p 40

[Macandy]

As far as I know the only issue the British had was with altitude performance, like the Mustang MkI that is an engine not the air frame problem

Yet the British recce Squadrons that had Mk1 Mustangs loved them and hung on to them as long as they could, the last ones only being replaced with Merlin engined Mustangs in late 1944.

It was fast on the deck, really fast - and had a very dependable and tough engine that endeared it to its pilots

 

[pbehn]

Yet the British recce Squadrons that had Mk1 Mustangs loved them and hung on to them as long as they could, the last ones only being replaced with Merlin engined Mustangs in late 1944.

It was fast on the deck, really fast - and had a very dependable and tough engine that endeared it to its pilots

I think that is what I said.

 

[Vincenzo]

If i'm not in wrong the 268 squadron use the Allison Mustang to August '45. and few other used it early in '45

 

[Admiral Beez]

It was sturdy and fairly manueverable, and lasted throughout the war, and it had a good COG. Not a bad aircraft, not a superior one, either. Just a good workhorse.

I agree, essentially the US' Hawker Hurricane.

 

[Shortround6]

The Merlins weren't doing much better in large part ...

Merlin XX: 1480 hp ... 6000 ft
Merlin 47: 1415 hp ... 14000 ft (less at 28500 ft)
Merlin 61 made 1565 hp . .12250 ft and 1390 hp at 23,500 ft.

The above were a bit more powerful than the Allison, but not by a lot. Not insignificant, but not a huge leap, either. Still, the Merlin was a better choice for the war in the ETO, which was definitely a higher altitude war than other theaters.

Merlin 76/77: 1233 hp ... 35000 ft. Now this one was a pretty decent jump in hp / altitude.

The Merlins were earlier in timing.
The Merlin XX was in production during the BoB (at a lower power rating) while the Allison -99 didn't show up until well into 1943. Granted the Allison's with 9.60 supercharger gears showed up well before the -99 but it was about the beginning of 1943.
Merlin 24's and above (Merlin 27 in Hurricane IVs) could offer 1510hp at 9500ft or about 100hp more at the same altitude as the Allison -99.

The Merlin 47 was a special high altitude single stage single speed engine. Or High altitude as far as single stage engines went.
Again the Allison -99 (single stage-single speed) offered 1150hp/15,500ft and went down as the altitude went up.
The Merlin 47 offered 1100hp at 22,000ft.
Yes the Allison -101 with it's two stage supercharger and variable speed speed drive could do a lot better than the Merlin 47 but then the Merlin 46/47 was in service in 1941, over 1 1/2 years before the -101 had hope of making it into service.

Testing an engine in July of 1943 when 4 or more squadrons of Spits using Merlin 61s were in service in Aug of 1942 shows how late the Allison was. In the summer of 1943 RR (and Packard) were producing engines of more power than the Merlin 61. (which was supposed to be limited to 15lbs of boost)
Packard V-1650-3 in Mustang Bs was good for 1330hp at 23,300ft, Packard built 184 of them in July of 1943 and about double the number in Aug and were approaching 700 a month in Dec.

The P-40Q was late (compared to the P-51) it was overweight, and it was slower than a P-51B using the same power.
It was better than any previous P-40 but the bar had been moved and being better than a P-40N in the spring of 1944 was not good enough.

 

[GregP]

The Merlins were earlier in timing.
The Merlin XX was in production during the BoB (at a lower power rating) while the Allison -99 didn't show up until well into 1943. Granted the Allison's with 9.60 supercharger gears showed up well before the -99 but it was about the beginning of 1943.
Merlin 24's and above (Merlin 27 in Hurricane IVs) could offer 1510hp at 9500ft or about 100hp more at the same altitude as the Allison -99.

