P40 Vs all other fighters in Europe

[Shortround6]

The Spitfire was around 2 years before the P-40 in development (based on first flights of prototype and production machines), I dont think there was any issue with what it could do as regards "G" forces in multi axes in flight was there? There were issues on landing loads. The late war Griffon versions were very heavy machines, but the wings were substantially modified, though they looked pretty similar.

Yes, No, Maybe. Perhaps?

The Curtiss 75 first flew April 1935.

The wing, landing gear, rear fuselage, tail surfaces were pretty close to what they would up with 1939/40.

After a crap load of engine changes (and those turbo charged YP-37s)

they wound up with the 10th P-36 off the production line finished off as the XP-40.

Work started in March of 1938, first flight was Oct 14th 1938.
The Prototype won the Army Pursuit competition in Jan 1939.
The big order was placed April 27th 1939 but the first production P-40 was not completed until May 1940.

This is what I meant by the "long, long" development.
The first few hundred P-40s were not ready for combat use.
Initial specifications called for Gross weight of 6807lbs but that was for two .50 cal guns with 200rpg, two .30s guns with 500rpg and 120 gallons of fuel (the tanks would hold 181 gallons at max gross). Curtiss built 778 P-40s in 1940, 582 of them just in the last 4 months.

The fuselage, tail surfaces, wings and landing gear were very close to the plane of 1935, although beefing up had been done. The Hawk 75/P-40 airframe does standup remarkably well against most of it's contemporaries. But it was no longer in the first tier in 1943.

 

[SaparotRob]

What was the rationale behind mixed caliber armament? Why not all .50's instead of a .30/.50 mix?

 

[GrauGeist]

What was the rationale behind mixed caliber armament? Why not all .50's instead of a .30/.50 mix?

During the interwar years and even into the early years of WWII, rifle caliber MGs (.30/7mm) were the mainstay of virtually every Air Force (and Naval Air Arm) on earth.
The USAAC used a .50 MG in the mix to provide a bit more hitting power. Around 1940-ish, the U.S. military determined that the hitting power of two .50 MGs was a preferable trade-off to the (then) slower rate-of-fire. The SBD was one of the first US aircraft to be designed with two fixed forward .50 MGs, though the rear gunner position still used a flexible .30 (or two).

 

[SaparotRob]

If a company is designing in .50's from the get go (P-40, right?), why not ditch .30's completely and simplify logistics?

 

[GrauGeist]

If a company is designing in .50's from the get go (P-40, right?), why not ditch .30's completely and simplify logistics?

It's not the company's decision.

They're building to Government specs.

Like I mentioned a bit ago, virtually every military on earth was using rifle-caliber machine guns in their aircraft designs before WWII.

U.S.: 30-06
Britain: .303
Italy: 7.7mm
Japan: 7.7mm
France: 7.5mm
Germany: 7.92mm
Netherlands: 7.9mm
Soviet Union: 7.62mm
Poland: 7.92mm
Romania: 7.92mm
Bulgaria: 7.92mm
Czechoslovakia: 7.92mm

The list goes on, but you get the idea...

 

[Shortround6]

The .50 cal gun weighed 3 times as the .30 cal gun and the ammo weighed 5 times as much per round.

In 1939 and into 1940 the US .50 gun had a rate of fire of 600rpm at best, the .30 cal gun had rate of fire of 1200rpm. They were able to increase the rate of fire of the .50 cal at some point in 1940 to about 800rpm.
They were able to retrofit existing guns with parts kits.

The supply of .50 cal guns was not enough to provide all the guns that the US wanted. It took time to provide the extra factories.

And lastly, the US was also in the middle of changing the .50 cal cartridge. New propellants allowed for increased velocity. This was also messing up logistics. Which guns would get the new ammo and which guns (Navy AA?) would get the old ammo.

That was in a perfect world.
The .50 guns gave quite a bit of trouble in 1940 and 1941 and beyond. In some cases the .30 cal guns worked and the .50 cal guns did not. The British Tomahawks took quite a while to get the .50 guns fire with any degree of reliability.

 

[drgondog]

The .50 cal gun weighed 3 times as the .30 cal gun and the ammo weighed 5 times as much per round.

