I just took the time to look up the engine on both the P-40 and the P-39. Yep the "wonderfull" Allison V-1710 powered both the P-39 and the P-40. The P-40 did benefit from the installation of a lower powered Merlin, around 1300hp, however the inprovement was very slight. The weight per horsepower for both the P-40 and the P-39 was about the same.
As a further sidenote the P-38 was also powered by a variant of the V-1710 engine. The P-38 however had supercgharging which would explain why the P-38 had such a stellar record versus the somewhat lacklaster performance of the P-39 and P-40 aircraft.
A final comment, I seem to recall reading soomewhere that the Luftwaffe pilots where somewhat wary of tangling with the P-40s at low altitude in North Africa as the performance of the P-40 was almost as good as the Me-109s.
Regards
alfa6 ;>}
It never happened my FRiend, for Kelly Johnson insisted that the RR Merlin be a part of the P-38 and it was denied politically for it was promoted that an American engine be used instead of the venerable Merlin.
The P-38 however toted a (new for the time) turbo charger but it was found to be plagued with issues that made the P-38 a more formidable fighter at low altitude, thus it's claim to fame came about in the Pacific and not in Europe.