V-1710 supercharger development potential

[kool kitty89]

My understanding is that P-63 was envisioned as an aircraft that would be able to outclimb the Fw-190 and Zero, or at least there is a line in the 'Vee's' about that. Small (for non-European measuring sticks) fuel tankage should contribute to that, the P-63 was a good climber. However in late 1943, when the P-63 entered production, the long range was a requirement for the USAF fighters, not the great RoC. For Bell to install greater fuel tanks would mean further delays to the production, since it would mean different ribs.

Right redesign a 'wet' wing to expand fuel. Shame they didn't catch on during the early prototype phase and put emphasis on that area. (especially in a modular fashion like the P39's fuel cells allowed) The P-59 had that problem too ... LOTS Of wing space for fuel, but only a moderate amount used. (granted a bit moot given it didn't develop into a useful combat ... or even recon aircraft)

The F2A actually comes to mind too for having the opposite problem. The poor thing started out with an exceptional fuel load for its size, but the designers seemed to put even more emphasis on fuel load over keeping weight down and climb performance up to the point of crippling it and exacerbating the build quality/manufacturing problems. (from 160 gallons on the F2A-1 to 240 gallons on the F2A-2/3 ... double the Wildcat's internal fuel load in spite of being a smaller, lighter built plane)

The Hispano worked okay aboard the P-38 - rigid fuselage mount benefits a bit? The ability to re-c0ck the Hispano would be a safety feture, of course.

The pilot ability to re-**** the cannon in-flight was a big part of this, most/all wing mounted variants didn't feature that until electrical cocking with the post-war M3. (electrical priming probably would have resolved many of the issues too ... not sure why that wasn't persued as the British did for their post-war Hispanos)

I will politely disagree with that. At 20000 ft, the better V-1710 (ie. 9.60:1 S/C gear) it with give around 950 HP, vs. ~1130HP of Merlin 45. It will enter production some 15-18 months later than Merlin 45. It will compare favorably under 10000 ft, though, not a great thing in a fighter aircraft.
What the V-1710 will have against early Merlin 45 might be the better carb (pressure injection vs. float-type) and use of 6-per-side exhaust stacks rather than 3-per-side; those are engine add-ons, though. The Merlins also received better carbs not long after the 9.60:1 V-1710s were introduced.

The cropped Merlin 50 series vs the 8.8 supercharged Allisons was a bit more favorable and available earlier (though official boost limit restrictions may have been raised later than on the Merlin).

We've already been over the unfortunate delay in higher alt supercharger speeds on the V-1710 though. 9.6:1 would have been VERY useful earlier on and having that along with the 8.8 blower to choose from would have made the situation much closer to the Merlin 45/50 series. (in terms of installing one or the other based on low/mid alt performance needs)

Aside from that the V-1710 had more economical cruise ability than the merlin and I think was a bit lighter and had a slightly smaller frontal area. (I know the single-stage Allison was lighter than the Merlin 20 series, not sure about the weight savings on the 45)

For that matter, I don't recall reading much of anything on overboosting V-1650-1 powered P-40s or Hurricane Mk.IIs. (low blower should have been fairly close to the low-alt Merlin 50 and high blower pretty much identical to the 45)

 

[Von Frag]

This might shed some light re. USN financing P&W to get a workable 2-stage engine to work:

I would like to know more about this P-40/R1830 test mule that could climb like that and hit 389 mph at 22,000 ft. Certainly it was stripped of armor and all uneccessary equipment. Why not use a P-36 airframe? It was designed for the 1830 from the start. Could they not fit the 2 stage SC onto it? Are there any photos or surviving test results?

 

[tomo pauk]

The P-40 with R-1830 is mentioned, for example, here, scroll down a bit. Picture has it as unarmed.

...
The cropped Merlin 50 series vs the 8.8 supercharged Allisons was a bit more favorable and available earlier (though official boost limit restrictions may have been raised later than on the Merlin).