The Merlin 47 was a special high altitude single stage single speed engine. Or High altitude as far as single stage engines went.
Again the Allison -99 (single stage-single speed) offered 1150hp/15,500ft and went down as the altitude went up.
The Merlin 47 offered 1100hp at 22,000ft.
Yes the Allison -101 with it's two stage supercharger and variable speed speed drive could do a lot better than the Merlin 47 but then the Merlin 46/47 was in service in 1941, over 1 1/2 years before the -101 had hope of making it into service.

Testing an engine in July of 1943 when 4 or more squadrons of Spits using Merlin 61s were in service in Aug of 1942 shows how late the Allison was. In the summer of 1943 RR (and Packard) were producing engines of more power than the Merlin 61. (which was supposed to be limited to 15lbs of boost)
Packard V-1650-3 in Mustang Bs was good for 1330hp at 23,300ft, Packard built 184 of them in July of 1943 and about double the number in Aug and were approaching 700 a month in Dec.

The P-40Q was late (compared to the P-51) it was overweight, and it was slower than a P-51B using the same power.
It was better than any previous P-40 but the bar had been moved and being better than a P-40N in the spring of 1944 was not good enough.

The P-40Q WAS good enough when compared with the standard P-40s actually being used, and they were used through the end of the war. All I said, and I stand by it, was that the P-40Q would have vastly preferred over the standard P-40s that were used in the war. I think that statement is hard to refute, and nobody who flew a P-40 would do so.

 

[Shortround6]

The P-40Q WAS good enough when compared with the standard P-40s actually being used, and they were used through the end of the war. All I said, and I stand by it, was that the P-40Q would have vastly preferred over the standard P-40s that were used in the war. I think that statement is hard to refute, and nobody who flew a P-40 would do so.

The P-40s were being phased out as fast as possible in 1944-45.
Most P-40s in 1944-45 in combat were being used for ground support or low altitude combat.

The question was NOT was the P-40Q better than a P-40N but was the P-40Q better than the P-51D, or the Bubble top P-47s or the P-38L.

No squadrons in 1944 were being formed up using P-40s.
No squadrons in 1944 were changing from any other type of aircraft to P-40s (nobody was re-equipping P-39 squadrons with P-40s, at least not long term) although a number of P-40 squadrons kept up their strength by transferring in old planes from other squadrons that were re-equipping with P-38s, P-47s and P-51 aircraft.

We also have two different engines used in the P-40Qs, The first P-40Q went through several cowlings and radiator/oil cooler changes but for most of it's life seems to have used the V-1710-101 engine. This was through 1943 and at least till March of 1944. It seems to have been fitted with the V-1710-121 in the last few months of the airframes life. The 2nd and 3rd P-40Qs used the V-1710-121 engines but they did not fly until March and April 1944 respectively and the V-1710-121 never seemed to run right for very long. A number of forced landings were recorded the aircrafts early history.

It seems like the V-1710-101 engine may be the only choice to get a P-40Q into service in the late spring or summer of 1944. It offered about 100hp less at take-off (1325hp) and around 300hp less at max power at altitude. Performance numbers of the P-40Q using this engine seems to be lacking as all the "Gee Wiz" numbers for P-40Q are for the troublesome -121 engine. Of course since only 2 (or 3) -101s were ever built and only 4 of the -121s were built perhaps not a lot should be read into such a small sample. The Middle P-40Q was loamed to Allison for quite some time to help sort out engine problems however (loan lasted from 1944 until 1946?)

The P-40Q had only four .50 cal guns with somewhere between 210, 235, 302 rounds per guns depending on source. Unless the last one is true then it held about 3/4s of the ammo that the P-51B did. It also held about 20 US gallons less internal fuel and yet it went about 9000lbs clean gross weight.

 

[GregP]

That isn't my question at all, Shortround. P-40s were flown in combat through the end of the war, and Curtiss could have shipped P-40Qs instead of P-40Ns, etc.. They could have done so without interrupting Dallas or Inglewood North American P-51 production and anyone flying a P-40M/N would have preferred a P-40Q over the P-40N.

