Manifold Pressure

[Hak]

Morning all

Can anybody PLEASE explain manifold pressure in layman's terms

[tandemtod1]

The manifold pressure gauge is perhaps the most misunderstood instrument in the cockpit and is used in only it's most basic form by most pilots of type certified aircraft and not at all by the rest, and this little instrument can give you an incredible amount of information, some of which can be an absolute life saver and even information that certain components on the engine need to be serviced.
To start with, you need to have a basic understanding of pressure. A simple description of pressure is that any body that has mass (weight) and when at rest on another body will exert pressure.(That is not an opening for all the perverts out there) In other words, mass creates pressure. The greater the mass, the greater the pressure.
The air around you (atmosphere) has mass and therefore exerts pressure on the earth. This pressure is referred to as atmospheric pressure.
Atmospheric pressure on an average day at sea level is equal to 100kpa or 1 bar or 14.7 psi or 29.97hg (inches of mercury) The instrument in an aircraft is usually calibrated in inches of mercury.
As your height above sea level increases, so there is less air above you and as we know that less air equalls less mass then a drop in atmospheric pressure is noticed. If you go high enough then your engine will quit and you will stop breathing.
The approximate drop in atmospheric pressure per thousand foot above sea level is 3%. Therefore if you operate at 5000 ft ASL then your gauge will read 25.48hg.
Wherever you operate from,if you have a manifold pressure gauge, get into the habit of reading the gauge BEFORE you start your engine. If it normally reads 26hg at your location on an average day, and one day you look at it and it only reads 20 hg, then you have an instant clue that you are in hot and high conditions and it's time to pull out the manuals and check to see if you have enough runway and power to get you and your loved ones safely airborn. That little gauge just became a life saver. It is obvious from this that before you start your engine, your manifold pressure gauge is reading atmospheric pressure and that the lower the pressue, the less power you have available from the engine.
When you start your engine, you will notice that the gauge will drop down and give you a low reading. This is beacause at one end of the manifold you have atmospheric pressure and at the other end you have an engine that is creating a low pressure zone in the cylinders and in between the two you have a blockage which creates a pressure difference and keeps the two apart. This blockage takes the form of a slide in a two stroke and a butterfly valve in a four stroke. This throttle is there so that we can have control over engine speed. So now you can see clearly that a low manifold pressure equals low power because at idle the engine is producing very little power and the reading on the gauge is at its lowest.
If at this point, you refer to your engine manufacturers handbook, you will find a nice graph which will show you exactly how much power your engine will deliver at all the different pressures and from this you can work out how much power will be available on a given day and what a 75% cruise setting will be and what an economy cruise setting will be and what your fuel burn will be so you can calculate endurance etc etc.
Again, take a reading before you start your engine, and lets say that it reads 26hg. Now you start bombing down the runway at full throttle and you notice that the gauge is reading 24hg. This indicates that there is a blockage between the atmosphere and your cylinders and we know that it can't be the throttle because that is set wide open. So what this gauge is now indicating is that your airfilters are partially blocked and it's time to clean them. So now it's also become a service indicator as well.
All that from one little basic pressure gauge.
I hope that the terminology was sufficiently 'LAYMEN' for you. If there is anything that I left out, feel free to ask.

[Hak]

Tandemtod 1 thank you i am sure there are other people that will understand it better as well

[franss]

Hak,
Valid points.After take off and BUMPH checks you can quikly see manifold pressure is still in turbo.I have to make use of turbo for take off and ease back as soon as i can on the turbo.
franss
Sycamore Mk 1
Rhino
ZU-CCN

[t-bird]

Hi Tandemtod1

Nice post

I am in a habit of flying at 28/29 inches of manifold pressure on a 914 Turbo.

If the pressure is effected by the atmospheric pressure, does this mean that I am flying at different power setting without knowing it ?

How do you fly at the same power setting ?
RPM is affected by the density of the air.
Manifold is affected by outside air pressure.

Regards

Callie

[tandemtod1]

Hi Callie,
All the info that I have posted relates to a naturally aspirated motor so only the basics apply to your engine.
As I said, pressure will determine power output and the higher the pressure the more power you will have. The maximum pressure available in a naturally aspirated motor will be prevailing atmospheric. In your case you have a compressor wheel in your turbo which in effect manufactures it's own value for atmospheric pressure so within reason you can have as much pressure as you want therefore you can have as much power as you want and atmospheric pressure really only comes into play at mega high altitudes. If you also refer to your manual, you will find a graph which gives you power output at all the various manifold pressures. Yours will naturally go higher than a naturally aspirated graph but the power output at a given pressure is an absolute and is not dependant on altitude. So if your graph says that you will get 100hp at 29inches then that is what you will get regardless of altitude.
Air temperature has a subtantial effect on air pressure, the lower the temperature, the denser the air will be and therefore the greater the pressure. This is one of the reasons why density altitude will always calculate out differently to actual altitude and density altitude will determine what atmospheric pressure will be and how much power will be available from a naturally aspirated engine, in your case, 29 inches is 29 inches and therin lies the beauty of a turbocharged aircraft engine. On a day when the prevailing pressure is high, all it means to you is that you will set your throttle slightly lower than usual. The only effect atmospheric pressure will have on your aircraft is the amount of lift produced by the wings.