The Merlins with cropped impeller (9.50 in, or same as V-1710) were indeed comparable with V-1710 that has 8.80:1 S/C gearing. Not all those V-1710 were created equal, though. The engines with 'regular' crankcase (prior early 1942) were good for ~1500 HP at 4500 ft on WER, once the reinforced crankcase was introduced the allowed boost was upped and power went to ~1590 HP at 2500 ft, WER. Those more powerful engines were contemporary with 'cropped' Merlins.
OTOH, the 'cropped' Merlin was maybe the 3rd category between available 1-stage Merlins, after 2-speed models and regular 45/46/47/50 marks.

We've already been over the unfortunate delay in higher alt supercharger speeds on the V-1710 though. 9.6:1 would have been VERY useful earlier on and having that along with the 8.8 blower to choose from would have made the situation much closer to the Merlin 45/50 series. (in terms of installing one or the other based on low/mid alt performance needs)

Indeed it was unfortunate. The 9.60:1 S/C on V-1710 means a P-40 that can do almost 380 mph, P-39 of circa 390, the P-51 above 400 mph.

Aside from that the V-1710 had more economical cruise ability than the merlin and I think was a bit lighter and had a slightly smaller frontal area. (I know the single-stage Allison was lighter than the Merlin 20 series, not sure about the weight savings on the 45)

'Vee's' list the Merlin 50M at 1385 lbs, and V-1710-39 at 1310 lbs.The same engine will have frontal area of 5.19 sq ft, the 2-speed Merlin is at 5.85; the 1-speed?
Smaller frontal area is good if the advantage can be taken of. If cockpit is wide enough for a pilot to comfortable sit in it and man the aircraft, going 'thinner than that is not going to yield any practical advantage. The lower consumption should pay off.

For that matter, I don't recall reading much of anything on overboosting V-1650-1 powered P-40s or Hurricane Mk.IIs. (low blower should have been fairly close to the low-alt Merlin 50 and high blower pretty much identical to the 45)

Not sure about V-1650-1. The Merlins in Hurricane II were overboosted up to +16 psi as war went on, here. The later 1-stage Merlins were using up to +18 psi.

 

[kool kitty89]

I would like to know more about this P-40/R1830 test mule that could climb like that and hit 389 mph at 22,000 ft. Certainly it was stripped of armor and all uneccessary equipment. Why not use a P-36 airframe? It was designed for the 1830 from the start. Could they not fit the 2 stage SC onto it? Are there any photos or surviving test results?

Mating the 2-stage R-1830 to the XP-42 style close-cowled fan-cooled installation (the final, satisfactory one similar to the Fw-190's, Tempest II, and XP-47J's) is something I've wondered about since first reading about the XP-42. Only real problem I can see is competition with the F4F for engines ... which already had shortages.

Hmm, though AS a navy fighter in place of the F4F entirely, yes that would seem like a much more capable aircraft but possibly have the issues of higher landing speeds due to wing loading.

The F2A with the same engine would have been interesting too but ... Brewster's mismanagement crippled the existing models as it was. (plus the R-1820-40 had relatively good altitude performance too, so the gain might not have been that dramatic)

 

[Shortround6]

Trouble with the R-1830 powered P-40 as Flown by P&W as a test mule was that it didn't set those performance figures until late supper/early fall of 1942. Nice achievement but a bit late to make a practical fighter. It took about 6 months to go from First flights of Merlin Mustang prototypes to start of production and in fact P-51Bs were ordered several weeks before prototypes flew (in either England or US).

They were still fooling around with cowls on the XP-42 at least until Dec of 1941 ( the plane was used for other research work later in the war) and maybe later.

The engine shortage for F4Fs was in 1941, it seems to have been resolved by early or summer of 1942.

