Manifold Pressure—What it Tells Us, What it Doesn’t

Move up from most training airplanes into high performance aircraft and you’ll confront a number of new gauges and devices. One of these, so very basic yet commonly misunderstood, is the manifold pressure gauge. Let’s look at what the manifold pressure tells us—and what it doesn’t.

Power Potential
“Manifold pressure” is just that—a measure of the air pressure available in the engine’s intake manifold. Combustion requires a proper mixture of air and fuel, ignited by a well-timed spark. The manifold pressure gauge tells you how much air is available to be combined with fuel; if you add the proper amount of fuel power will result. Manifold pressure, then, represents the potential for power development. All the fuel flow in the world will not give you more power than what’s appropriate for the air available.

Just a Barometer
The manifold pressure gauge is just an unusual barometer, directly sensing the induction manifold air pressure downstream of the throttle plate. The gauge is unusual because it displays pressure in inches of mercury (or for many of my international friends, hectoPascals—formerly millibars, or mb), but unlike weather barometers the MP gauge is not corrected to sea level.

We learn about the atmospheric pressure lapse rate while preparing for the Private Pilot written test. In the lowest 10,000 feet or so of the atmosphere, air pressure drops at the rate of about one inch of mercury (Hg) per 1000 feet above sea level. Standard air pressure at sea level is 2.92 inches Hg, or for our purposes about 30 inches Hg. Sitting on the ramp before engine start an airplane’s manifold pressure gauge reads about 30 inches, then, at a sea-level airport. In Wichita, Kansas, the same MP gauge indicates roughly 28.5 inches, or 1.5 inches below sea-level standard for the 1500 foot field elevation. At Denver, Colorado, roughly 5000 feet above sea level, the MP reading is 25 inches before engine start.

INSIDER’S TIP: 300, 180 or even 65 horsepower, the engine’s manifold pressure gauge reads exactly the same for a given altitude—MP, again, shows the potential for power development, but from there it’s a matter of total engine displacement and its ability to efficiently combine air with fuel.

Notice we’ve talked about MP readings before engine start. Once the engine is running MP is not exactly the same…but it is still quite predictable. Most light airplane engines have enough bends and turns in the induction system to reduce MP just a bit below ambient pressure; the throttle plate itself, an obstruction in the intake manifold, reduces MP some as well. Hence at full throttle a running engine tends to read about one inch below ambient air pressure. Start the engine at sea level and you’ll see about 29 inches MP. At Wichita it’ll run at around 27.5 inches; full throttle nets about 24 inches MP takeoff off out of Denver—five inches, or roughly 17% less power than available at sea level. No wonder it takes so much more runway for takeoff at high elevations! (Other factors, including wing and propeller efficiency, further inhibit high elevation performance).

This predictability continues in cruise flight, assuming you’re at full throttle (and full propeller rpm; more on that in a moment). Of course, reduce throttle and the manifold pressure will be some value less than this altitude-derived maximum.

So What?
So what’s the big deal about predicting MP at high elevations? Mainly, it allows you to predict where the MP should read during takeoff, to prevent that “nagging feeling” something’s wrong if this is the first time you’ve seen such a low MP at full throttle. It helps you detect a throttle or obstruction problem if the expected value isn’t reached. Knowing the expected takeoff MP also helps you anticipate the amount of mixture leaning required to achieve optimal takeoff performance.

Other Characteristics
Manifold pressure varies with propeller speed. Think of your engine as a big air pump. The faster the pump turns, the more rapidly air flows through it. As air speeds up its pressure drops (remember Bernoulli?), so the faster the engine is turning the lower its MP. As you reduce prop speed air “backs up” in the engine and the MP increases. Advance the prop and the MP drops. This is why a given percentage of power can be obtained at a variety of MP/RPM combinations (for a given mixture leaning technique)—for instance, roughly 65% power comes at 23 inches/2300 rpm or 25 inches/2100 rpm in many airplanes.

As you reduce throttle setting the MP of course drops, but since most MP gauge-equipped airplanes have constant-speed propellers, the RPM will not change as a result. Eventually you’ll get to low enough a throttle setting that the propeller is below its governing range and from there throttle controls RPM as well as MP…but that’s a function of prop mechanics and not the physics of manifold pressure.

Engine Failure
Here’s a key concept in understanding MP: What happens to MP if the engine quits? Initially, nothing The propeller will at least for a while windmill at its pre-failure rpm and, since rpm and physical characteristics of altitude, the throttle plate and the induction system determine the MP, no change in the variables means no change in the indicated manifold pressure. Eventually the propeller begins to spin slower, and the airplane loses altitude (assuming a single-engine airplane)—both these variables cause an increase in manifold pressure. The MP gauge will show no change initially when an engine dies, and the indication will gradually increase during the emergency!

INSIDER’S TIP #2: Info for another day’s discussion: exhaust gas temperature (EGT), if you have such a gauge on board, is your best indicator of engine operation in flight.

Turbo Supercharging
Through the years some piston engines have enjoyed several mechanical means of artificially boosting manifold pressure. The most common form of “supercharging,” today, i.e., increasing MP above natural levels to provide the potential for more power, involves spinning a turbine in engine exhaust which in turn spins a compressor in the induction manifold. With the exception that MP will increase to a predictable, albeit boosted level for a given throttle position, and except at very high altitudes MP may automatically maintain a set level with a change in propeller rpm, most else holds true with these “altitude engines” as well. In a total engine failure, MP will drop to ambient pressure (minus throttle and obstruction-driven reductions) and again increase as the prop slows and altitude is lost.

BOTTOM LINE: One of the big differences we encounter when moving up to “high performance” airplanes is the manifold pressure gauge. We often spend far too little time in these checkouts, however, becoming familiar with what MP tells us…and what it doesn’t.