In 1939 and into 1940 the US .50 gun had a rate of fire of 600rpm at best, the .30 cal gun had rate of fire of 1200rpm. They were able to increase the rate of fire of the .50 cal at some point in 1940 to about 800rpm.
They were able to retrofit existing guns with parts kits.

The supply of .50 cal guns was not enough to provide all the guns that the US wanted. It took time to provide the extra factories.

And lastly, the US was also in the middle of changing the .50 cal cartridge. New propellants allowed for increased velocity. This was also messing up logistics. Which guns would get the new ammo and which guns (Navy AA?) would get the old ammo.

That was in a perfect world.
The .50 guns gave quite a bit of trouble in 1940 and 1941 and beyond. In some cases the .30 cal guns worked and the .50 cal guns did not. The British Tomahawks took quite a while to get the .50 guns fire with any degree of reliability.

All good SR - but think you meant 30 cal = 5 per pound and 50 cal = 3 per pound

Additional to one of the posters re: Logistics. The AAC/AAF was still part of US Army and in 1940 there were a hell of a lot more 30 cal/30-06 rounds in US Army stores and field than 50 caliber rounds (or M2).

 

[drgondog]

Bill,

I understand the comparison of Combat GW between the planes, but how did the climbs compare when they met in battle? How about a Mustang with approx half gas, compared to the 109 / 190 both at half fuel load (weight)? I would think less 1/2 gas in a P-51 would result in a greater percentage of weight gone, and a much better climb rate.

Cheers,
Biff

Of course you are correct. The fuel fraction of a P-51B/D with 85 gal internal fuel is much more than Bf 109 and Fw 190A, so comparing at 50% fuel fraction as combat GW is advantage P-51B. That said, that is kinda where the P-51B was when engaging in combat over Munich, Berlin, Posnan.

The ROC for the P-51B at full combat weight (with 269 gal) was ~ 3600fpm at 67" 3000 rpm at SL (about same as Bf 109G-6 with DB605AS with 503 rack for drop tank) but for 8600#GW at 75" and 3000 rpm ~ 4500fpm at SL and 3600 at 18,000 feet.

When the P-51B was approved for 67", the full transition to 85 gal tank had not occurred and the 75 gal tanks were dropped before reaching Brunswick. Given draw down for warm up, take off, assembly and initial climb, the 51B at target was probably around 130-140 gallons or about 8400 pounds (vs 9600 at full GW take off w/269 gal). A WL of 36psf compared to 41 at T.O. is a huge difference - more in Spitfire class and significantly below full internal combat load of a Bf 109G6 and Fw 190A-7. Given that 60 gal drop tanks were being used in 1944 on 109G and Fw 190A, they were entering combat with a higher % of their fuel fraction than the P-51B.

 

[PAT303]

If a company is designing in .50's from the get go (P-40, right?), why not ditch .30's completely and simplify logistics?

Because the .50's didn't work if fitted in the wings, they would jam as soon as the plane pulled G's, likewise, as SR has posted what .50's and ammunition are you using and where?, 1939-40 see's a change from low velocity slow firing guns to high velocity quicker firing guns.

 

[drgondog]

Because the .50's didn't work if fitted in the wings, they would jam as soon as the plane pulled G's, likewise, as SR has posted what .50's and ammunition are you using and where?, 1939-40 see's a change from low velocity slow firing guns to high velocity quicker firing guns.

First comment I have heard that P-40 had jamming problems during maneuvers? They were mounted upright whereas the A-36/P-51A/P-51B had canted mounts.

 

[Shortround6]

All good SR - but think you meant 30 cal = 5 per pound and 50 cal = 3 per pound

Additional to one of the posters re: Logistics. The AAC/AAF was still part of US Army and in 1940 there were a hell of a lot more 30 cal/30-06 rounds in US Army stores and field than 50 caliber rounds (or M2).

I may not have stated it clearly. but,
.30 cal was about 6lbs/100 rounds
.50 cal was about 30lbs/100 rounds

Or the .50 cal round was about 5 times as heavy as a .30 cal round.