Weight savings between Single speed Merlin and the Allison are too small to worry about. You could sometimes get a bigger change in weight by changing propellers. The engines rejected different amounts of heat to the coolant and oil systems and needed slightly different radiator/s and oil coolers. Dry weight doesn't include the starter system and that can change things also (not to mention generators, P-39s were pretty much an all electric airplane, electric propeller and may need bigger generator than a Plane using a hydraulic propeller. )

Right redesign a 'wet' wing to expand fuel. Shame they didn't catch on during the early prototype phase and put emphasis on that area. (especially in a modular fashion like the P39's fuel cells allowed) The P-59 had that problem too ... LOTS Of wing space for fuel, but only a moderate amount used. (granted a bit moot given it didn't develop into a useful combat ... or even recon aircraft)

The F2A actually comes to mind too for having the opposite problem. The poor thing started out with an exceptional fuel load for its size, but the designers seemed to put even more emphasis on fuel load over keeping weight down and climb performance up to the point of crippling it and exacerbating the build quality/manufacturing problems. (from 160 gallons on the F2A-1 to 240 gallons on the F2A-2/3 ... double the Wildcat's internal fuel load in spite of being a smaller, lighter built plane)

Integral tanks or "wet wings" had been a common feature (or at least not uncommon) in 1939/40 but quickly went away with the demand for self sealing tanks/fuel storage. You can't have both. P-43 tried and leaked fuel even without combat damage.

In fact that is part of the F2A story. Original 160 gallons was held in two tanks made up of the mains spars in the wing as front and back walls, wing skinning as top and bottom and solid ribs as tank ends. Trouble was trying to put self sealing liners in such a structure and/or repairing combat damage. Extra fuselage tank and wing leading edge tanks replaced one of the wing tanks normal capacity ( other tank was kept in use to simplify(?) fuel piping system, the reserve taps and overflow piping being connected to it. The other tank was sealed and could only be unsealed with the authorization of the squadron commander. poor climb was NOT the result of trying to carry 240 gallons of fuel. BTW the early F4Fs with unprotected tanks held the same 160 gallons as the early F2As.

 

[tomo pauk]

Problem with that P-40 with 2-stage R-1830 is that speed figures are without any military stuff on board. Once the guns, ammo, armor, military radio navigation systems are installed, much of the claimed speed will be lost. It is unlike that a resulting battle-worthy fighter will be faster than P-40F (Packard Merlin V-1650-1), that one was good for 360+ mph just above 20000 ft. Good for Asia/Pacific, bad for ETO MTO of 1941 and further.

 

[Shortround6]

Hmm, though AS a navy fighter in place of the F4F entirely, yes that would seem like a much more capable aircraft but possibly have the issues of higher landing speeds due to wing loading.

The F2A with the same engine would have been interesting too but ... Brewster's mismanagement crippled the existing models as it was. (plus the R-1820-40 had relatively good altitude performance too, so the gain might not have been that dramatic)

A P-36 as a Navy fighter instead of the F4F has a lot of problems. Stall speed is part of it. Another part is is the vertical impact speed at which the plane hits the deck. Higher on average than land planes. Early P-36s had problems with wing wrinkling/bending as it was. slamming them onto decks with arrestor landings was not going to turn out good.

Everybody wants to keep the P-36 weight as built but fails to realize that Curtiss ( and Brewster) Management did NOT add weight for the fun of it.
A Hawk 75 had a wing that weighed about 840lbs. I don't know what the weight of a P-36 wing was ( and you have the before and after modification weights). The 5th P-40 built had a wing that weighed 1003lbs (provision for but not including a single .30 cal in each wing), later P-40s had wings that weighed around 1120-1130lbs. How much was the change in structure due to the six .50 cal guns and how much was to beef up the wing so the plane was still stressed for an ultimate 12G load factor at over 8000lbs vs the the under 5700lbs of a Hawk 75 I don't know. The sales brochure for the Hawk 75 says the Cyclone powered Hawk was stressed for 12 Gs while the Twin Wasp powered model was stressed for 11.5Gs (it weighed 230lbs more).
However there is a line right below that states "The Airplane can be furnished at standard load factors at an increase in weight and price".
The Hawk 75 with Cyclone engine had a powerplant weight of 1985lbs. early P-40 had a power plant weight of about 2100lbs.
Hawk 75 had one .50 cal with 200 rounds and one .30 cal with 600 rounds.

