Some of the aircraft are detailed in the manual. Even if you don't have the printed manual that comes with the collector's edition, you have the PDF manuals of all the available languages in the following folder:
c:\(your program files or games folder)\steam\steamapps\common\IL2 sturmovik cliffs of dover\manual
The data starts with the Spitfire at page 83. It is by no means complete and some aircraft are missing altogether, but you can probably use the settings from one aircraft on another one that uses the same engine.
From what i can remember, the Stuka, Ju88 and He-111H all have Jumo engines, so the data for the Stuka would probably be the one to use. The Ju88 also has the limits clearly marked on the instrument bezels of the RPM and manifold pressure/ata indicators: green is maximum continuous, yellow is probably climb power (this is usually restricted to a certain amount of time but i don't know how much, i just watch my temperatures) and red is emergency and/or take-off power.
The hurricane and spit also have similar engines, so i use the same settings on both. According to the Spit manual in my collector's edition the advised power setting during combat is climb power which is 6 1/4 lbs of boost at 2600 RPM for 30 minutes. The next lowest setting (which is probably the continuous one) is cruise with rich mixture, which is 4 1/4 lbs of boost at 2600 RPM. I think this is also the setting marked with a little triangle on the instrument and labeled as "boost for level flight" if you zoom in close to the instruments.
In practice i just drop my RPMs to 2600 and keep my boost on the little triangle marker, then i add extra power in short bursts whenever i need to instead of running at higher settings all the time. This way the engine cools down between sessions of increased power and i don't have to worry about the 30 minute time limit.
Finally, the 109, 110 and He-111P use daimler benz engines, but i think that the one used by the 109 is geared differently and has different limits (the manual says max continuous is 1.35 ata and 2300-2400 RPM).
However, if you fly the 110 you will see that the operating limits are clearly marked on the instrument bezels of the RPM and manifold pressure/ata gauges and they are slightly lower than the ones used on the 109: a red "d" letter is the maximum continuous setting, a red "30" is allowed for 30 minutes and is probably climb power and a red "1" is allowed for 1 minute and is probably emergency and/or take-off power.
I haven't tried the He-111P a lot, but i would probably start by using the 110's settings on it.
Hope this helps a bit
