Engine RPM's vs. manifold pressure

av8or91

av8or91

Well-Known Member
Ive always been taught to keep manifold pressure at or below what the rpm setting is. I was told by my last instructor that increasing manifold pressure past what your rpm setting is can put serious strain and wear on the engine. I believe he called "overboosting". This was in a non turbo aircraft so not sure if that s the correct term. IS this the general consensus and is the information correct?
 
av8or91 said:
This was in a non turbo aircraft so not sure if that s the correct term. IS this the general consensus and is the information correct?
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It's usually called "oversquare" and is an old wive's tale. You can find numerous oversquare settings in the POH of most airplanes with constant speed props.

There are limits, though. Very low RPM with a very high manifold pressure can be stressful on the engine. On the Lycoming IO-360, the spread can be something like 5" on the high side, something like 2000 RPM and 25" MP. The only Continental I looked at was lower, but in general, anything in your POH should be fine.

Beware of these bizarre rules that flight instructors give you.
 
"Oversquare" is a rule-of-thumb, not an absolute.

In fact, if you fly a Beech Baron (or, I expect, a Bonanza as well) many of the tech order power settings violate the oversquare ROT. So much so that my CFI explained it to me as "Beech backwards"!
 
Sometimes I wish they would calibrate manifold pressure gauges in PSI or millibars instead of inHg. That would make it less tempting to find some relationship there just because the numbers are close.
 
Berkut said:
Sometimes I wish they would calibrate manifold pressure gauges in PSI or millibars instead of inHg. That would make it less tempting to find some relationship there just because the numbers are close.
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No kidding!

Here is a picture a friend of mine took several years back. We had just departed (just under gross that afternoon if I remember correctly). Take a look at the MP vs RPM. Keep in mind though that these are P&W R-985s though... (hint...MP is 32", RPM is 2100). If I remember correctly, the bottom arc for the green on MP is 17" (top is 32.5) and the top of the green on RPM is 2100 (bottom is 1700).

picture.php
 
av8or91 said:
So just as an example, running 27 inches of manifold pressure at 2500 RPM is ok?
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Only if 915 millibar at 2,500 RPM is OK. They're just numbers. Check Section 5 of your POH for the "Cruise Performance" tables. At higher altitudes you might have some power settings that have the MP (in inHg) be higher than the RPM (in hundreds).
 
av8or91 said:
So just as an example, running 27 inches of manifold pressure at 2500 RPM is ok?
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You can't make a statement that applies to all airplanes. According to the Lycoming engine manual for the IO-360, you can take the RPM all the way down to 1800 for a MP of 25". This may not be true in all airplanes. If you don't have the engine guide for the airplane, you simply don't know. Used published settings in the POH as a guide; I'm sure there's a lot of safety built in.

Here are some good articles that will touch on these issues:

http://www.avweb.com/news/pelican/182544-1.html
 
Turbocharged airplanes are almost always over square. 40'' 2700 rpm's on a mooney 231. Over square is bs do what the book says its in there for a reason.
 
It's impossible to overboost a nonturbocharged engine. In fact, the highest manifold pressure reading you'll probably ever see on it is when it's shut down!
 
aloft said:
It's impossible to overboost a nonturbocharged engine. In fact, the highest manifold pressure reading you'll probably ever see on it is when it's shut down!
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You don't realize it, but a C-172 is ALWAYS "oversquare" when at full throttle. 2500 RPM, and about 29" MAP. Only there is no MAP guage, so nobody notices.


There are four completely wrong "rules" out there that keep getting passed down from CFI to CFI. PLEASE help put a stop to this.


1, Never run your engine oversquare, doing so will overstrees the prop and crank.

Not true, operate with power setting as found in the POH. The concidence that the most efficient prop RPM and the airpressure at cruising altitudes are nearly the same is just that, a concidence.


2, Pull the power back to 25" inches durring climb.

Closeing the throttle slightly also closes the power enrichment valve. Therby leaning the mixture dramaticly durring a peroid that is creatign the most heat already. Full throttle all the way to cruise altitude actually keeps the engine cooler.


3, If you run your engine too lean, it will burn up.

The most dangerous mixture is 50 deg ROP, that is where detonation is the most likely. You want to either be 100 ROP, or 50 LOP. Eitherway results in the same amount of heat and strain on the engine.


4, If you pull the popwer back too fast on a descent, your enigne will shatter due to "shock cooling"

It's made of aluminum, not glass. The only case of damage I have ever heard of that might have been realted to overcooling, was the result of gross abuse, well beyond what any sane pilot would do. I used to fly skydivers in a 182 where we climbed to altitude with the CHTS climbing over 450, then gave it a mear 2 minutes to cool off, followed by a Vne descent. It ran fine well past TBO, if that didn't create "shock cooling" nothing will. As long as you make a modest power reduction for about a 500FPM descent, you'll be fine.



