The P-40 was a bit altitude and power challenged as well as heavy. But the robust standards did give it good dive acceleration and superior roll performance. Through the early China and desert campaigns these attributes –with the appropriate tactics- gave it the ability to best the best of the opposition. With certain pilots, this seems to be true also in the Soviet theater.
It was more the aerodynamics of the control surfaces that allowed better high-speed control rather than structural limits.
To Curtiss's discredit, it, as well as Bell, politicked during the original design against a bulkier, more cumbersome properly boosted Allison for fear of it compromising their sleek designs. 15,000 was thought to be more than adequate.
It was the USAAC that refused to fun Allison's supercharger development and was initially even reluctant to introduce the 8.8:1 medium altitude supercharger gearing. (rather than only producing the low-altitude/turbocharger optimized 7.48:1 gearing)
Had they had full support and funding from the start, the issues with the 9.6:1 gear should have been worked out much sooner and the auxiliary supercharger stage would have been introduced much sooner as well. (the intake kinks might have been avoided or fixed as well ... partially ignored due to being less significant with turbocharging and low added compression provided by the 7.48:1 impeller speed -the intake bottleneck became a bigger issue with the 8.8:1 and especially 9.6:1 superchargers, though perhaps not meaning as much for the 2-stage variants)
It made a good 1100 - 1200 HP for a long time and later got to 1,325 - 1,600 HP. People were actually USING it at 75" MAP and getting good power, but only in the Soviet Union and China with some of the guys in the AVG, such as General (Ret.) Davey Allison (no relation to the engine company at all). He said he demonstrated the P-40 at 75" MAP regularly. In the standard USAAF, people mostly stayed within the factory limits.
Commonwealth use in North Africa and possibly the PTO noted use of overboost as well, but this was in the context of the P-40E and K and in the 66-70" boost range, with the latter only possible at sea level. 60-66" at low level seemed to be the more useful limit though I believe Allison formally limited the -73 of the P-40K to 60" and 56" on the older -39. It just took several years to get those official ratings. If the USAAC/AAF had been under the same pressure as the RAF from 1939 onward, it might have changed things somewhat but even pre-war the USAAC had tended towards more conservative design constraints.
As I've mentioned before, I'm not sure how 75" manifold pressure would be possible without significant overrev and ram at SL with the 8.8:1 supercharger ratio and would lead to detonation if attempted on the 9.6:1 engines unless they were rev limited to maybe somewhere around 2800 RPM. (approximately the same supercharger speed as an 8.8:1 engine pushed to 3200 rpm)
I believe the Tomohawk's V-1710-33 was limited mainly by the reduction gear strength (overboosting tending to cause stripped gears), the later -39 limited mostly by the crankshaft (overboosting tending to cause failures there), and I'm not sure if the stronger crankshaft of the -73 was still the limiting factor or not.
The most consistent power gains in the V-1710's operational life appear to be with the P-38, though even there not pushed into WEP ratings nearly as rapidly as British engines tended towards. (I'm also unsure if Allison's eventual 2000 HP 75" MAP clearance for the P-38L using 100/150 fuel ever extended to USAAF operational clearance)
As far as the USAAF goes it SHOULD be noted that the V-1650-1 lacked a WEP rating as well and didn't formally adopt the high emergency boost employed operationally by the Hurricane II. (or similarly on the Spitfire V's Merlin 45) The P-40F and L should have been faster than the M and N even with a bit of added weight (less dramatic when the P-40N was loaded down with full armament and fuel load unlike the stripped-down initial production configuration). The P-40F also should have had more power down low in WEP than the P-40E or K, at least complying with the official 56/60" manifold pressure limits.
All that said, even with the added conservative margins for safety, with more funding for exhaustive testing and R&D (and actual interest in extended supercharger designs beyond turbo installation back before the V-1710 even entered mass production) the engine should have competed far better and should have been a better investment than most or all of the Army's hyper-engine projects. (especially the XI-1430)
The Allison CAN make power and CAN turn WAY faster than 3,000 rpm. The tractor pull guys turn them at 4,600 rpm and they don't break .... but it's for a short time. Aircraft engines have to run for the entire mission. So ... a P-40 pilot COULD turn it faster ... up to about 3,600 rpm ... at which point he would be close to stretching and maybe losing the prop due to breakage, not to mention possible sonic issues. Had the Allison been cleared to 3,400 rpm in normal service with an attendant gearing change to account for prop speed, the power would have ratcheted upward a bit.
Is any of that done with the F series engines that lacked the additional counter weighing on the crankshaft? (I think it was the G series that introduced 3200 RPM take-off)
I have seen pilot and (I believe) engineer or possibly mechanic/crew chief notes from RAF operations noting the smoother running of the V-1710 compared to the merlin and ability to run smoothly at lower RPM for more efficient cruise. I have no idea how well it would have coped with heavy abuse being officially cleared for emergency use, but I do wonder how far the engines might have been pushed if rated as aggressively as the RAF/Rolls Royce seemed to. (how far might the C series have been pushed if it had been in the same situation as the Merlin III in the BoB?)
Had they thrown a 2,100 HP, 2-stage Merlin into one with NO other changes, it would go from 360 mph to 426 mph with nothing other than the power increase at the same altitude and faster up higher. Change the aerodynamics a bit and give it a better prop (necessary) and the top speed may have been quite competitive with the great fighters of the war. It could NOT have done any harm for the climb rate either.
Given the P-40 had both more space for fuel from the start and a larger margin for structural strength along with good aerodynamic qualities, I'd even argue it would have seen significantly greater gains than the Spitfire's engine evolution. (it'd stay a heavier plane, but the gradual weight creep would likely make up a smaller percentage of overall aircraft weight leaving the 1945 P-40 and Spitfire variants much closer in overall weight than the 1940 ones)
The P-51 just ended up better in nearly every way than the P-40, so it made such comparisons superfluous. (though I'd still argue a P-40 derivative might have been a better use of those war-time 2-stage allison engines than the P-63, more useful for the USAAF at least -ie something the USAAF might actually find operationally useful -shorter ranged than the P-51 but still decently ranged compared to the P-36 and with the added centerline hardpoint over the P-51 to make it more flexible as a fighter-bomber -also a
They also would have HAD to abandon the Curtiss Electric 3-blade prop for a better unit ... but that's another post.
The P-39 actually got 4-blade props on some of the later production models but seemed to cope poorly with the added torque ... not good on a plane with finicky spin characteristics. The P-40 might have handled it better, though. (being a heavier and generally more stable aircraft)
