Bell P-39 Airacobra Variants/ideas

[Conspirator]

so the Bell P-38 has been one of two favorite misfit aircraft that came from the brainchild of WW2. i have read a while back that there were proposals for certain fighter, fighter/bomber and escort variants. however i do not have the book or remember the name unfortunately. so any help would be great

Great Thanks
Hunter

 

[Conspirator]

oh okay didnt know

 

[Conspirator]

so the Bell P-38 has been one of two favorite misfit aircraft that came from the brainchild of WW2. i have read a while back that there were proposals for certain fighter, fighter/bomber and escort variants. however i do not have the book or remember the name unfortunately. so any help would be great

Great Thanks
Hunter

 

[_Del_]

https://www.secretprojects.co.uk/threads/some-little-known-cobras-of-bell-aircraft-corporation.2380/

 

[isayyo2]

Probably the P-63D being the ultimate "What if" with the more powerful M9 cannon and single piece bubble canopy.

P-63D

Powerplant: One Allison V-1710-109 (E22) water-cooled engine rated at 1425 hp for take off . Performance: Maximum speed was 437 mph at 30,000 feet, service ceiling was 39,000 feet, and an altitude of 28,000 feet could be reached in 11.2 minutes. Normal range was 950 miles, and maximum ferry range...

forum.warthunder.com

Redesign the wing to hold four .50s and drop the "gondola" guns and I bet it could it hit 450mph; with its drop tanks and twin-stage supercharging It'd be a stand out for sure. What's even more impressive were the plans to mount the Turbo-Compound Allison that had significantly more power and fuel efficiency.

Perhaps @tomo pauk has some more ideas?

 

[Conspirator]

View attachment 666721Probably the P-63D being the ultimate "What if" with the more powerful M9 cannon and single piece bubble canopy.

P-63D

Powerplant: One Allison V-1710-109 (E22) water-cooled engine rated at 1425 hp for take off . Performance: Maximum speed was 437 mph at 30,000 feet, service ceiling was 39,000 feet, and an altitude of 28,000 feet could be reached in 11.2 minutes. Normal range was 950 miles, and maximum ferry range...

forum.warthunder.com

Redesign the wing to hold four .50s and drop the "gondola" guns and I bet it could it hit 450mph; with its drop tanks and twin-stage supercharging It'd be a stand out for sure. What's even more impressive were the plans to mount the Turbo-Compound Allison that had significantly more power and fuel efficiency.

Perhaps @tomo pauk has some more ideas?

thanks! didnt think about the Allison that would have made all the difference

 

[_Del_]

Redesign the wing to hold four .50s and drop the "gondola" guns and I bet it could it hit 450mph; with its drop tanks and twin-stage supercharging It'd be a stand out for sure. What's even more impressive were the plans to mount the Turbo-Compound Allison that had significantly more power and fuel efficiency.

The inner portion of the Kingcobra wing held the gear and radiators. The outer portion was where all the fuel was carried. Not sure what the aft fuselage held, but I'm sure there was equipment of some kind. I'm sure the decision for gondolas wasn't haphazard. If there was an easy way to carry them internally, it probably would have been done.

 

[isayyo2]

I'm sure the decision for gondolas wasn't haphazard. If there was an easy way to carry them internally, it probably would have been done.

Weren't .30 Cal guns in the wings originally and the gondolas were used since the .50s were too large for the same space? But I totally agree that there wasn't an easy way with the war going on; you could stretch the wingspan out and end up with some P-63/Ta 152H hybrid.

 

[_Del_]

P-63 had a whole new wing compared to the -39's. Never had internally mounted (wing) guns to my knowledge.

 

[Justo Miranda]

Hi

 

[tomo pauk]

Probably the P-63D being the ultimate "What if" with the more powerful M9 cannon and single piece bubble canopy.

P-63D

Powerplant: One Allison V-1710-109 (E22) water-cooled engine rated at 1425 hp for take off . Performance: Maximum speed was 437 mph at 30,000 feet, service ceiling was 39,000 feet, and an altitude of 28,000 feet could be reached in 11.2 minutes. Normal range was 950 miles, and maximum ferry range...

forum.warthunder.com

Redesign the wing to hold four .50s and drop the "gondola" guns and I bet it could it hit 450mph; with its drop tanks and twin-stage supercharging It'd be a stand out for sure. What's even more impressive were the plans to mount the Turbo-Compound Allison that had significantly more power and fuel efficiency.

Perhaps @tomo pauk has some more ideas?