I'm suggesting the P-40Q not supplant P-51 production at all. Instead, it could supplant P-40M/N production. Alternately, Curtiss would have produced P-40Qs instead of the very few P-47Gs they made. They only made 354 in total out of more than 15,000 P-47s built. Who would have missed them? Most stayed stateside anyway.

 

[Shortround6]

That isn't my question at all, Shortround. P-40s were flown in combat through the end of the war, and Curtiss could have shipped P-40Qs instead of P-40Ns, etc.. They could have done so without interrupting Dallas or Inglewood North American P=51 production and anyone flying a P-40M/N would have preferred a P-40Q over the P-40N.

I'm suggesting the P-40Q not supplant P-51 production at all. Instead, it could supplant P-40M/N production. Alternately, Curtiss would have produced P-40Qs instead of the very few P-47Gs they made. They only made 354 in total out of more than 15,000 P-47s built. Who would have missed them? Most stayed stateside anyway.

They only made 77 (?) P-47Gs in 1944, Production stopped in March of 1944, the rest of the P-47Gs were built in Dec 1942 and all of 1943.
Now since Allison does not have the -101 production ready (or indeed any two stage Allison production ready) until Nov of 1943 (-93 engine in the P-63) one is left to wonder what they could have powered the P-40Qs with? It didn't matter what the P-47G line did, it was being shut down before the 2nd and 3rd P-40Q prototypes flew.

In 1943 Curtiss built 400 or over P-40s in month at least 4 times. In 1944 they never built more than 275 P-40s in one month and only exceeded 200 planes a month 4 times.
Yes they probably could have exceeded historical production limits if anybody had really wanted the planes but shipping space was limited, many of the P-40s built in 1944 went nowhere. The dwindling number of P-40 units were often getting used airplanes from squadrons that were equipped with the 3 higher performance planes. They were gearing up to support the P-38, P-47 and P-51. P-39s and P-40s were on the way out and presented complications to the supply lines.

And one wonders what the P-40Qs would have brought to the table.
Too much depends on which engine they may have gotten.
A P-40Q in one test is 700lbs heavier than a P-40N.
The P-40N can carry two 500lbs and 75 gallons drop tank and still be a few hundred pounds lighter than a P-40Q with a pair of 500lbs and internal fuel only.
The P-40N is carrying six guns and not four.
Apparently none of the P-40Qs were tested with underwing loads. Since they were all converted P-40Ks and and N the wings were strong enough.
Is an extra 125 hp enough to restore take off performance with the extra weight and clipped wings?

 

[GregP]

The P-40Q flight performance was very good and it was more maneuverable than a P-51. Had they put in a 1425 hp Allison with 1600 WER hp and made it a bit lighter, it would have handily out-performed a P-40N. Of that I have no doubt. This assumes Curtiss started on it a bit sooner than they did in real life, but the entire thing is a "what if" to start with, so I might as well suggest one.

There was nothing especially difficult about making a P-40Q airframe, so it well could have flown in time to help the war effort, even if it happened with a different engine from the real-world prototypes.

 

[Wild_Bill_Kelso]

P-40Q could have certainly been more competitive as a fighter in places like Italy, and the better high altitude performance would have been particularly helpful in Theaters like Burma where they were using P-40Ns to escort transports over 'the hump' in the Himalayas (assuming they could get them there in time).

The faster climb rate and 50 -70 mph speed increase would have made them more useful in the interceptor role that older P-40 variants were still sometimes being used for in 1944 in the Pacific, by the US and also Australia, New Zealand, Chinese, Dutch East India company etc.

As P-51s were needed most for the Strategic bomber escorts, they were still using P-40F and L as fighters for tactical (medium bomber and fighter-bomber) escort in Italy pretty late (several squadrons still flying them at Anzio), and they weren't getting creamed either. So I think it's a cinch the P-40Q could have fit into that niche pretty easily assuming they could arrive in numbers, in time, and didn't have substantial teething problems.