Brewster was sort of in the same boat. First planes built were designed to have one .50 and one. 30. the wing guns were added in. The Early Planes also used not only and single speed supercharger but the engine did not use a reduction gear. Prop turned the same speed as engine crankshaft. Later Buffaloes not only got an engine with a reduction gear but got engines with more cooling fins and with steel crankcases instead of aluminium and other changes. They also got bigger, heavier props to handle the extra power. The Buffaloes wing went up by 172lbs. Landing gear got heavier, the better engine/powerplant went up by 547lbs. There is some dispute about the amount of ammo for the wing guns, .50 cal ammo weighs about 30lbs per hundred. changing from 200rpg to 400rpg for just the two wing guns is an increase of 120lbs. Changing the Fuselage guns from 200rpg to 500rpgs is another 180lbs.

 

[kool kitty89]

Integral tanks or "wet wings" had been a common feature (or at least not uncommon) in 1939/40 but quickly went away with the demand for self sealing tanks/fuel storage. You can't have both. P-43 tried and leaked fuel even without combat damage.

I meant more wing fuel cells ... the 'wet wing' term seems to get used loosely at times. (the P-47N is described as such)

In fact that is part of the F2A story. Original 160 gallons was held in two tanks made up of the mains spars in the wing as front and back walls, wing skinning as top and bottom and solid ribs as tank ends. Trouble was trying to put self sealing liners in such a structure and/or repairing combat damage. Extra fuselage tank and wing leading edge tanks replaced one of the wing tanks normal capacity ( other tank was kept in use to simplify(?) fuel piping system, the reserve taps and overflow piping being connected to it. The other tank was sealed and could only be unsealed with the authorization of the squadron commander. poor climb was NOT the result of trying to carry 240 gallons of fuel. BTW the early F4Fs with unprotected tanks held the same 160 gallons as the early F2As.

I was under the impression the F2A-2 had no self sealing and 240 gallons internal capacity while some of the export models and F2A-3 got additional/redesigned self-sealing tanks.

The F2A-3 was heavier still overall, but the F2A-2 has seen a major weight increase as well but seemed to focus more on long range than compromising to keep weight down. (that or integral modifications to the airframe wouldn't actually save much/any weight compared to simply limiting fuel and ammunition load)

Brewster was sort of in the same boat. First planes built were designed to have one .50 and one. 30. the wing guns were added in. The Early Planes also used not only and single speed supercharger but the engine did not use a reduction gear. Prop turned the same speed as engine crankshaft. Later Buffaloes not only got an engine with a reduction gear but got engines with more cooling fins and with steel crankcases instead of aluminium and other changes. They also got bigger, heavier props to handle the extra power. The Buffaloes wing went up by 172lbs. Landing gear got heavier, the better engine/powerplant went up by 547lbs. There is some dispute about the amount of ammo for the wing guns, .50 cal ammo weighs about 30lbs per hundred. changing from 200rpg to 400rpg for just the two wing guns is an increase of 120lbs. Changing the Fuselage guns from 200rpg to 500rpgs is another 180lbs.

Even with all that, the F2A-3 seemed to perform favortably compared to the similarly powered F4F and F4F-4 and had trade-offs with the F4F-3. (longer range vs poorer altitude performance vs weak bombload -the F4F certainly made for a better fighter-bomber)

Manufacturing volume and maintanence/reliability issues seem to be the real disadvantages compared to the F4F.