P-63D over the C had a number of changes. Wing was bigger, with greater fuel tankage (168 vs. 126-128 US gals). Engine was the latest of the 2-stage supercharged V-1710s, the -109, able to run at 3200 rpm, with carburetor between the compressor stages, and with faster-turning impeller of the auxiliary S/C; water injection was also used. All of those changes gave significant boost to the hi-alt power, and indeed the P-39D was supposed to do ~450 mph at ~30000 ft (at least per Bell's figures, so discretion is advised). However, P-39D was too late for ww2; even the -C was too late for European part of ww2 to matter.

P-63 section on Mike Williams' site: link

As for the guns - I'd delete the 37mm cannon, install the 20mm belt-fed Hispano (same as on the P-38s and P-61s), keep the nose HMGs. Delete the wing guns and their ammo, use the freed volume for even more fuel.

 

[Conspirator]

Hi

hol up..... is that a VTOL version with counter rotating props and a ramjet?

 

[Conspirator]

Probably the P-63D being the ultimate "What if" with the more powerful M9 cannon and single piece bubble canopy.

P-63D

Powerplant: One Allison V-1710-109 (E22) water-cooled engine rated at 1425 hp for take off . Performance: Maximum speed was 437 mph at 30,000 feet, service ceiling was 39,000 feet, and an altitude of 28,000 feet could be reached in 11.2 minutes. Normal range was 950 miles, and maximum ferry range...

forum.warthunder.com

Redesign the wing to hold four .50s and drop the "gondola" guns and I bet it could it hit 450mph; with its drop tanks and twin-stage supercharging It'd be a stand out for sure. What's even more impressive were the plans to mount the Turbo-Compound Allison that had significantly more power and fuel efficiency.

Perhaps @tomo pauk has some more ideas?

P-39D over the C had a number of changes. Wing was bigger, with greater fuel tankage (168 vs. 126-128 US gals). Engine was the latest of the 2-stage supercharged V-1710s, the -109, able to run at 3200 rpm, with carburetor between the compressor stages, and with faster-turning impeller of the auxiliary S/C; water injection was also used. All of those changes gave significant boost to the hi-alt power, and indeed the P-39D was supposed to do ~450 mph at ~30000 ft (at least per Bell's figures, so discretion is advised). However, P-39D was too late for ww2; even the -C was too late for European part of ww2 to matter.

P-63 section on Mike Williams' site: link

As for the guns - I'd delete the 37mm cannon, install the 20mm belt-fed Hispano (same as on the P-38s and P-61s), keep the nose HMGs. Delete the wing guns and their ammo, use the freed volume for even more fuel.

honestly i feel that this could have been a great fighter if it was built better. the tail control reactions would be a bit slower than a spitty but it would have better stability and protection of the engine because of where it is. a marvelous idea if i do say so myself. but this would have made a great fighter/bomber because of a mid-engined layout. however a weapons bay would be small or nonexistent because of the space taken up by the engine. what i see is a pretty concentrated armament in the hollow nose of this aircraft as a fighter variant. i was thinking like the Mitsubishi AM Zero with the calibrated guns on the cowling but more of them. either two on the sides of the nose and one on top or some other setup. however you would have to keep weight down so a few M3 Brownings or Vickers .303s would be sufficient in my opinion

 

[Justo Miranda]

Hi

hol up..... is that a VTOL version with counter rotating props and a ramjet?

On 8 January 1941, an engineer of the Bell Aircraft Corporation named Arthur Young applied for a patent of a tail sitter VTOL airplane. It was propelled by one radial engine located behind the pilot driving two big contra-rotating propellers by means a geared drive shaft.

It was a clever application of the technology developed by Bell in 1937 to propel the P-39 fighter.

For security reasons the patent Nº 2382460 was not published until 14 August 1945.

In 1946 the Juriev OKB bureau of design developed the Juriev KIT-1, a VTOL point defence interceptor based on the propulsion system of the Bell P-39 Airacobra.

The novelty, compared with the Young design, consisted of the usage of two propellers with different diameter.

The forward two bladed metallic propeller, with 4.28 m of diameter, was used for propulsion purposes only, the rear one, with 9.28 m of diameter, had a light structure partially covered by fabric and was used to balance the rotation of the other during take-off and landing.

The stability in vertical flight was achieved by the wing and tail control surfaces.

In horizontal flight, the rotor was disconnected from the drive shaft and blocked in parallel to the wing, with zero-degree pitch, acting as a canard plane.