I think it's also clear that the P-40N production run went on longer than it needed to, an improved version could have definitely been helpful.

I think it's unlikely though just because of all the problems Curtiss was having. The late run P-40Ns had a lot of minor production issues which the USAAF was well aware of and quite angry about (this is mentioned in some test documents). And of course there were the various other Curtiss scandals.

The Q also came too late, they are still doing flight testing in mid-1944, that's too late for an aircraft of this capability. So in that sense Shortround is right. The only way this changes is if Curtiss could have been persuaded to give up some of their other failed projects much earlier than they did and go through some kind of reorganization of management to improve their overall production issues.

One other question I have about the Q, is how much fuel did it carry? I couldn't find any numbers on that. If the range was reduced compared to the N that would be a problem.

 

[Shortround6]

aco AQC-6 ?

One other question I have about the Q, is how much fuel did it carry? I couldn't find any numbers on that. If the range was reduced compared to the N that would be a problem.

Tests say 160 gallons. Now it you the extra power your range gets shorter

A big problem with the P-40 as it was used was that it basically used 3 engines. Yes there were a lot of type numbers but you basically had basic set ups with variations. The next problems was that

A. all the early ones from the Long nose -33 through the -73 in the P-40Ks were sort of equivalent to the the Merlin III. More on this later.
B, The P-40F and the L used Merlin XX engines but introduced them about 1 3/4 years after the Hurricane II.
C. The P-40M ((lend lease for the British ) and the P-40Ns used the -81, the -99 and the -115 but in terms of power production they were equivalent to the Merlin XII used in the Spitfire II of 1940. And building such fighters in 1943-44 at over 8000lbs gross weight (A P-40N-5 was 100lbs lighter than the prototype Spitfire XIV with a 2 stage Griffon) shows how far out of date the P-40N was by the 2nd half of 1943.

The Merlin III compares to the early Allisons as follows.

engine..................................Merin III....................................V-1710-33.....................-33 alternate...................-39.......................-73
HP at altitude................1030/16,250...............................1040/14,300...................1090/13,200.............1150/11,700..........1150/12,000
HP extra boost..............1310/9,000..................................????/??????......................????/?????..................1490/4300............1580/2500
HP WER?..........................1440/5,500...................................????/?????.......................????/?????...................????/?????.............????/?????
Take-off................................880...............................................1040....................................1040.............................1150......................1325

A few notes
The 1440hp Merlin rating was for Sea Hurricanes ONLY. It does help show what the supercharger was capable of.

The Allisons all used just about the same supercharger, there were some differences between the _39 and the -73 as far as backfire screens and intake manifolds and a few other details. Please note that the later engines were allowed to make more power becasue of better crankshafts and crankcases and other details. There was very little difference in power at altitude. If you brought a -39 or -73 engine up to 14,300ft you were going to get very close to 1040hp as that is all the supercharger would flow.
The Allison soon showed it was rather tolerant of excessive boost, at least after the first 300 or more built which had to be reworked. Many squadrons in both British Empire service and in US service after Dec 7th were operated well over pilot's handbook pressures, BUT it could not exceed the pressures-power ratings at altitude in the first line of the chart, only at lower altitudes was more power available and all four engines showed the could make just about the same power at the same altitudes. that is to say that a -33 engine could be over boosted to make very close to 1580hp at 2500ft. Since most of the -33 engines did not have all the high strength parts of -73 this power output might have been for a short duration (the long nose engine reduction gears were also a weak spot).

