The problem is the altitude. Lets say, just for arguments sake (Illustration) that the AM-35 was making 1500hp in the cylinders and was using up 100hp of that in friction and was using 200hp to drive the supercharger leaving it with 1200hp to the prop. Changing the gear ratio of the supercharger to one that spun the impeller 70% as fast (I haven't bothered to look them up) would require 1/2 the power to the supercharger leaving you with 1300hp at the prop with no other changes, except you now are heating the intake charge well under 1/2 as much so you get a denser intake charge. You can also use more boost before detonation sets in. That may well explain a fair amountof the extra power of the AM-38. Remember that a Merlin VIII was good for 1080hp for take off at 5 3/4lbs boost on 87 octane compared to the Melrin III 880hp at 6 1/4lb boost on 87 octane. An extra 200hp (22.7%)and it all didn't come from just the difference in power needed to drive the supercharger and they weren't even using more boost.
Fine points.
The AM 38 was indeed to the AM 35A what basically the Merlin VIII was to the Merlin III - an engine with lower S/C grearing, with most of everything else remaining the same. The difference was a bit lower compression ratio (7:1 down to 6.8:1), and the AM 38 was rated for extra 100 rpm more for take off. The S/C gear ratio for the AM 35A was 14.6:1, for the AM 38 it was 11.05:1.
BTW, the AM 38F was turning 2350 rpm for take off (and emergency; the impeller was of smaller diameter; CR was down to 6:1), 2150 was for the AM 38 and 2050 was for AM 35A.
Thank you, I thought there was an early air frame with pressure cockpits ?
I'm not sure there were pressurized cockpits on the 'aircraft 103', it featured trainable MGs from get go, manned by 2 crew members. The 'aircraft 100', predecessor of the Pe-2 seems to be with pressure cockpits.