Bibliography: Soviet magazine Technika-Molodiozhi, 1983.

Juriev KIT-1 Technical data

Power plant: one 1,100 hp. Klimov VK-108 liquid cooled piston engine, wingspan: 30 ft. (9.12 m), length: 25.8 m (7.89 m), wing surface: 300 sq. ft. (27 sq. m), armament: two wing mounted 20 mm ShWAK cannons or four 12.7 mm Beresin BS heavy machine guns.

 

[Conspirator]

Hi

hol up..... is that a VTOL version with counter rotating props and a ramjet?

On 8 January 1941, an engineer of the Bell Aircraft Corporation named Arthur Young applied for a patent of a tail sitter VTOL airplane. It was propelled by one radial engine located behind the pilot driving two big contra-rotating propellers by means a geared drive shaft.

It was a clever application of the technology developed by Bell in 1937 to propel the P-39 fighter.

For security reasons the patent Nº 2382460 was not published until 14 August 1945.

In 1946 the Juriev OKB bureau of design developed the Juriev KIT-1, a VTOL point defence interceptor based on the propulsion system of the Bell P-39 Airacobra.

The novelty, compared with the Young design, consisted of the usage of two propellers with different diameter.

The forward two bladed metallic propeller, with 4.28 m of diameter, was used for propulsion purposes only, the rear one, with 9.28 m of diameter, had a light structure partially covered by fabric and was used to balance the rotation of the other during take-off and landing.

The stability in vertical flight was achieved by the wing and tail control surfaces.

In horizontal flight, the rotor was disconnected from the drive shaft and blocked in parallel to the wing, with zero-degree pitch, acting as a canard plane.

Bibliography: Soviet magazine Technika-Molodiozhi, 1983.

Juriev KIT-1 Technical data

Power plant: one 1,100 hp. Klimov VK-108 liquid cooled piston engine, wingspan: 30 ft. (9.12 m), length: 25.8 m (7.89 m), wing surface: 300 sq. ft. (27 sq. m), armament: two wing mounted 20 mm ShWAK cannons or four 12.7 mm Beresin BS heavy machine guns.

i feel like the speed would be horrible but the mobility would be amazing.... a questionable but interesting design. thanks Justo.

 

[RyanC]

Redesign the wing to hold four .50s and drop the "gondola" guns and I bet it could it hit 450mph; with its drop tanks and twin-stage supercharging It'd be a stand out for sure.

Try 500 MPH.

NARA II RG 18 Decimal 452.1 Unclassified Records​

IN REPLY ADDRESS BOTH
COMMUNICATION AND ENVELOPE
TO ATTENTION OF FOLLOWING
OFFICE SYMBOL:
TSENG ( TSESE-2C )

HEADQUARTERS
ARMY AIR FORCES
AIR TECHNICAL SERVICE COMMAND​

TSESE-2C:JFA/fkk
WRIGHT FIELD, DAYTON, OHIO
29 June 1945​

SUBJECT: Interim P-63 Interceptor Fighter

TO: Commanding General
Army Air Forces
Washington 25, D. C.
Attention: AC/AS, M&S.,
Aircraft Projects Branch

1. During the past few weeks a number of conferences and discussions have been held at the Air Technical Service Command concerning the aspects of procuring “interim” fighters to fill the gap between present production types and contemplated future types which are as yet several years away. As a result of these conferences and discussions, a general conclusion has been reached that such a program will benefit the Government by maintaining uniform and continued development of fighter type airplanes. In general, the program contemplates the procurement of improved versions of the present production type fighters which will incorporate the latest developments of the engine manufacturers. The first detail proposal to partially fulfill the needs of this contemplated program has been received from the Bell Aircraft Corporation and involves an improved version of the P-63 airplane. The details of this proposal are as follows:

a. Install the Allison E-30 (AC-V-1710-135) engine which is essentially an E-22 (AC-V-1710-109) engine with the following exceptions: Compression ratio changed from 6.65:1 to 6.0:1, Auxiliary stage supercharger gear ratio changed from 7.32:1 to 7.64:1. This engine could be available in production quantities by 1 January 1946.

b. Remove the 37mm cannon and install a .50 caliber 5-gun nose , one .50 caliber gun to be free firing through the prop spinner with 300 rounds of ammunition provided and the other four guns to be synchronized through the propeller and each provided with 250 rounds of ammunition. The externally mounted wing guns will be removed and provisions made for two free firing .50 caliber wing tip guns, each provided with 300 rounds of ammunition. These guns will have provisions for automatic corrections in azimuth and elevation to compensate for wing deflection in maneuvers, and to harmonize at different ranges when used in conjunction with the A-1 gunsight.

c. Install the Davis-Draper A-1 gyro computing radar ranging gunsight.