 

[Shortround6]

I was under the impression the F2A-2 had no self sealing and 240 gallons internal capacity while some of the export models and F2A-3 got additional/redesigned self-sealing tanks.

The F2A-2 had no self sealing and 160 gallons internal capacity. Some of the export models got rudimentary self-sealing tanks, Like treated leather coverings of the existing standard tanks. F2A-3 got the additional tanks which were self sealing.

The F2A-3 was heavier still overall, but the F2A-2 has seen a major weight increase as well but seemed to focus more on long range than compromising to keep weight down. (that or integral modifications to the airframe wouldn't actually save much/any weight compared to simply limiting fuel and ammunition load)

F2A-2 saw no increase in range over the F2A-1, what it saw was a change in engine which resulted in a 360lb increase in both empty and normal gross weight. However the Early F2A-2 was NOT combat capable as defined by US standards in the summer of 1941. The US Navy was also playing games with different load-outs ( 2 gun fighter, 4 gun over load fighter, 2 gun bomber, 4 gun overload bomber and zero gun ferry condition) which might have masked the real situation. Also masking the situation was some of the load/weight charts are for 660lbs of fuel (110gal) and not even the full 1080lbs (160 gal) capacity.

Not much to choose in bombload until the FM-2 shows up. F4F-3 adn F4F-4 usually rated for a pair of 100lb bombs.

Trying to back a little more on topic, what hurt the American fighters as much as "a less than the best supercharger design" was that they were heavy!!.

A MK V Spit was just under 6500lbs. A P-40C without drop tank went 7500lbs, A P-40E without tank went about 8300lbs and a P-39D-2 without tank could go 7700lbs.
A MK IX Spit was just under 7500lbs.

Changing the supercharger and picking up 100-150hp at altitude might not be enough to even things out. Close the gap somewhat certainly.

 

[kool kitty89]

The F2A-2 had no self sealing and 160 gallons internal capacity. Some of the export models got rudimentary self-sealing tanks, Like treated leather coverings of the existing standard tanks. F2A-3 got the additional tanks which were self sealing.

Thanks for the correction, I forget where I first saw the 240 gallon figure, I was thinking it was on Joe Baugher's page (which, granted, isn't always accurate either), but looking again, it's only the F2A-3 that he lists at 240 gallon maximum capacity. (which, as you mentioned, wouldn't have been a normal loadout at all)

Baugher's page also shows a moderate increase in range for the F2A-2 over the F2A-1, but that might not be comparing similar loadouts, or could be a consequence of the R-1820-40's fuel economy. (I recall mention to that model having particularly efficient cruise capabilities)

Not much to choose in bombload until the FM-2 shows up. F4F-3 adn F4F-4 usually rated for a pair of 100lb bombs.

Thanks again. Both the F2A-3 and F4F-3/4 had the 2x 100 lb bombload, and the F2A was never fitted for drop-tanks.

Trying to back a little more on topic, what hurt the American fighters as much as "a less than the best supercharger design" was that they were heavy!!.

A MK V Spit was just under 6500lbs. A P-40C without drop tank went 7500lbs, A P-40E without tank went about 8300lbs and a P-39D-2 without tank could go 7700lbs.

Yep, heavier, sometimes heavier armmed, and usually longer range aircraft. P-39 was the closest to its European counterparts, but still usually heavier aside from some stripped-down configruations. (but the same would apply to the P-40N and some field modifications made on earlier models)

 

[tomo pauk]

