Mixture and Engine Pitch..

[Azimech]

Also, there is a big difference between carburetted engines and the ones with mechanical fuel injection. All combat planes in IL2 have superchargers or turbochargers.

You will notice that some planes will start to produce black smoke and engine power drops/irregular RPM above a certain altitude. That altitude is the supercharger's critical altitude, usually around 5000 meters for most planes. This means that the supercharger cannot deliver the volume of air to the engine as requested. To keep it simple, the volume of air drops but the volume of fuel stays the same. That's why in IL2 on most planes you only need to adjust mixture above the critical altitude, although it's done in a very crude way.

Fuel injected systems like those on the FW190 and Bf109 adjust themselves automatically. And by the way the FW190 adjusts it's supercharger stage automatically as well; the Bf109 has the most elegant construction because the speed of that supercharger is adjusted the whole time by means of a engine-driven dynamic hydraulic clutch and this provides maximum boost until supercharger RPM and clutch RPM are equal. This is also less efficient than gear-driven superchargers because friction in the hydraulic clutch produces heat, and that's loss of engine power. In a car with older types of automatic transmission this loss increases fuel consumption by 20% compared with the
same car with a manual gearbox.

Turbochargers are a different story, their critical altitude (usually much higher) is the maximum allowable RPM: the higher the altitude, the more RPM the turbo has to make in order to supply the requested manifold pressure. In IL2 turbochargers are set automatically, in real life pilots had to adjust their engine RPM, manifold pressure AND turbo RPM while climbing or descending. As you climb, the pressure differential between the exhaust system and the outside air increases, which speeds up the turbo. Turbo RPM was/is regulated by a valve that controls the amount of exhaust gas that flows through the turbine.