your MP, without over-revving of course, via mixture control and other goodies.

Come to think of it.. I haven't checked whether any a/c has an exhaust temp/mixture controller thing. Maybe on the bombers ?

03-15-2010, 09:26 PM
The way I always thought of it/was taught was that MAP is an indicator of how much work the engine produces per cycle, i.e. how well it is performing.

At low manifold pressures, the engine is not producing as much horsepower as it could per cycle.

At high manifold pressures, the engine produces much horsepower per cycle but if you overboost (let the pressure get too high) you can damage the engine due to the strength limits of the parts inside. Overboost damage is not modeled in IL2, IIRC.