A bit on topic.
The 2-stage V-1710, like the V-1710-121 (military designation; F28R Alison designation - R denoting the rotation to the right) being test flown in early 1944 were comparable with V-1650-7 in altitude power. The military power was about 1100 HP at 25000 ft for both engines; the V-1650-3 was still a better hi-alt powerplant than either. For WER the F28R used water-methanol to help out (like almost all V-1710, it didn't have an inter-cooler ) managing 1750 HP at 9500 ft; 1500 HP at 15000 ft. When over-revved to 3200 rpm (that being one of rare thing where V-1710 excelled over Merlin, but it is questionable whether service engines ever used it legally), the WER went to 1680 HP at 17000 ft, and 1190 HP at 25000 ft. All values for the F28R are for climb, ie. with minimum ram available.
For comparison sake - V-1650-3 was good for 1300 HP at 26400 ft, so not that much better. Quirk being that V-1650-3 was available about a year earlier, the British 2-stage Merlin 61 about 2 years earlier.
The F28R is one of rare things where the 'Vee's for victory' got it wrong, claiming that power of 1700 HP on 3200 rpm was available at 26000 ft, or about 10000 ft higher than it is documented in this pdf. That power at altitude was maybe 'reachable' with late war big engines with 2-stage S/C and inter-cooler, like R-2800-18, RR Griffon or Jumo 213E.

 

[wuzak]

Maybe the V-1650-9 would be the US Merlin contemporary of the two stage V-1710?

Certainly the V-1650-3 was a high altitude version, but it was an earlier one, like the Merlin 63, without the strengthening that came with the later engines.

The V-1650-7 was the US equivalent of the Merlin 66 - a "low altitude" version.

 

[GregP]

Let's see ... low altitude Merlin 66 equivalent for the P-51D?

Best speed was 437 mph at 25,000 feet and service ceiling was 41,900 feet.

I don't think "low altitude" applies here.

Could be wrong ...

 

[Edgar Brooks]

The L.F. designation, on the IX, was due to the supercharger, of the Merlin 66, going over to high gear at 14,000', instead of the "normal" 21,000' of the other 60-series Merlins.

 

[wuzak]

Let's see ... low altitude Merlin 66 equivalent for the P-51D?

Best speed was 437 mph at 25,000 feet and service ceiling was 41,900 feet.

I don't think "low altitude" applies here.

Could be wrong ...

The L.F. designation, on the IX, was due to the supercharger, of the Merlin 66, going over to high gear at 14,000', instead of the "normal" 21,000' of the other 60-series Merlins.

As Edgar said.

Comaprison between the V-1650-7 and V-1650-9, as posted elsewhere.

Alt (ft)	V-1650-7 MAP	V-1650-7 hp	V-1650-9 MAP	V-1650-9 hp
0	67	1780	67	1503
4800	67	1730		1546
5000	66.5	1720	67	1548
10000	56.3	1470	67	1590
13800		1289	67	1622
15000	47.2	1232		1564
16000	67	1580		1515
19000	67	1500		1369
20000	65.2	1455	67	1320
25000	55.5	1225	67	1340
26700		1157	67	1347
30000	46.4	1025	59.4	1209
35000	37.7	855	49.4	1012
40000	29.8	700	40.6	830

As you can see, the V-1650-7 loses MAP from a relatively low altitude, then is back to 67inHg MAP from 16,000ft. The difference between the 14,000ft Edgar stipulated and the 16,000ft here may be due to the ram effect.

The FTH for the V-1650-7 appears to be 19,000ft.

 

[kool kitty89]

When over-revved to 3200 rpm (that being one of rare thing where V-1710 excelled over Merlin, but it is questionable whether service engines ever used it legally)

With over and unferreving changing supercharger speed, you'd need to account for added charge heating at higher RPM even if the engine itself could handle the strain perfectly well. Though that does bring up another aspect I'd forgotten to consider. Maybe it would be too fiddly to manage in service, but wouldn't rev-limiting also allow higher maximum boost at low alt on engines limited by charge heating?

The F28R is one of rare things where the 'Vee's for victory' got it wrong, claiming that power of 1700 HP on 3200 rpm was available at 26000 ft, or about 10000 ft higher than it is documented in this pdf. That power at altitude was maybe 'reachable' with late war big engines with 2-stage S/C and inter-cooler, like R-2800-18, RR Griffon or Jumo 213E.