NOTE: In order to provide space for the radar equipment connected with the A-1 gunsight, the bomb shackle located in the center of the fuselage will be removed.

d. Provide additional self-sealing fuel cells giving the airplane a total internal fuel capacity of 240 to 250 gallons.

e. Provide a bubble canopy of the free-blown type.

f. The quoted performance of this airplane based on standard grade 130 fuel and the anticipated grade 150 to 170 fuel is as follows:

Performance - (Estimated)​
Present Grade 130 Fuel​	Grade 150 - 170 Fuel​
1. High Speed MPH a. Sea level - 415 b. Critical Alt. 460 (13,900 Ft.) c. 30,000 Ft - 441 2. Rate of Climb F.P.M. a. Sea level - 5720 b. Critical Alt. - 5120 (11,000 Ft.) c. 30,000 Ft. - 2100 3. Time to climb to 30,000 Ft. - 7.5 min.	1. High Speed MPH a. Sea level - 453 b. Critical Alt. - 502 (14,700 Ft.) c. 30,000 Ft. - 479 2. Rate of Climb F.P.M. a. Sea level - 7460 b. Critical Alt. - 6430 (11,000 Ft.) c. 30,000 Ft.- 2870 3. Time to climb to 30,000 Ft. - 5.8 min.

g. At such time as the E-27 (V-1710-127) engine becomes available in production quantities (approximately 18 months) it

will be possible to replace the E-50 with the E-27 by modification of the airplanes resulting in the following quoted performance based on the use of standard 130 grade fuel and on the anticipated 150 to 170 grade fuel.

Performance - (Estimated)​
Present Grade 130 Fuel​	Grade 150 - 170 Fuel​
1. High Speed MPH a. Sea level - 432 b. Critical Alt. - 490 (14,000 Ft) e. 30,000 Ft. - 463 2. Rate of Climb F.P.M. a. Sea level - 6380 b. Critical Alt. - 6150 (11,500 Ft) c. 30,000 Ft. - 2320 3. Time to Climb to 30,000 Ft. - 6.6 min.	1. High Speed MPH a. Sea level - 471 b. Critical Alt. - 519 (14,800 Ft.) c. 30,000 Ft. - 481 2. Rate of Climb F.P.M. a. Sea level - 8120 b. Critical Alt. - 7000 (11,000 ft.) c. 30,000 Ft. - 2770 3. Time to Climb to 30,000 Ft. - 5.6 min.

2. The development of the P-65 airplane along the lines set forth above is considered a logical and desirable step in the interim fighter program. It will provide an airplane superior to the present production type and which can be placed in limited production with small additional expenditures for tools, jigs, dies, etc.

5. It is therefore requested that a directive be issued covering the procurement of two proto-type airplanes of the P-63 series powered by the E-50 engine incorporating the modifications outlined above. By proceeding with the procurement of the proto-type now, limited peace time production can be undertaken with minimum delay and disruption of the Contractor's facilities. The airplanes could be further improved when the E-27 engine becomes available.

FOR THE ACTING DIRECTOR:​

/S/
F.O. CARROLL,
Brig. General, USA
Chief, Engineering Division​

 

[sienar]

b. Remove the 37mm cannon and install a .50 caliber 5-gun nose , one .50 caliber gun to be free firing through the prop spinner with 300 rounds of ammunition provided and the other four guns to be synchronized through the propeller and each provided with 250 rounds of ammunition. The externally mounted wing guns will be removed and provisions made for two free firing .50 caliber wing tip guns, each provided with 300 rounds of ammunition. These guns will have provisions for automatic corrections in azimuth and elevation to compensate for wing deflection in maneuvers, and to harmonize at different ranges when used in conjunction with the A-1 gunsight.

Do you have any more information about this?

 

[Tony Williams]

My favourite P-39 fantasy mods:

1. Use a gearbox to divide the engine power between twin wing-mounted propellers, leaving the nose clear for any desired armament configuration without needing any synchronisation (thereby doubling the rate of fire compared with the synch .50s). This would also boost the airflow over the wings to the benefit of increased lift for reduced landing/take-off speeds.