Now the P-40 went about 6780lbs with about 380lbs worth of guns and ammo and 120 US gallons (720lbs) of fuel. NO protection.
The P-40B went up to 7326lbs with about 600lbs worth of guns/ammo and 120 US gallons of fuel but had 93lbs of armor and some sort of protection on the fuel tanks.
P-40C added Better protection to the fuel tanks and a few other details.
The P-40D/E is where things really began to slide. With no increase in power at over 13,200ft and only about 60hp more from 11,700ft down to the ground (WER wouldn't be approved until late in the Fall of 1942) and with the guns/ammo load going over 800lbs with over 900lbs with full ammo the designed gross weight for the P-40E was 8011lbs with 235rpg of ammo and only 120 US gallons in the fuel tanks.
A Spitfire VC had about 650lbs of guns and ammo (120rpg for the cannon) and had around 610 lbs of fuel and would go 6900lbs or less clean. We won't even go into the performance of the Merlin 45 engine
The Merlin XII was strengthened and allowed to use 12lbs of boost for take-off for 1175hp or just 25hp less than the Allison of 1943. The MK XII was only allowed to use 12lbs of boost at altitude for WEP and gave 1280hp at 10,500ft.

When the Allisons with the 9.60 gears show up (winter of 1942/43) the altitude rating goes from 12,000ft to 1125hp at15,500. Take-off goes down to 1200hp, WEP is either given as 1410hp at 9,500ft or 1360hp at sea level. 1943 Allison single stage, single speed engine only shows around 100hp at best at certain altitudes over the 1940 Merlin XII engine.

Except for under 400 P-40N-1 with limited equipment and only 4 guns the rest of the P-40N series was only a few hundred pounds lighter than the E & K. or well into the 8000lb range with full internal fuel and normal ammo load.

The P-40 needed to be lighter and it needed a better engine without going to the two stage engine.
Perhaps it was NIH but the Merlin XX used in the P-40F and L was not developed over it's 1940 beginnings for use in the P-40 and Allison may not have had enough engineers to fool around with and "in-between" engine when the Brass were calling for two stage engines or turbocharged engines.

 

[Wild_Bill_Kelso]

I'm not sure the P-40 needed to be lighter, or necessarily that it was the biggest problem with that aircraft... a lot of late war fighters were heavier than a P-40, I think all of the later war US types, they just had bigger engines.

Spit V had a slightly different job and it did not always do better than P-40s in various Theaters where they were both used.

 

[Shortround6]

Basically the P-40 was trying to lug too much armament without enough engine.
Yes the Corsair and Hellcat were bigger and heavier but they carried the same number of guns with less than double the ammo.
They also used engines that were hundreds of pounds heavier to do it. Which is also what ran the weight up.
You don't stick an R-2800 into an 8000lb fighter.
The airframe and engine/ power plant have to work as a team.
Trying to stick the Allison two stage engine in the P-40 showed the limitations. The extra power plant weight ( prop, cooling system, etc.) Added around 600lbs to the aircraft despite carrying only 2/3rds the armament, at best.
The mistakes started being made back in 1940 with the P-40D & E. If they had limited the armament to closer to 600lbs instead of 900lbs they might have been able to save 50-75lbs in the wing, they might have been able to save a bit more weight in the landing gear, perhaps a bit in the fuselage/tail?
The P-40E wing was 250lbs heavier than the radial Hawk 75 wing.

 

[nuuumannn]

The question regarding the P-4Q despite its virtues is what it offers compared to existing and future fighters within the time period? And this last point is worth noting for two reasons. The first one is that a production variant would have been a considerable redesign over existing P-40Ns on the production line, there wouldn't be a simple change to existing types on the production line, like fitting a new engine to the basic frame, it's a whole new airframe and would be essentially different. That's gonna take time to implement tooling to get to speed in production, which takes our first production examples off the lines to around mid 1945, giving a rough estimate once existing contracts for P-40Ns are fulfilled. The next point is that the P-80 Shooting Star is entering production and service in the same time period. Where does this leave our burgeoning production of an aircraft that offers little if any advantage compared to existing piston-engined fighters and none whatsoever compared to jet fighters in terms of performance?

