Hallo all,
Thats an interessting discussion going on here.
With a more in deep engineering background I would like to add the following thoughts:
The shape of the power curve is ( in my understanding) given by the designer - which of tries to meet the demands of the customer. So if the customer wishes a "Volldruckhöhe" (altitude were the power starts to drop of) of lets say 2000m or so the designer tries to match supercharger gear ration in that way.
For ww2 engines, most of the engines with single speed superchargers have had similar shape of power curve: raising up to the rated altitude, then falling down. Distinctive 'kink', or angle was to be observed by the rated altitude. With AM-34FRN and later Mikulin engines, the inlet guide vanes were employed just before supercharger's impeller. How those benefited the engine, can be read in the report that I've posted the link in post #23. The power curve did not have the distinctive kink, but of a more smooth shape. Jumo copied the device for their Jumo 213 series of engines, ditched the throttle plate(s) all together, and, at least it does seem to me like that, 'earned' even more power at ~2 km under each rated height. I was trying to describe all of those workings in this 2-page long thread
In the picture, one can note the differences in power with and without 'swirl throttle' (ie. intake guide vanes). In case the maifold pressure is 1.35 ata, and 1900 rpm, the difference is more than 160 PS at sea level (red vs. green line). For 1.415 ata and 2050 rpm, the difference is lower, just better than 50 PS between SL and 6.5 km (full black line vs. blue line):
Here we have a more complex situation were the supercharges has a two stage gear or even a fliud coupling like in some DB-60x engines.
There was no 2 stage gear in ww2 piston engines' superchargers - maybe you think of two speed gear?
These things will give (the engine) or take (supercharger) power non linear with varring rpm speed. On top of this with altitude pressure and temperature will drop (also non linear). On the BMW 801 the work of fuel management, supercharger gear speed and so on was orchestered by an computer. Ok, not a computer as we know them today but a mechanical analog calculating one. Have a look:
http://www.focke-wulf190.com/images/bmwkommandogeraet.jpg
Pretty complex aye?
Not sure what to say here - in most of clutched multi-speed supercharged engines, the power curves were similar. The power curves of the Jumo 213s and AM-39 were different that those majority.
I guess that the power at low altitude was lower because due to the higher intake temperature the engine would start knocking(detonating). Only a guess....
Best regards
Guess you're right. The use of Polikovsky's device on the AM-35A reduced the temperature of compressed air, eg. from 165°C to 125°C, for 1.415 ata and 2050 rpm at sea level.