To all CFIs, read and re read Deakins articles on Avweb.
 
Thanks for all the responses I checked out the performance section for the piper seminole and found some manifold pressure settings that are higher than the RPM setting. Thanks USMCmech for nullifying some CFI myths.
 
USMCmech said:
You don't realize it, but a C-172 is ALWAYS "oversquare" when at full throttle. 2500 RPM, and about 29" MAP. Only there is no MAP guage, so nobody notices.


There are four completely wrong "rules" out there that keep getting passed down from CFI to CFI. PLEASE help put a stop to this.


1, Never run your engine oversquare, doing so will overstrees the prop and crank.

Not true, operate with power setting as found in the POH. The concidence that the most efficient prop RPM and the airpressure at cruising altitudes are nearly the same is just that, a concidence.


2, Pull the power back to 25" inches durring climb.

Closeing the throttle slightly also closes the power enrichment valve. Therby leaning the mixture dramaticly durring a peroid that is creatign the most heat already. Full throttle all the way to cruise altitude actually keeps the engine cooler.


3, If you run your engine too lean, it will burn up.

The most dangerous mixture is 50 deg ROP, that is where detonation is the most likely. You want to either be 100 ROP, or 50 LOP. Eitherway results in the same amount of heat and strain on the engine.


4, If you pull the popwer back too fast on a descent, your enigne will shatter due to "shock cooling"

It's made of aluminum, not glass. The only case of damage I have ever heard of that might have been realted to overcooling, was the result of gross abuse, well beyond what any sane pilot would do. I used to fly skydivers in a 182 where we climbed to altitude with the CHTS climbing over 450, then gave it a mear 2 minutes to cool off, followed by a Vne descent. It ran fine well past TBO, if that didn't create "shock cooling" nothing will. As long as you make a modest power reduction for about a 500FPM descent, you'll be fine.



To all CFIs, read and re read Deakins articles on Avweb.
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Man I'm so glad I'm not the only one that thinks shock cooling is a myth.

Also, idle mixture is set rich for a reason.
 
trafficinsight said:
Man I'm so glad I'm not the only one that thinks shock cooling is a myth.
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I don't think he said "shock cooling" is a myth, it's just not as big a deal as many people think. You really have to work to damage the engine in this way. Even an iconoclast like Deakin doesn't think shock cooling is a total myth.
 
Berkut said:
Sometimes I wish they would calibrate manifold pressure gauges in PSI or millibars instead of inHg. That would make it less tempting to find some relationship there just because the numbers are close.
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Or the fact that in every other circle where internal combustion piston engines are used its called "manifold vacuum" or "boost" (in a forced induction engine) and is typically calibrated in either PSI or BAR.
 
tgrayson said:
I don't think he said "shock cooling" is a myth, it's just not as big a deal as many people think. You really have to work to damage the engine in this way. Even an iconoclast like Deakin doesn't think shock cooling is a total myth.
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Yeah, I agree that is possible to dammage an aircooled engine by cooling it to fast. However the conditions required to make that happen are WAY outside normal operations. As I said, skydiving 182s run reliably in most cases for plenty of hours, and they are probably the most at risk for this problem. Your basic take off, cruise for two hours, descend, and land pilot will never be in danger of "shock cooling" (unless something is drasticly out of sorts).


Airflow is the main variable for engine cooling, power is second. As long as your airspeed and cowl flaps remain constant, CHTs will change only gradually even with large power changes.

If your CHTs are below 350, then the engine is already "cool", and you can do whatever you want.

350-400, make a modest power reduction giving a constant airspeed 500 FPM descent, and you'll be fine.

400-450, slightly open your cowl flaps, and/or adjust the mixture 2-3 minutes before descent. Then when you land, find a mechanic to investigate why your engine is running so hot.


Exceptions are out there, but those guidlines will keep 95% of engines happy for decades.
 
tgrayson said:
I don't think he said "shock cooling" is a myth, it's just not as big a deal as many people think. You really have to work to damage the engine in this way. Even an iconoclast like Deakin doesn't think shock cooling is a total myth.
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I would venture to say shock cooling is more of a "pseudo-myth" than just a myth. It does exist and you can damage an engine by cooling it too rapidly, but not to the extent that some individuals would want you to believe.

Not sure about other manufacturers, but Lycoming recommends CHT changes of not more than 50 degrees per minute during descent. Looking at it that way, you almost need one of those "one in a million" days where everything necessary for shock cooling to happen is prevalent. Even in the middle of the winter in KC, we normally wouldn't see CHT changes of 50 degrees per minute in the Commander.
 
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