1700 HP at 25,000 ft sounds more like a turbocharged V-1710.

 

[tomo pauk]

Maybe it would be too fiddly to manage in service, but wouldn't rev-limiting also allow higher maximum boost at low alt on engines limited by charge heating?

The 3200 rpm rating was allowed only when water injection was used, since indeed it would heat the charge too much. We might recall that DB 601A and 601N were allowed for over-revving only above critical height, probably to avoid overheating the charge.

Maybe the V-1650-9 would be the US Merlin contemporary of the two stage V-1710?

I don't have accurate data about when the V-1650-9 went into production, so it's a bit hard to compare it with a 2-stage V-1710.
However, sticking with production engines - the P-51H was 1st delivered in February 1945, so we can compare it with the engine from the P-63C, that fighter 1st delivered in December 1944. The V-1710 in question would then be the E21 (Allison designation; V-1710-117 military designation). It was good for 1100 HP at 25000 ft, military rating. Still less than V-1650-7 and V-1650-3, and much less than -9, that one making 1200 HP at 30000 ft on military rating.

The P-63E, 1st delivered in May 1945, finally managed to equal the V-1650-3 and -7 (those being old news by then), with 1100 HP at 28000 ft with V-1710-109 (E22; main distinction vs. the E21 being relocation of the carburetor between compressor stages, while allowed for 3200 rpm for WER). The V-1650-9 can use water injection, that makes above 1400 HP at 30000 ft, and above 1800 HP at 20000 ft. Vs. that, the E22 can use 3200 rpm and water injection, but still makes only 1340 HP at 27500 ft with ram - that would amount to ~23500 ft without ram, and won't cut it vs. the V-1650-9.

 

[tomo pauk]

As seen above, the Allison was managing for the 2-stage V-1710 to come out with ever better 'altitude power', so the supercharger development potential was there, even if it was running late vs. 2-stage Merlin or R-2800. The main obstacles for earlier development of 2-stage V-1710 was probably in the US Army fixation with turbos as means for hi-alt power, followed with quite a bit of resources poured in the hi-per engines the Army was trying to 'co-develop' with Continental and Lycoming.
Some of the things might get the finger pointed on the Allison, too - lack of inter-cooler on the production 2-stage engines; that, coupled with deletion of backfire screens that meant the engines used on the P-82E were in problems when greater boost was to be used.
Installing of the carburetor in the location between the S/C stages necessitated the use of 'speed density' fuel pump in the place of the carb if inter-cooler is to be used, as it was the case for the rare V-1710 F32R. That fuel pump needed more development, and the F32R was limited to only 54 in Hg during flying tests. In light of that, we don't know whether the XP-51J, the Mustang with the F32R, actually ever managed the claimed 491 mph. The max power was to be 1700 HP @ 3200 rpm at 20700 ft ('WER wet') or 1200 HP @ 3000 rpm at 30000 ft (military power).
Gruenhagen notes: The water injection system was not used on the XP-51J due to calibration problem with the speed density unit.; he lists the take off power 1500 HP @ 3200 rpm on 58 in Hg.

 

[GregP]

Hi Tomo,

3200 rpm was allowed only when equipped with a 12-counterweight crankshaft from the first G-series onward. None of the 6-counterweight engines were ever allowed 3200 rpm. At Reno, we've seen 3600 rpm on a stock warbird in the Bronze class for a couple of laps at the end and that was at the limit of the Curtiss Electric prop.

In tractors today in Europe, they are running them a 4800 rpm without damage ... but it's only for short tractor-pull runs. Nothing like a flight. Tractors rarely pull for more than 15 - 20 seconds. Still, that much overspeed without damage is indicative of VERY good primary balance coupled with strong rods and cranks. Some of them are not even runiing G-series rods!

 

[tomo pauk]

The V-1710-121 was run on 3200 rpm already in early 1944, I'll re-post the link to the XP-40 testing: pdf.