Tomo has covered just about all of the technical changes to the Allison engines.
Some of the pilots manuals don't quite line up with the Allison factory specifications. But some of the pilot manuals tend to get a little sloppy with ram and no ram altitudes
There were some refits with changes with the backfire screens, too. But the superchargers and gear ratios stayed the same. until the 9.60 gears came along.
I'm well aware of all the changes done to the Allisons. The actual altitude where WEP was available varied based on atmospheric conditions, temperature etc., but it was obviously not literally at Sea Level, I think that is just a euphemism for "as low as possible" as a general guideline.
This specific engine flight chart from Dec 1942 shows that War Emergency power (57" Hg) on a P-40N is available up to 7500 feet. This is with a V-1710-81 or -99, according to the chart.
This 1944 RAAF test showed WEP rating (also looks like 57" again) available to 9,200 ft, and still managing 50" at 13,500 ft. The chart also shows what a huge improvement in speed that meant compared to military power (285 mph vs. 315 mph at Sea level, 315 vs 355 mph at 9200 ft., 330 vs 350 mph at 14,000 ft)
But the main impeller when driven with a 8.80 (or 8.77) supercharger was only going to do so much and it didn't matter if you had the plain steel crankshaft, the shot peened crankshaft or the shot peened and nitrided crankshaft. The engine would hold up longer without breaking at low altitude but it was not going to make any more power at 12,000-15,000ft. (aside from changing out the backfire screens).
See the chart referenced above!
The 9.60 supercharger gears increased the temperature of the mixture and the higher temperature is what caused the caution in restricting the Pressure. An engine running 9.60 gears was always going to be operating closer to the detonation limits.
I would note that the WER were also what the engine should make no RAM (like climbing or coming out of a steep turn)
The P-40 also got lucky. How many threads do we have on things the Japanese could have done differently. (or what the Italians could have done differently)
If the Japanese had looked around and realized that the Ki-43 needed to be retired at the end of 1942 then a lot of the P-40 saga against the Japanese facing Ki-44s ( or Ki-44s with big wings) would have been way different.
This is an interesting line of speculation. There was a lot of higher potential all around, that was always impacted by field conditions, political and economic realities.
Another one of the big Tropes that always settles in over all the key early / early-mid war Allied fighter types: P-40, F4F, Yak-1 and 7, Hurricane, D.520, Spit I through V etc. is that as relatively new aircraft, they suffered teething problems and needed to be adapted to field conditions. Their pilots weren't well trained in many cases (USN being somewhat of an exception, maybe RAF in England too) and new tactics had to be developed to make them work.
P-40N does 378 mph in a test in Buffalo, but then in Australia or at Boscombe down, with bomb shackles, direction finder, rear view mirror, full load of ammunition and fuel, some rivets and metal plates not smoothed over, and maybe not the cleanest paint job, it makes 355 mph.
Obviously the Japanese had the same problems. But they start the war with very well trained pilots, and some field experience in getting their new aircraft ready for battle. Same with the Germans, who also add particularly sophisticated tactics.
Later, when the Japanese are struggling with making some of their new types work on remote islands in Tropical conditions, we sometimes give them a pass. Oh if only the Ki-61s and Ki-44s were running right etc. (let alone getting a new wing!). They had the same struggles the Allied Air Forces did. Various US and Australian P-40 squadrons did indeed face Ki-44s and they didn't exactly get slaughtered. (maybe NZ too I'm not sure). The 23rd FG even seem to have done alright against Ki-84s in one encounter in China that I know of.
Of course by then, the Japanese were suffering from declining pilot quality / training. But we don't really give this same pass to the Allies in 1941-42.
The P-40N (and the L) were both attempts to lighted the plane up when they realized they weren't going to get much in the way of increased power. But with the N the difference in performance wasn't great and the trade-offs limited the actual utility of the airplanes. There were thousands of P-40Ns built (over 1/2 the production run)and no more of the 400 were hotrods were built, if that many and a number of them were converted back to normal Ns, at least somewhat. The Japanese and Italians and to some the extent the Germans, were not able to come up with next generations fighters (or even good improvements to existing generation fighters) that would have knocked the P-40s out of the feild.
Yeah, but this again is veering out into speculation and 'What -IFs'. The US could have gotten jets or bearcats into the Theater earlier, or developed the P-40Q. What matters though really is what happened. P-40s (not Q) turned out to be more useful in 1944 than anyone would have thought in early 1942. That's partly because they were able to work out tactics and mechanical improvements and maintenance procedures so that 'not quite good enough' (ala early 1942) was turning into ('definitely good enough') by late 1942 and most of 1943, and was still 'just about good enough' in 1944. To the surprise of many then, and I would suspect, of many people today as well.
Getting your existing, or new and improved technology working up to spec in the wretched kind of environments you would find in Rabaul or Guadalcanal is part of the whole contest. And this is one of the interesting threads of the story of aviation in WW2, I think.