2 Optional extra in addition to (1): Redesign the nose so that the cockpit is closer to the front, improving the forward/downward view and moving the guns so that the barrels run under the cockpit (to either side of the nose wheel) with the magazines behind it, to keep the muzzle flashes out of the pilot's eyeline and maintain CoG as the ammo is used up.

 

[Colonial-Marine]

What was the rate of fire of the .50 caliber AN/M2 with synchronization gear? I've heard claims that it significantly reduced the rate of fire, more than what is usual.

Maybe for a hypothetical ultimate P-63 (without any drastic changes) it would have been better to relocate those .50 caliber guns to the wings instead. Using the 37mm M9 in the nose instead of the M10 should also provide some extra utility in ground attack.

 

[Justo Miranda]

L-39

 

[Apophenia]

What was the rate of fire of the .50 caliber AN/M2 with synchronization gear? I've heard claims that it significantly reduced the rate of fire, more than what is usual.

Maybe for a hypothetical ultimate P-63 (without any drastic changes) it would have been better to relocate those .50 caliber guns to the wings instead. Using the 37mm M9 in the nose instead of the M10 should also provide some extra utility in ground attack.

Here, the M2's synchronized rof is given as "300 rounds/min instead of 650" (although, elsewhere, I see the AN/M2 cyclic rate of fire given as 750–850 rpm).
-- https://www.mission4today.com/index.php?name=Downloads3&file=details&id=2537

But there is a problem moving the AN/M2s out to the wings. Historically the Kingcobra had a single 'fifty' mounted in underslung fairings on each wing. There simply wasn't enough room in the wings to fully-enclose the larger .50-cals (as had been done with the twin .30 guns on early P-39s).

This was mainly due to the wingspar arrangement - with its forward spar at ~10% chord and the centre spar at ~40% chord (with a third, false spar just in front of the flaps and ailerons). As compared with the Airacobra, the P-63 had an entirely new, laminar-flow wing design. But that new wing still mated to the centre fuselage in the same locations as the P-39.

To mount all four .50-cals within the wings, this hypothetical P-63 would need completely redesigned wings (and centre fuselage pickup points). So, either accept the disruption to the production line and redesign the spars to accommodate the bigger guns (preferably closer to the c/g) or leave the underslung guns and simply delete the cowl guns. After all, a 37 mm plus twin 'fifties' still provides a pretty decent weight of fire.

 

[sienar]

The kingcobra didn't have a spar at 10%

I suspect the underslung guns had more to do with fuel. The area where 50s could have been placed was taken by a fuel tank. Range on internal fuel was an issue for the aircraft and there was hardly any excess internal volume outside the wing.

 

[T. A. Gardner]

View attachment 666721Probably the P-63D being the ultimate "What if" with the more powerful M9 cannon and single piece bubble canopy.

P-63D

Powerplant: One Allison V-1710-109 (E22) water-cooled engine rated at 1425 hp for take off . Performance: Maximum speed was 437 mph at 30,000 feet, service ceiling was 39,000 feet, and an altitude of 28,000 feet could be reached in 11.2 minutes. Normal range was 950 miles, and maximum ferry range...

forum.warthunder.com

Redesign the wing to hold four .50s and drop the "gondola" guns and I bet it could it hit 450mph; with its drop tanks and twin-stage supercharging It'd be a stand out for sure. What's even more impressive were the plans to mount the Turbo-Compound Allison that had significantly more power and fuel efficiency.

Perhaps @tomo pauk has some more ideas?

An easier fix to firepower would have been to fit it with 3 nose mounted (one through the propeller) 20mm cannon and dump the .50's entirely. Bell had access to both British and Russian technology on 20mm cannon and could have used that if a US variant couldn't be fitted. But I think there's sufficient room for three. That's a lot of highly concentrated firepower.

Fitting the Allison E series like the -117 or -119 with the auxiliary supercharger stage would likely have been sufficient for power.

 

[Tony Williams]

Using the 37mm M9 in the nose instead of the M10 should also provide some extra utility in ground attack.

Yes, it definitely would. It would also make for a better ballistic match with the .50 or 20mm.

 

[Tony Williams]

An easier fix to firepower would have been to fit it with 3 nose mounted (one through the propeller) 20mm cannon and dump the .50's entirely.

The problem with that is that the Hispano was not initially capable of being synchronised. Only with the postwar M24 model was this possible.

 

[T. A. Gardner]

An easier fix to firepower would have been to fit it with 3 nose mounted (one through the propeller) 20mm cannon and dump the .50's entirely.