The next thing is that does the USAAF need another type doing the same job as three existing types it has under production and in service, in the P-38, P-47 and P-51? Does it offer significant advantage over these three types? Not really, not when you take the likes of the performance of the P-51H into consideration. And as a ground pounder there is the AD1 Skyraider under development, too. It's simply not worth the time or effort to put the P-40Q into production. With hindsight, most would end up in scrapyards. There might have been foreign export options post-war, but foreign countries that operated P-40s, such as Australia, Canada and New Zealand bought (built, in Australia's case) P-51s. Latin and South American nations benefitted from surplus ex-WW2 types from foreign aid deals that saw P-47s and P-51s (as well as Avengers, F4Us, F6Fs etc) exported in numbers, so they might have been useful for sales.

Essentially it's far too late to be useful in a World War Two context.

 

[Wild_Bill_Kelso]

Basically the P-40 was trying to lug too much armament without enough engine.
Yes the Corsair and Hellcat were bigger and heavier but they carried the same number of guns with less than double the ammo.
They also used engines that were hundreds of pounds heavier to do it. Which is also what ran the weight up.
You don't stick an R-2800 into an 8000lb fighter.

The P-51 was also a good bit heavier too, at least the Merlin versions, as was the P-38 of course. The Fw 190 was heavier too, so was the Typhoon and Tempest.

The airframe and engine/ power plant have to work as a team.
Trying to stick the Allison two stage engine in the P-40 showed the limitations. The extra power plant weight ( prop, cooling system, etc.) Added around 600lbs to the aircraft despite carrying only 2/3rds the armament, at best.
The mistakes started being made back in 1940 with the P-40D & E. If they had limited the armament to closer to 600lbs instead of 900lbs they might have been able to save 50-75lbs in the wing, they might have been able to save a bit more weight in the landing gear, perhaps a bit in the fuselage/tail?
The P-40E wing was 250lbs heavier than the radial Hawk 75 wing.

P-40s were close to a tipping point in weight, per the engine power available, but I wonder if there may have also been some other design features (wing?) which affected the climb rate.

They often, when they needed to, lightened P-40s by taking out the oil cooler armor, a pair of guns, IFF and some other gear, to get it below that 300-400 lbs which allowed them to perform better (especially climb rate) and increased the critical altitude a bit. The main factor though was increasing boost and RPM on the engine, which certainly improved performance quite a bit, though only at lower altitudes. This is why the P-40 which was really a medium altitude fighter by design became a low altitude fighter in the field.

The Merlin engined variants though remained capable in the medium altitude level, up to around 25,000 ft, and presumably a Q variant would have been better still, maybe 20% better overall, which should have been sufficient through 1944 at least in Europe, and for the whole war in the Pacific.

And four 12.7mm guns isn't necessarily so bad as armament, it was the armament for the early Wildcat, for the early Merlin P-51 (B/C) which did spectacularly well, for many of the Merlin P-40s flying in Europe / Med, and I think it was the original armament for the late war / too late for the war Bearcat. Unless you are attacking heavy bombers i think four .50 caliber guns is pretty good armament.

 

[Shortround6]

They often, when they needed to, lightened P-40s by taking out the oil cooler armor, a pair of guns, IFF and some other gear, to get it below that 300-400 lbs which allowed them to perform better (especially climb rate) and increased the critical altitude a bit. The main factor though was increasing boost and RPM on the engine, which certainly improved performance quite a bit, though only at lower altitudes. This is why the P-40 which was really a medium altitude fighter by design became a low altitude fighter in the field.

A lot of that stuff was sticking band-aids on the problem.
Planes are designed to carry a certain "load" and a lot of the strength calculations are done taking that load into account. If you scale back the load some of the structural components don't have to be as heavy but the guys in field cannot do that. They are "stuck" with the structure and the only they can do is take out "load" items, like guns, armor, not fill the fuel tanks for short range missions, etc.
Over boosting engines and/or over revving them certainly has draw backs too.
The P-40D & E were in preliminary design work in the summer of 1940, they wouldn't get into production until the summer of 1941 so nobody actually knew what was needed for guns or ammo. They were learning lessons from the BoB as things went along.
However as I have tried to point out, the Short nose Allison -39 was not actually that much of an improvement over the long nose. Without over boosting it was only good for about an extra 60hp at altitudes below around 12,000ft, RAM does help some.