As you state there were millions more rounds of .30 cal ammo than .50. But when they are trying to build airplanes with 1000-1100hp engines trying to stick in the wing four to six .50 cal guns can run up the payload real quick.
By the end of 1940 Curtiss was building over 200 airplanes per month. Trying to build planes at that rate while incorporating the "flavor of the month" armament has it's own problems.
The P-40D did try with making provisions to mount a 20mm cannon underwing in addition to or in place of one the .50 ca guns. It is in the manual for the plane but I don't think any planes actually went operational with the 20mm guns.

 

[GrauGeist]

First comment I have heard that P-40 had jamming problems during maneuvers? They were mounted upright whereas the A-36/P-51A/P-51B had canted mounts.

I don't recall hearing that either and in a bit of irony, it was the .30 MG that jammed on Lt. Rasmussen's P-36, leaving him with only a .50 MG to engage the Japanese that morning at Pearl Harbor.

 

[Venturi]

In Europe, the movement was towards cannon for bomber work, in combination with fast firing mg for fighter work. All about rate of fire (anti fighters, high deflection shots), vs individual shell hitting power (anti bombers, low deflection shots) for the cannon. For example the ROF for oerlikon (sp?) cannon was quite low (Japanese zero also had a variant of this weapon, of course it was also mounted on Bf109E).
The US viewed the 50cal as almost a cannon shell, and that is pretty much correct when you look at the bullet weights of a .30-06 and the .50BMG as shortround points out. Pre war, not too many 20mm cannon were out there.
In this way, the US was following standard principles with "heavy" weapons for bombers and "light" weapons for fighters. Later in 1940 of course the British proved 8 or 12 rifle MGs are sufficient if not optimal for both, combat experience reflected in the P40D and E designs. Of course the US navy along with multistage supercharging had already had gone to 4x 50BMG with the Wildcat.

 

[GrauGeist]

Though in the 1930's, the USAAC was looking at cannon for their bomber interceptors (designs like the YFM-1, P-38 and P-39 reflect this) and the Army did try and adapt cannon to just about everything at one point or another.

 

[Schweik]

This covers a lot of area/time.

Actually the P-40Ls were produced from Jan 1943 to April of 1943.
Allison was not producing enough more powerful engines. The 1943 engines had about 4,000ft more altitude than the 1942 engines. Change over was actually at the end of Nov 1942 with the P-40M model. But 15,000-15,500 altitude for an 1150hp engine wasn't good enough for a general purpose fighter in 1943. It was good enough to fill in and it was good for low altitude work.

The P-40 could be improved but it was going to take a lot of work and it wasn't going to be as good as the P-51B.

P-40 Myths
Most people confuse single-speed supercharger with no supercharger. I know you do not suffer from this misconception I just mention it because it is one of the typical sticking points.
The other myth is that it was slow and unmaneuverable but rugged (it was faster than almost every Japanese fighter it faced, and could out turn all of the German and Italian monoplane fighters). It's real flaws were that it climbed poorly and had a low effective ceiling. I.e. it didn't perform at anywhere near high altitudes.

In spite of this, P-40s were well liked by most (not all) of their pilots because once they learned it's strengths and weaknesses, it had a pretty high pilot survival rate. This was partly because it was indeed rugged, but ruggedness in a military aircraft is far less conducive to pilot survival than maneuverability. In the case of the P-40 this meant a good roll and turn, high dive speed and dive acceleration, and ability to continue to turn and roll at high speeds. The latter meant it was often able to disengage from combat when it wasn't going well, both in the Pacific and in the Med and Russia.

Australian Ace Clive Caldwell before his heavily damaged Tomahawk, just after a close call with a German Experten

An American pilot with the 79th FG in Italy examines a 20mm shell hole in his P-40L. He had 5 cannon strikes on this aircraft, and in the video this image is from he found the plug from one of the shells stuck in his wing. He wore that plug as a pendant for the rest of the war.

