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Never Fly Your Engine "Oversquare!" is an old recommendation. This wisdom cautions pilots that selecting a power setting with a manifold pressure in inches higher than the RPM in hundreds is damaging to the engine -- e.g., 2300 RPM dictates using no more than 23 inches manifold pressure.

Pilots are instructed by flight instructors to always reduce the manifold pressure before the RPM on power reductions, and increase the RPM before the manifold pressure on power increases. From this, it seems even short periods of "oversquare" is harmful during takeoff and landing. What about cruise?

Is it harmful to operate an engine for long periods "over square"?

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  • $\begingroup$ This question is not about a rule of thumb or generalized operation of an engine. It is about a specific setting of the engine controls. $\endgroup$
    – jwzumwalt
    Commented Jul 12, 2018 at 16:50
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    $\begingroup$ It says in the accepted answer "It actually may be beneficial to run an engine "oversquare" in some cases!" and goes on to further explain. $\endgroup$
    – fooot
    Commented Jul 12, 2018 at 16:51
  • $\begingroup$ Related: How do different RPM–MAP settings affect range? $\endgroup$
    – user14897
    Commented Jul 13, 2018 at 21:00

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No it's an old pilots' spouse's tale. Those are just two numbers. Having said that, Brake Mean Effective Pressure in the cylinder is higher when manifold pressure is high and rpm is low, for a given horsepower output, than the opposite way around, so there is a bit more stress on the cylinder head and piston. And you do want to avoid that condition at the extremes to avoid really excessive BMEP, which could result in detonation.

The key word is "extremes". With wide open throttle with the propeller at min RPM, it could be damaging, so it is normal practice to always lead with rpm on the way up and throttle on the way down.

But in the normal operating range it's not a big deal. Look at a supercharged engine like a R985. Redline is 36" of manifold pressure at 2200 or 2300 rpm. It's jugs should be blowing off left and right. And you cruise it at maybe 28" and 1600. Oversquare enough?

So on the one hand you do have lower BMEP running higher rpm/lower MP, so somewhat lower stress on the cylinder. On the other hand, heat and wear from friction is higher because of the higher rpm itself (and very importantly, the higher piston speed), so that tends to favour higher MP/lower rpm, for the same horsepower output. My own preference in a normally aspirated engine is to cruise at say 2000 rpm and 22" instead of the other way around, because there is more benefit from the lower piston speed (piston rings wear out after traveling a specific distance, so the lower the piston speed the better, within reason) than from marginally lower internal cylinder pressures.

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It’s an old wives tale about reciprocating engines, similar to shock cooling and a 1001 other ignorant myths perpetuated by misinformed pilots. I’ve also heard this as ‘prop on top’ as well. The idea was to prevent overstressing the engine at high power settings with a high load applied, but it does not necessarily mean one can not operate an engine that way. Consult the power charts published in your airplane’s POH or engine manual for the correct range of power settings.

When reducing power, the generally recommended procedure is to reduce MAP to a cruise power setting, then adjust the propeller speed, then make a final adjustment to the MAP to compensate for changes in power output due to the increased workload. The process is reversed when increasing power.

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    $\begingroup$ I strongly disagree with the shock cooling part. That does cause damage. Lycoming has a maximum cool down rate (if I recall it's about 3 degC/sec). It's all from the cylinder head casting shrinking faster than the valve seats which creates massive hoop stress in the casting around the seats and generally will cause cracks between the exhaust valve and spark plug hole. Glider tugs experience shock cooling more than any other airplane because of the full power climb then sudden descent on every flight. Glider clubs all use power reduction procedures now to mitigate it. $\endgroup$
    – John K
    Commented Jul 12, 2018 at 17:04
  • $\begingroup$ On the other hand, you can't "shock heat" a cylinder, because the differential expansion is the other way around and can't cause a crack. So it's no problem adding large amounts of power, if the engine can handle it without stumbling. $\endgroup$
    – John K
    Commented Jul 12, 2018 at 17:06
  • $\begingroup$ I have never once in my life witnessed a shock cooled engine on an aircraft. And this comes from experience dealing with twins like Dutchesses and Seminoles which have engines routinely shut down during flight for training purposes. We don’t see a change in TBOs for each engine. I suppose if you ran an aircraft engine on a load cell for 45 mins to heat soak it, then quickly dunked the crankcase into a vat of ice water, there’s a chance of shock cooling, but during normal operations, the answer is no. $\endgroup$
    – Romeo_4808N
    Commented Jul 12, 2018 at 18:56
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    $\begingroup$ Training airplanes don't come close to the duty cycle that glider tugs do. A glider club I was in was plagued with cracked cylinders and did a series of instrumented tests and found that WOT to idle exceeded Lycoming's 50F/min cooling rate. After a power step down procedure was adopted, the problems went away. I'll admit it's not common on normally operated aircraft, but don't say it can't happen or that it's not hard on a cylinder to cool it suddenly. The stresses I talked about are there; they're not imaginary. lycoming.com/content/how-avoid-sudden-cooling-your-engine. $\endgroup$
    – John K
    Commented Jul 12, 2018 at 20:00
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    $\begingroup$ The problem is that cracked cylinder heads are more like kelt caused by high CHTs than shock cooling of an engine. And this would simply indicate an engine which has been extensively operated at high power settings at low airspeeds for prolonged periods, decreasing the engine life and resulting in these maladies. Again, if shock cooling were a root cause here I would expect to see a lot more damage to engines in light twins regularly used for engine out and restart procedures as that is literally the worst case scenario for a shock cooled engine. Is it shock cooling? Maybe, but I doubt it. $\endgroup$
    – Romeo_4808N
    Commented Jul 12, 2018 at 23:12

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