Now the US decided to to stick 380rpg in the P-40B &C which ran the weight up about 100lbs right there. This was more than the extra .30 cal/.303 gun in each wing PLUS the ammo.
Since the cowl guns were lucky they could hit 500rpm cycle rate is over 45 seconds worth of ammo. Maybe the guys in the field stuck wooden blocks in the ammo bins to lighten things up?

Now with this extra 60hp available the British decided to fit four .50 cal guns in the wings of the P-40D (the new reduction gear meant no cowl guns) and load in 250rounds per gun, which seems very sensible,...........except............they made "provision" to hold 615 rounds per gun as an "alternate" load. Problem with this is you have to build the wing (and the landing gear) to hold this extra load. The British also wanted the ability to mount a 20mm Hispano cannon under each wing just outboard of the outer .50 cal gun with cutouts made in the lower wing panel for equipment. No mention is made if they were using drums up in the wing or belts or????????

The Americans, not to be left out, didn't make any arrangements for 20mm guns but decided to mount six .50 cal guns from the start with 235 rounds per gun "normal" and an alternate load of 312rpg for the No 1 gun, 290rpg for the No 2 gun and 240rpg for the No 3 gun. The pilots manual is in disagreement with itself as the weight charts show an extra 200rounds above even this amount.
You can also put six 20lb bombs under the wing, stick another 25.5 gallons of fuel in the rear fuselage tank, hang a drop tank with 52 gallons of fuel under the fuselage. (and other 3 gallons of oil) for a gross take-off weight of just about 8800lbs.

all of the planes you mentioned were designed to be heavier aircraft, except the Merlin Mustangs (and it was extensively redesigned). Having 2000hp available in the design stage allows you do a number of things you cannon do with an 1150-1200hp engine, of course when you are designing a plane with 2600lb (P-38, bare engines) or 2400-2600lb engines (Sabres) means you make rather different choices than if you are using a 1300-1400lb engine.

 

[Wild_Bill_Kelso]

A lot of that stuff was sticking band-aids on the problem.
Planes are designed to carry a certain "load" and a lot of the strength calculations are done taking that load into account. If you scale back the load some of the structural components don't have to be as heavy but the guys in field cannot do that. They are "stuck" with the structure and the only they can do is take out "load" items, like guns, armor, not fill the fuel tanks for short range missions, etc.
Over boosting engines and/or over revving them certainly has draw backs too.

yes but there is a signficiant difference between 'overboosting' and gradually increasing the normal / rated boost levels which was done for many aircraft including Spits and Hurricanes. Overboosting was an emergency measure and when done with the V-1710 aircraft (including P-40 and Allison P-51s) they were getting as much as 1700 hp down low. But that was indeed somewhat reckless, certainly until they knew what they could get away with. Apparently this kind of boost became normal for some of the RAF Mustang I units.

Increasing the normal boost and RPM more moderately for the V-1710-39 and -73 mark P-40s, they were able to go from the ballpark of 1,100 hp at 42" Hg up to 1,240 hp at 45" Hg, to by the second quarter of 1942, 1,470 hp at 56" hg, (with corrresponding improvements in climb and acceleration, and without apparently putting undue strain on the engines). The engines were gradually strengthened by Allison (crank case, crank shaft, bearings) and the official boost level went up to 60" by the third quarter of 1942, which gives 1550 hp for WEP for the V-1710-73 (P-40K and up-engined P-40E). That is in the P-40K manual. And that is plenty of power for the amount of weight they were carrying.

