What is manifold pressure?
Here is a simple enough question that made it to my inbox today:
What exactly is meant by Manifold Pressure?
The manifold pressure gauge is an engine instrument typically used in piston aircraft engines to measure the pressure inside the induction system of an engine. In other words, it literally reads the pressure inside the induction system.
Correction: A good pilot is always learning right? Well, since writing this post I have learned that the manifold pressure gauge is really NOT about pressure but about suction! Think about it. Your whole engine (especially the cylinders) is a big vacuum pump. Every time the piston drops into the “intake” stroke it is literally pulling or sucking air into the cylinder. Your manifold pressure gauge is actually reading suction not ram air pressure. That’s why at idle power your manifold pressure gauge might read 10 or 12 inches when the outside ambient pressure is 30 inches. Your engine is literally starving for air! It is creating a vacuum or negative pressure inside the intake manifold.
The induction system of couse being the air / fuel mixture that is between the throttle and the cylinders.
This measurement, which is read in inches of mercury or “in hg”, is one of the best methods to determine just how much power is being developed by the engine. The more air and fuel we can pump or pull into the cylinders, the more power the engine can develop (which makes us fly faster). When you can measure how much air pressure is in the induction system, just before the air / fuel mixture enters into the cylinders, you will have a good idea of how much power you are developing.
In normally aspirated engines (non turbo-charged), the manifold pressure gauge has a range of anwhere between 10 – 40 in. hg (or inches of mercury). In a turbocharged engine, the manifold pressure is allowed to go as high as the engine manufacturer allows. When the engine is shut down, the manifold pressure gauge should read very close to the current atmospheric pressure setting.
In order to equate manifold pressure to aircraft performance we need to look in section 5, or the performance section of our Airplane Flight Manual:
Take a look at this sample above. You can see that at 8,000 feet pressure altitude, -2 C, and 2,450 RPM we would be developing about 19.5 inches of mercury in the induction system. You can also see how that would then be related to fuel flow and our true air speed.
Douglas Adams on Mar 11, 2009
Paul,
This is a very good explanation. I was actually looking to see they made such a device for automobiles when I ran across your article. Very easy to read and understand. Thank you for your time and keep up the good work!
Douglas
Thyrso Guilarducci on May 24, 2009
It’s really good to read and comprehense explanatiuons from website yours. Thank you.
ghafoor khan on Sep 04, 2009
it is realy good and comprehensive explation . i saw first time in my 29 years of working as aircraft instrument technican being always remain in doubts in its operation and behavour of work.
Sonny Cropley on Oct 02, 2009
Paul
I have a SEL certificate with just over 200 hrs. I plan to move up to complex with constant speed props. I understand how to adjust the throttle and rpm but I don’t know why and how this changes the manifold pressure. My problem is that I’m a mechanic and I feel the need to understand the mechanics of this mechanism
John on Mar 17, 2010
How doe the reading on a vacuum gage correlate to the manifold pressure gage?
I assume zero on the vacuum gage is atmospheric then if I see a drop from 24 in hg on the vacuum gage to…let’s say 5 in hg then what would that be equivelent to on a manifold pressure gage?
Sumone on Mar 26, 2010
In a normally-aspirated engine (not turbocharged or supercharged), the manifold “pressure” gauge is _always_ measuring a vacuum, i.e. a pressure _less than_ atmospheric. This is because an internal combustion engine acts as a vacuum pump during the intake cycle when the piston is going down and the intake valve is open. The piston is trying to _suck_ fuel and air into the combustion chamber. It is restricted from doing so by the throttle plate (more on that later), creating vacuum behind the throttle plate and as measured by the MP gauge. At all other times the intake valve is closed and what is going on with the piston and cylinder does not register on the MP gauge. Since you have 4 or 6 cylinders going, you always have some cylinder in an intake phase.
The MP gauge is measuring in _absolute_ pressure. That is why it says ABS on the face. Absolute pressure means relative to absolute zero pressure, total vacuum. It is measuring pressure in inches of mercury (in. Hg), the same type of unit you enter into your altimeter. With the engine off, the MP gauge should read the same as the “altimeter” pressure, minus any adjustment for altitude. So John, an MP gauge reads 30 or so at atmospheric while a vacuum gauge would read zero.
At low power settings, e.g. idle, MP is low, say 10-12 in. Hg which indicates that the throttle plate is closed and we are generating vacuum behind the throttle plate. As we increase load, we see the MP increase because the throttle plate is open and “throttling” less, i.e. choking the flow less, so there is less vacuum (more absolute pressure).
Hope this helps.
Peter on Jul 11, 2010
I don’t understand the relationship between the temperature, MP and TAS showed in the cruise power setting table.
Can you help me?how do i have less MP and TAS with ISA – 20?
Thanks
mateo on Jul 21, 2010
how possible is it to have low MP and high engine temperature?
Why does manifold pressure increase during a prop check? | Ask a Flight Instructor on Jul 26, 2010
[...] This is a follow-up question to an earlier post entitled “What is Manifold Pressure ?” [...]
Mike Geronime on Aug 27, 2010
Great answer, and great site. I’m now watching a WWII training film for pilots about to fly the TBF Avenger. Manifold pressure is brought up over and over again. Now I understand what they’re talking about! Thanks.