The problem with that is that the Hispano was not initially capable of being synchronised. Only with the postwar M24 model was this possible.

Then use the Berezin B-20 or the British Polsten. Even an Oerlikon 20mm might be a choice. Build the ones going to the Russians with a copy of their cannon in the nose. As with the P-400, the USAAF might well see the value of the set up and okey doak it for their use too.

 

[Tony Williams]

An easier fix to firepower would have been to fit it with 3 nose mounted (one through the propeller) 20mm cannon and dump the .50's entirely.

The problem with that is that the Hispano was not initially capable of being synchronised. Only with the postwar M24 model was this possible.

Then use the Berezin B-20 or the British Polsten. Even an Oerlikon 20mm might be a choice. Build the ones going to the Russians with a copy of their cannon in the nose. As with the P-400, the USAAF might well see the value of the set up and okey doak it for their use too.

Unfortunately the Oerlikon and Polsten were even less suitable for synchronisation than the Hispano, and could not be modified to make them suitable. The B-20 would work, but was not released until the end of the war.

 

[Justo Miranda]

Hi

 

[Grey Havoc]

View: https://www.youtube.com/watch?v=I3nx5MMjfqs

 

[Colonial-Marine]

I know the Merlin engine was in high demand throughout the war, but was any P-39 or P-63 ever flown with one on an experimental basis? There was a planned P-63B series which would have used the Merlin, but this was cancelled so those engines could be used elsewhere.

In a way it seems surprising that they didn't even get a chance to test that combination once. Then again, the final variants of the Allison V-1710s seemed very comparable even if they weren't quite as good at high altitude. So perhaps it wouldn't have made much a difference on the P-63. Earlier in the war I imagine it might have helped performance on the P-39, similar to how some P-40s received early Packard Merlin engines for that purpose. I'd guess the P-40 had priority due to the spin issues the P-39 had.

 

[T. A. Gardner]

Probably the #1 issue with the P-39 and use by the USAAF was range. For the USAAF the P-39 simply didn't have the legs or means to carry drop tanks to increase that to a point where it was useful offensively most of the time. The P-40 did better so it was the plane of choice for early fighter designs.

For the Russians, range wasn't generally an issue. Their P-39's were either operating over friendly territory or close to the front. You really couldn't hang drop tanks on the wings when there were already those .50 gondolas there. The result was the P-39, for the USAAF simply couldn't reach the fight most of the time.

 

[Antonio]

Probably the #1 issue with the P-39 and use by the USAAF was range

In fact it was originally designed as point high altitude / high speed climbing bomber interceptor. (Circular Proposal X-609 from 1937)

 

[tomo pauk]

I know the Merlin engine was in high demand throughout the war, but was any P-39 or P-63 ever flown with one on an experimental basis? There was a planned P-63B series which would have used the Merlin, but this was cancelled so those engines could be used elsewhere.

The V-1650-5 was supposed to be the reworked V-1650-3 (that was used on the P-51B), the rework consisting of the redesigned crankcase that was now without the reduction gear. Thus the extension shaft, connecting the engine and the remote gearbox for the prop, will fit just under the pilot of the P-63, otherwise it would've been too high to fit.
I'm not sure that the -5 ever materialized, though.
The P-39 was never mooted to be using the Merlin.

In a way it seems surprising that they didn't even get a chance to test that combination once. Then again, the final variants of the Allison V-1710s seemed very comparable even if they weren't quite as good at high altitude. So perhaps it wouldn't have made much a difference on the P-63. Earlier in the war I imagine it might have helped performance on the P-39, similar to how some P-40s received early Packard Merlin engines for that purpose. I'd guess the P-40 had priority due to the spin issues the P-39 had.

The P-39 had the problem that it had too much of ... stuff, and not enough of free and CoG-neutral volume.
FWIW, the P-39M/N/Q were even better performers than the Merlin-powered P-40s, or even the Spitfire V; the P-39 being one of the sleekest and lightest V12-powered fighters certainly helped.

What would've helped even more would've been a further improved V-1710 still with a 1-stage engine (= big S/C, hopefully with variable speed drive; water-alcohol injection, too) since the 2-stage V-1710 will not fit, due to it being way too long; also the CoG issues would've been worsened still, and with wing free of guns & ammo, but with more fuel.

If there was the V-1650-5 powered P-63, we'd probably see the 430+ mph figure on it already in the late 1943/early 1944, even if that meant removal of the gondola HMGs.