The P-40D & E were in preliminary design work in the summer of 1940, they wouldn't get into production until the summer of 1941 so nobody actually knew what was needed for guns or ammo. They were learning lessons from the BoB as things went along.
However as I have tried to point out, the Short nose Allison -39 was not actually that much of an improvement over the long nose. Without over boosting it was only good for about an extra 60hp at altitudes below around 12,000ft, RAM does help some.

It wasn't an improvement at all initially - the P-40D and E (as Kittyhawk I and Ia) had inferior performance to the earlier Tomahawk types, as in 1,000 fpm worse climb rate. However, they increased the engine rating from it's initial very conservative ratings, performance improved considerably, again at least for lower altitudes (6,000 ft or lower, basically) which the DAF adapted themselves to. This is what RAAF 10 victory Ace Bobby Gibbes said:

"Well it was basically the same aeroplane. We were a little disappointed when we first got the Kitty, we thought it'd be way ahead of the Tomahawk. In actual fact, it was a little bit better. One thing I personally didn't like about it was the Tomahawk had fairly high sides and you'd be sitting behind a thin sheet of metal but you felt safer. The Kittyhawk had perspex coming way down and you felt as if you were sitting up, very vulnerable, because you could see out so much. That was one feature I do remember. However, later when we got our Kittyhawks running properly - were getting better performance - they were a better aeroplane."

Now the US decided to to stick 380rpg in the P-40B &C which ran the weight up about 100lbs right there. This was more than the extra .30 cal/.303 gun in each wing PLUS the ammo.
Since the cowl guns were lucky they could hit 500rpm cycle rate is over 45 seconds worth of ammo. Maybe the guys in the field stuck wooden blocks in the ammo bins to lighten things up?

Some Soviet units took all the wing guns out of their Tomahawks (P-40B/C) and apparently thought they had enough firepower with the two HMGs, despite the low ROF. They seem to have done pretty well with them.

Now with this extra 60hp available the British decided to fit four .50 cal guns in the wings of the P-40D (the new reduction gear meant no cowl guns) and load in 250rounds per gun, which seems very sensible,...........except............they made "provision" to hold 615 rounds per gun as an "alternate" load. Problem with this is you have to build the wing (and the landing gear) to hold this extra load. The British also wanted the ability to mount a 20mm Hispano cannon under each wing just outboard of the outer .50 cal gun with cutouts made in the lower wing panel for equipment. No mention is made if they were using drums up in the wing or belts or????????

The Americans, not to be left out, didn't make any arrangements for 20mm guns but decided to mount six .50 cal guns from the start with 235 rounds per gun "normal" and an alternate load of 312rpg for the No 1 gun, 290rpg for the No 2 gun and 240rpg for the No 3 gun. The pilots manual is in disagreement with itself as the weight charts show an extra 200rounds above even this amount.
You can also put six 20lb bombs under the wing, stick another 25.5 gallons of fuel in the rear fuselage tank, hang a drop tank with 52 gallons of fuel under the fuselage. (and other 3 gallons of oil) for a gross take-off weight of just about 8800lbs.

all of the planes you mentioned were designed to be heavier aircraft, except the Merlin Mustangs (and it was extensively redesigned). Having 2000hp available in the design stage allows you do a number of things you cannon do with an 1150-1200hp engine, of course when you are designing a plane with 2600lb (P-38, bare engines) or 2400-2600lb engines (Sabres) means you make rather different choices than if you are using a 1300-1400lb engine.

Sure but there was kind of a medium level, like the Mustang, with the ~1,500 hp engines, and the P-40 basically became that, at least at certain altitudes. I think this is the real reason (or one of the reasons) why the type remained in front line use for so long.

The most common field mod was just removing a pair of wing guns with their ammunition, that seemed to get it pretty close to the power they needed. And it was sufficient for shooting down Axis fighters and most Axis (light and medium) bombers.

 

[Wild_Bill_Kelso]

I should add - higher boost ratings were for 'Grade 100' fuel.

 

[space dodo]

lovely plane, best early war us fighter in my opinion.