Early P-40s in US use were restricted to lower altitudes than most people realize
Allison-engined P-40s ranged widely in terms of their functional combat ceiling, which was a bit above the critical altitude for the engine. It varied mainly based on the supercharger gears. The typical altitude mentioned is 15,000 or 16,000 ft, but that was for the early Tomahawk variants, which (aside from Pearl Harbor) were really only used by the British and the Soviets. The P-40E, the one almost always featured in synopsis, movies, video games, and models, actually had a lower 'performance altitude' of about 12,000 ft. It was also slower than typically reported, at least at the original power settings, topping out a bit over 340 mph. They could be used at higher altitudes, as they did successfully during the defense of Darwin up to 25,000 ft, but the aircraft is so starved for power at that height they are really taking a big risk, and that was only done out of desperation.

Later the up-engined P-40Es and K (which is just an improved E) got the Allison 1710-73 engine, which (which using higher octane fuel) made them faster especially down low (and conferring the oft quoted top speed of ~360 mph at their highest altitude), and yet the performance altitude was still about 12,000 ft. At 16,000 these planes were already struggling. This meant in the Med they were contending with German fighters that could dive down from 8-12,000 feet above them.

The P-40M and some marks of the N had a different supercharger gear ratio and pushed the critical altitude up to about 16,000 ft, and the performance limit to about maybe 18,000 ft. This was the 'hot' interceptor configuration of the P-40N, which included the first few hundred and some from later marks, which could make ~380 mph, but these were comparatively rare. By the time the N model came out Allison P-40s were only being used as fighters in the Pacific and Burma, and by the Russians. In the MTO the British were using them exclusively as fighter bombers and max speed was about 345 mph.

But because it was still always a single speed supercharger, there was no real fix here, as P-40 marks with the higher gear ratio (and thus higher altitude limits) had much less power at low altitude. Whereas P-40K and later model P-40E could produce up to 1570 hp at WEP down around 2,000 ft or lower (from the manual, not counting overboosting) which made them very effective at escaping pursuit in a diving escape maneuver, (and very dangerous to tangle with down low) - the higher geared Allisons couldn't produce much more than 1,200 hp at any altitude. So many pilots preferred the K. There were more Aces flying the K in China, the Solomons and in Russia than any other P-40 type. Later in the war (1944) the N and M ended up being used in India to cover the 'hump' air corridor into Burma because they could make it over the mountains better.

Restored merlin P-40 "Lee's Hope" at Duxford.

P-40F with Merlin was a major improvement
By comparison, the P-40F (available from late 1941) and L was much more versatile, because it had the two speed supercharger. So it had good speed at low-medium altitude speed, around 4 -6,000 ft, and then the higher middle altitude, with a critical altitude about 17,500 ft and good performance up to around 21-22,000 ft. (370 mph at 20,400 ft according to this British test) This was of crucial importance in the MTO, which is why the Merlin-engined version was the only type used by the American fighter squadrons in that Theater (with a few brief exceptions of some K models due to temporary shortages). German and Italian fighters often attacked from as high as 25,000 ft and the high altitude performance of the Merlin Engined P-40s did well.

All five of the US P-40 fighter groups (33rd, 57th, 79th, 324th, and 325th), plus one independent squadron, (99th FS / Tuskeegee) active in the MTO had good to excellent combat records, and this is verified in the daily combat accounts in Mediterranean Air War. For a brief period from mid-1942, the P-40F was in heavy demand, until the middle of 1943, and arguably it was the best Allied fighter in the MTO overall until the Spit IX and later VIII arrived.

As for being a year behind, I don't think the P-40 let alone the F was ever really behind in the Pacific or China (it wasn't necessarily ahead either but when you look at realtively little known units like the 23rd FG in China and 49th FG in the Pacific they did very well). There was a general lag in the MTO where it was used against German and Italian fighters, in the sense that all nations seemed to take a while to send their most modern types down there, and it was as comparable to the Bf 109F and early G models that it faced, as well as the MC 200, 202 and 205, and the Spit V it flew with. The P-38 was probably a little bit better overall but it was having trouble in that Theater. I think the P-40F does deserve a bit more attention than it gets. It's one of those cases where a single type (P-40) has such a wide variance in subtypes in terms of it's effectiveness and combat record, that this model and the equally important K model tend to get overlooked. If it had a different designation it would probably be more famous.

