So somewhere in our world of numerous documents and research someone must have tried to quantify the extra wear and tear on an engine operating at 100% as opposed to 75%. In the world of physics the difference of 25% is I am sure not a linear relationship with regard to engine wear.
The difference between a bullet flying by your ear and hitting you in your head is also small.
But why would this matter? You are not allowed to operate the engine at the stoichiometric mixture at 100% and is there any real world situation where you would consider doing this for any prolonged period?
The fuel burn to get 180hp out of my engine is about 50l/h. The fuel burn to get 235hp out of the engine is 100l/h.
There are loads of specific limitations on engines, e.g. limits on max RPM (as Silvaire says), limits on power for how long, limits on specific RPM ranges even if well below max RPM (Continental's controversial SB, concerning crankshaft counterbalance de-tuning supposedly) and there are even limits on the MP versus RPM e.g. here
where (LH chart) if above 27.2" MP you should not be below 2300 RPM.
But I have never seen the often mentioned "over square" prohibition i.e. the MP must not exceed the RPM/100, because that makes absolutely no physical sense. Is there an engine which has a MP/RPM limit which is thus expressed?
Cad Many of the things that we do and don't know why are going back to the days of the big radial engines. Many if not all of these engines were good only for 2min. full power, then power had to be reduced, the same goes for "Don't fly over square", most engines can do it. The last word is always with the aircraft manufacturer.
Ben
The thing which seems definitely wrong is reducing the throttle opening or the prop RPM during climb. There is no reason to do that - other than perhaps noise abatement and then perhaps picking a different departure route is better.
Another reason would be a propeller with a 2500 rpm max continuous limitation, like me. Its good for 2700 rpm for "take-off". I'm not sure what that means but turn it down to 2500 rpm on the crosswind, with throttle maintained wide open.
So somewhere in our world of numerous documents and research someone must have tried to quantify the extra wear and tear on an engine operating at 100% as opposed to 75%. In the world of physics the difference of 25% is I am sure not a linear relationship with regard to engine wear.
That's another old question 
I am not aware of any systematic testing ever having been done.
As Silvaire says, the smaller engines don't seem to mind getting thrashed. Most of the flying school ones make TBO.
The bigger engines (e.g. the 360 and 540) seem to have more trouble with temperature management. I think there is a fair consensus that running at about 65% of max rated power gives you the best result, especially if running at/near peak EGT (for best economy). The engine will almost certainly last longer at say 40% power but then you fly so slowly that you will need a lot more hours in the air. When you get to say 75%, temperature management gets harder, and also you lose the peak-EGT option so you waste another 10+% in fuel.
But you will never find any research where somebody put a load of engines on a bench and ran them for 2000hrs, at different power settings, then took them apart and measured the bits with a micrometer. The cost of the fuel would be absolutely massive, for a start; about €200000 per engine. Even initial engine certification testing involves c. 150hrs on a dyno.
Also different engines can have specific weaknesses. For example Lycos have the camshaft at the top, which is fine when it's running, but the camshaft gets poor lubing during startup. Also the cam followers suffer from disintegration of the surface. There are also some valve guide issues. Contis have different issues. A lot has been written about this aspect, but nobody has managed to define an ideal operating profile AFAIK.
Frequent flying (once every 2 weeks, or more often) is important. And always flying for at least 1 hour, to make sure the oil gets a chance to boil off all the water condensed into it when the engine last cooled down.
The thing which seems definitely wrong is reducing the throttle opening or the prop RPM during climb. There is no reason to do that - other than perhaps noise abatement and then perhaps picking a different departure route is better.
The rental fleets of the world are powered by O-320s that have been operated very hard for their whole lives, the old joke being that a rental 172 comes equipped with a two position throttle. I think an O-320 will last when run that way - CHT and oil temp issues that affect larger engines in going places airframes are not nearly as evident.
Air racers do get short engine life - because they are often running a (nominally 100 HP) O-200 Continental at 4000 rpm, and about 140 HP. They'll last longer than you'd think even when doing that.
Thanks for all the feedback, I was really trying to establish why this back to 25/25 is taught. POH says nothing however at the back of my mind I am sure I had seen documented the reason behind it all. I am aware of all the in and outs of CHT/EFT mixture, fuel consumption etc and I am also aware that air racers despite them denying everything experience shortened engine life times when operating at WOT. mr Lycoming rate their engines to operate at WOT with max RPM but no one in this world can believe that a Lycoming could get even close to TBO if it was operated like this continually. So somewhere in our world of numerous documents and research someone must have tried to quantify the extra wear and tear on an engine operating at 100% as opposed to 75%. In the world of physics the difference of 25% is I am sure not a linear relationship with regard to engine wear.
The noise level inside and out reduces quite a bit between max RPM and 2500. And the lower the RPM, the more "pleasant" the engine sounds. Personally, with every engine in every vehicle I operate I reseve max. power for situations that really require max. power, e.g. emergencies.
For cruise, I agree, sure. I fly c. 65%.
But for climb, the engine needs to run very rich of peak, to keep CHTs down. That is achieved by adjusting the fuel servo such that when all 3 levers are fully forward, the engine is running around 150F ROP.
By reducing power to say 2500/25" during climb, you are compromising this adjustment. It also doesn't save fuel (per distance travelled in the climb, or height gained). Usually it increases the CHT, because the airspeed drops disproportionately, and engine cooling is much reduced at even a small airspeed drop.
A typical Lyco is rated to operate at 100% power indefinitely.
That is more of academic value. More than 75% BHP requires full rich and who in his right mind would cruise full rich?
The stuff about reducing to 2500/25 is OWT, pointless, and is generally nonsense.
The noise level inside and out reduces quite a bit between max RPM and 2500. And the lower the RPM, the more "pleasant" the engine sounds. Personally, with every engine in every vehicle I operate I reseve max. power for situations that really require max. power, e.g. emergencies.