They made the right decision with the Merlins - P-51 (and Spitfire) were better regardless
P-51 however was a better candidate for the 2 stage merlins both because of it's cooling configuration and because it was such an aerodynamically efficient / low drag design. Even with the Allison Engine the P-51 was 30-40 mph faster and it had the potential for much greater range. The P-38 and P-47 were also better than a P-40 once they worked out the various issues. The P-40Q was an interesting design which might have had a small role if not for the crash of several prototypes and overall major problems with Curtis Aircraft which put them on the outs with War Dept. officials, but I think the P-51 was still better. The Spitfire VIII and IX were also far and away better fighters than any mark of P-40.

 

[Schweik]

Greg's Airplanes and Automobiles on Youtube has a recent video on the P40.
I don't know how to make the movie work here but a link is below.



General things I've read here involve how it's use started before Dec 1941 when the USA was not putting a lot of money into research. The British used it in North Africa in 1941 and they were trying to use it with dogfighting tactics that it was not totally successful at. Then it was Lend Leased to Russia and their airforce had been hit hard by Germany so pilot skill and tactics could have suffered there also.

Greg's video linked above impressed me with how all the added boost and power only increased top speed 20-25mph. That was in part because it still ran out of power before it was able to benefit from reduced drag up high due to the single stage supercharger situation.

I watched that a few weeks ago and liked it (I like most of his videos) but I believe he greatly underestimates the importance of the higher altitude performance of the Merlin engined types, at least against German and Italian fighters. If you look at the war records from the Med, the British still had a lot of Kittyhawk I and Ia squadrons flying as late as mid 1943, and the poor South Africans even had some Tomahawks. The Kittyhawk Is were getting slaughtered by 109s and MC 202s. The (Merlin Engined) P-40F/Ls and Kittyhawk IIs however, were more than holding their own.

The top speeds are roughly the same, and 8,000 feet of performance ceiling doesn't sound like that big of a deal, but it absolutely was. By the end of 1942 the P-40E / Kittyhawk Ia was no longer really viable as a fighter. It couldn't handle Bf 109F-4 and was doomed against a G-2 most of the time. But the merlin types were flying escort over Sardinia on their own and doing very well. Even by the time of Anzio, when they were clearly getting obsolete, the F and L models were still not taking the kinds of heavy losses that Hurricanes or Kittyahwk I did over North Africa.

 

[Schweik]

One other point to make - For the MTO, the P-40B/C (Tomahawk) was actually a better fighter than the original P-40D and E (Kittyhawk I and Ia), at least when the latter were flying according to their manual at a max of 46" Hg. The Kittyhawk was much heavier, had problems with many of it's new features like the heavy wing guns, and was definitely underpowered. A Tomahawk IIa had a maximum climb rate of about 3,000 fpm, whereas a Kittyhawk I had a maximum climb rate of less than 2,000 fpm and was slower. It wasn't until they worked out how to increase boost (to 56 or 57 Hg) and RPM (to 3,000 - 3,200), which took a few months, that they were able to compete with their early Kittyhawks against the Bf 109F2s and MC 202s. When the F-4s were coming in they were starting to get into trouble again.

 

[tomo pauk]

P-40 Myths
Most people confuse single-speed supercharger with no supercharger. I know you do not suffer from this misconception I just mention it because it is one of the typical sticking points.
The other myth is that it was slow and unmaneuverable but rugged (it was faster than almost every Japanese fighter it faced, and could out turn all of the German and Italian monoplane fighters). It's real flaws were that it climbed poorly and had a low effective ceiling. I.e. it didn't perform at anywhere near high altitudes.

That P-40 was slow was no myth. 90% of Japanese fighters in service were also slow.
Most of today's people, when they see no turbo attached to an engine on a ww2 A/C, reckon that there was no supercharger whatsoever. That a S/C migth be driven by engine itself is probably seen as an attempt to point out to a perpetuum mobile.

The P-40E, the one almost always featured in synopsis, movies, video games, and models, actually had a lower 'performance altitude' of about 12,000 ft. It was also slower than typically reported, at least at the original power settings, topping out a bit over 340 mph.

Engines on the P-40E and P-40C were making the same power at 15000 ft, ie. 1000-1050 HP there. The -39 (on P-40D and E) was rated for higher boost by the manufacturer, so it's military power was 1150 HP at 12000 ft, vs. the -33 (on P-40, -B and -C) having the mil power of 1040 HP at 14000 ft (a tad lower with backfire screens installed). Take-off power was also greater on the -39: 1150 HP vs. 1040 for the -33.
Unfortunately, the V1710-39 was a lesser engine than it was the Merlin III in 1939. It was comparable to the low-level Merlin 45M - the one with smaller impeller (9.50 in diameter, ie. same as on the V-1710).

P-40B, -C, -D and -E saw incremental weight creep (almost 1100 lbs for the basic weight), rendering the V-1710-39 obsolete the day it was introduced.

Later the up-engined P-40Es and K (which is just an improved E) got the Allison 1710-73 engine, which (which using higher octane fuel) made them faster especially down low (and conferring the oft quoted top speed of ~360 mph at their highest altitude), and yet the performance altitude was still about 12,000 ft. At 16,000 these planes were already struggling. This meant in the Med they were contending with German fighters that could dive down from 8-12,000 feet above them.

Unfortunately, the -73 engine still had the same supercharger and it's drive as it was the case with -39, and barely different than the -33. Changes in engine (mostly related to crankshaft and crankcase) allowed for increased boost and thus increase of power with the 100/130 grade fuel.

But because it was still always a single speed supercharger, there was no real fix here, as P-40 marks with the higher gear ratio (and thus higher altitude limits) had much less power at low altitude. Whereas P-40K and later model P-40E could produce up to 1570 hp at WEP down around 2,000 ft or lower (from the manual, not counting overboosting) which made them very effective at escaping pursuit in a diving escape maneuver, (and very dangerous to tangle with down low) - the higher geared Allisons couldn't produce much more than 1,200 hp at any altitude.

It was about the S/C being with just one speed drive, but it was also about the S/C being physically small on the V-1710 - not a good thing if the engine is also small. Spinning it up faster, as it was the case on the engines on P-40M and N, gave only so much. That improvement was late by perhaps 12 months, if not 18 months?
Higher-geared V-1710s did a lot more than 1200 HP at any altitude, eg. almost 1500 HP on the P-40N (chart here); less on the late P-39s (not sure about the reasons).

By comparison, the P-40F (available from late 1941) and L was much more versatile, because it had the two speed supercharger. So it had good speed at low-medium altitude speed, around 4 -6,000 ft, and then the higher middle altitude, with a critical altitude about 17,500 ft and good performance up to around 21-22,000 ft.

Merlin XX (the V-1650-1 being the member of the family) was outfitted with a bigger S/C (10.25 in impeller diameter) that was also turning fast, at 9.49 times the crankshaft speed. That it was a 2-speed engine had no bearing on high-alt capability, it could've been with just 1 speed S/C drive (the high one) and still beat a V-1710 on a P-40/39/51.
We can see the Merlin 45 - an 1-speed S/C from Merlin XX, also with the much improved inlet, the impeller turning at 9.089 times the crankshaft speed. It gave about the same power above 12000 ft as the Merlin XX.

 

[PAT303]

First comment I have heard that P-40 had jamming problems during maneuvers? They were mounted upright whereas the A-36/P-51A/P-51B had canted mounts.

Didn't matter how they were mounted, the guns were tested with the plane flying straight and level, RAAF .50's didn't work and neither did the same guns fitted to FAA F4 Martlets, we have covered this in other threads.

 

[Schweik]

That P-40 was slow was no myth. 90% of Japanese fighters in service were also slow.
Most of today's people, when they see no turbo attached to an engine on a ww2 A/C, reckon that there was no supercharger whatsoever. That a S/C migth be driven by engine itself is probably seen as an attempt to point out to a perpetuum mobile.

Right but my real point is that they weren't slow compared to their opposition. I would also say, 360 mph in early 1942 (once they are pushing the engines a bit harder) is not slow, it's about average for a fighter. Some were faster but not necessarily in the field. Another issue with the P-40 (in it's favor) is that unlike the Spit and the 109 it did not seem to suffer that much from it's Tropical Filter.

Engines on the P-40E and P-40C were making the same power at 15000 ft, ie. 1000-1050 HP there. The -39 (on P-40D and E) was rated for higher boost by the manufacturer, so it's military power was 1150 HP at 12000 ft, vs. the -33 (on P-40, -B and -C) having the mil power of 1040 HP at 14000 ft (a tad lower with backfire screens installed). Take-off power was also greater on the -39: 1150 HP vs. 1040 for the -33.

My understanding is that the -33 had a slightly higher ratio, I'd have to go pull down some books to check. But for example according to this Sept 1941 data sheet, it had a critical altitude at military power of 13,600, where it is producing 1040 hp, and performance is usually still pretty good for another 2,000 feet above that. Which puts you at about 15,600

This similar chart from Dec 1942 for the P-40D/E shows a critical altitude at military power of 11,800 ft. Which puts you at 13,800 (but I think that was pushing it!). It's interesting though that the chart does show 56" Hg WEP setting for 1470 HP at Sea Level. I don't think a lot of units in the field got that memo until a bit later (like March).

P-40D/E was also for sure heavier. Though it was also much better protected (not just with armor but better self sealing tanks, better redundancy for flight controls etc.)

Unfortunately, the V1710-39 was a lesser engine than it was the Merlin III in 1939. It was comparable to the low-level Merlin 45M - the one with smaller impeller (9.50 in diameter, ie. same as on the V-1710).

P-40B, -C, -D and -E saw incremental weight creep (almost 1100 lbs for the basic weight), rendering the V-1710-39 obsolete the day it was introduced.

Well yeah, until they improved it

Unfortunately, the -73 engine still had the same supercharger and it's drive as it was the case with -39, and barely different than the -33. Changes in engine (mostly related to crankshaft and crankcase) allowed for increased boost and thus increase of power with the 100/130 grade fuel.

The -73 seems to basically be identical to the -39 except it was toughened (crank case, crank shaft, and bearings) for higher boost. Many of the -39s were apparently refitted with some of these same parts or the aircraft using them were given -73s which is all a bit confusing, but the net result is you got some souped up P-40Es which were really more like Ks.

The K also got some improvement to gun and/or ammunition storage which resulted in fewer stoppages while shooting.

It was about the S/C being with just one speed drive, but it was also about the S/C being physically small on the V-1710 - not a good thing if the engine is also small. Spinning it up faster, as it was the case on the engines on P-40M and N, gave only so much. That improvement was late by perhaps 12 months, if not 18 months?
Higher-geared V-1710s did a lot more than 1200 HP at any altitude, eg. almost 1500 HP on the P-40N (chart here); less on the late P-39s (not sure about the reasons).

My understanding is that there were different types of the later generation Allisons, and the variant they put on the 'Interceptor' configurations of the P-40N was the higher boost type, however the 'Fighter Bomber' type P-40N and the P-40Ms were limited to lower boost, this chart for a P-40M with the V-1710-81 shows 1360 for WEP and 1200 for takeoff. The infamous Allison Memo also mentions this, that the higher geared Allisons were going to have more trouble with high boost (due to heat, from what I gather).

Merlin XX (the V-1650-1 being the member of the family) was outfitted with a bigger S/C (10.25 in impeller diameter) that was also turning fast, at 9.49 times the crankshaft speed. That it was a 2-speed engine had no bearing on high-alt capability, it could've been with just 1 speed S/C drive (the high one) and still beat a V-1710 on a P-40/39/51.

Right but without the second speed it would then be relatively a dog down low. As it was the P-40K was considered much faster than the P-40F below 4,000 ft.

We can see the Merlin 45 - an 1-speed S/C from Merlin XX, also with the much improved inlet, the impeller turning at 9.089 ties the crankshaft speed. It gave about the same power above 12000 ft as the Merlin XX.

But all the single speed superchargers had that problem - you pick where you want to go fast. Down low means poor at high altitude, up high means the reverse. It's just a bit less of an issue with a Spitfire because that is a much lighter aircraft (at least pre-Griffon)