WW2 without V-1710: options for the Allies?

A P-51-sized airframe might be too much for the R-1830 (P-66 being a better choice?) - the R-2800 being much better alternative? The main shortcoming being the combat radius?

Another alternative might be the Mustang's prototype flying on British-supplied Merlin (III/XII/X/XX)?

I didn't mean to suggest a R-1830 powered P-51, but something superior to other R-1830 powered fighters.
Something non-naval (i.e., light) and really clean (ala P-51).

PS: This would be something to supercede the P-36.

Then it's the P-66? It was about as fast as the P-40, despite the radial engine of modest power, ie. the R-1830 with single stage supercharger. Maybe we should ditch the fuselage MGs so the installation of the 2 stage R-1830 is easier, while substituting the 4 wing LMGs with HMGs?

A P-51-sized airframe might be too much for the R-1830 (P-66 being a better choice?) - the R-2800 being much better alternative? The main shortcoming being the combat radius?

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Main problem is the R-2800 is way too late in timing.

By the end of 1942 the US built over 11,000 Allison powered fighters, almost 1700 of them P-38s, about 1900 R-1830 powered fighters and 720 R-2800 fighters.

Even trying to use the singe stage bomber/transport R-2800 engines and stopping all B-26 production is going to leave you way short of engines until late in 1942.

I might point out that some of the possible Allison alternative engines weren't necessarily failures ... they weren't proceeded with.

Had the Allison failed, the desired to peoceed with them might have been of somewhat higher priority.

We have been over the possible (or at least the probable) Allison alternatives. Alternatives being liquid cooled V-12s, either upright or inverted (or flat). of around 1200-1600lbs. 2200-2400lb H-24s are NOT Allison alternatives in the sense that you could re-engine a P-36/P-40 with them. Since P-40s made up about 43% of the fighters built by Jan 1st 1943 coming up with different airframes to take bigger engines would seriously impact fighter production in 1941-42 and 43.

The Lycoming was wasn't "proceeded with", why? It didn't offer enough power without a turbo-charger ( and barely enough with one), it was longer in the power section (basic block/crankcase) than an Allison and it weighed just as much. Before we start talking about "development" consider that the Allison at 3000rpm will flow 22% more air per minute than the Lycoming will at 3400rpm. Assuming equal manifold pressures and equal breathing through the valves and manifolds it means the Lycoming needs a LOT of "development" to equal the Allison. The smaller cylinders do mean it can use a bit more boost on the same fuel but that 22% airflow gap may be too big to get around. If an Allison with 1150hp at 12,000ft wasn't considered quite good enough then 950hp at 15,000ft (about 100hp less than a Merlin III) gets you what for performance?

The Continental ***-1430 went through SIXTEEN different Army type numbers/models before it was not "proceeded with" how many were paper studies I don't know but the last one was a turbo-compound with the turbo-charger geared to the crankshaft so the turbine could add power to the prop. Some models had two speed prop drives or counter-rotating propellers. Since this was truly the Army's "baby" they didn't give up on it for quite some time. Even the simple versions (single prop, single prop speed, no turbo,etc) went 100-200lbs more than an Allison and were about 20in longer. Most models were rated at either 1250 or 1350 hp for take-off. Maybe if they spent a lot less time with the "tricks, bells and whistles" they might have actually got a workable engine.

After that the field changes from "probable" to "possible". Ford, even if they pull a miracle, won't have a production engine in time for most of 1942. That kind of leaves Packard trying to build a modernized version of the V-2500 or air version of the Torpedo boat engine.

Unless I am missing something that is about it for US designs that can "substitute" for the Allison. The other Liquid cooled engines were all more like Liquid cooled substitutes for the R-2600, R-2800 and R-3350.

Yes, you ARE missing something. The Allison didn't fail and WAS ordered. If it HAD failed, another alternate engine would have been developed and proceeded with. It NEVER happened, so I don't know which one would have been developed and neither do you ... but one would have beeen developed.

GregP said:

Tes, you ARE missing something. The Allison didn;t fail and WAS ordered. If it HAD failed, another alternate engine would have been developed and proceeded with. It NEVER happened, so I don't know which one would have been developed and neither do you ... but one would have beeen developed.

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Greg, the ones most likely (the IV-1430 and O-1230) didn't stop their development because the Allison was a success. The IV-1430's development continued well into the war, long after the Allison was being churned out by the thousand. The O-1230 was deemed to be too small and not have enough power, so it was doubled up as the H-2470 and continued for a few more years.

The US government built a factory in which to build the IV-1430. The IV-1430 was to built as well as the V-1710, not just as a backup.

They only made 23 ... I say it was a "backup" engine that did not get proceeded with after the Allison had success ... but COULD have worked out just fine. I say that because development was stopped, somethinhg that might NOT have happened if the Allison had failed.

One can only wonder what engine they would have developed had the Allison failed. I wouldn';t choose one from the crowd myself ... the desisions at the time were just too policitical to predict. Purportedly they ignored warning of the attack on Pearl Harbor just to get us into the war! How crazy is that?

I guarantee it would have been SOME U.S. engine, not the Merlin, just due to attitudes at the time ... not due to the Merlin, which was and is a great engine among successful WWII engines. If we could go back with hindsight, we probably WOULD build the Merlin into our fighters ... but not at the time in any great numbers.

GregP said:

They only made 23 ... I say it was a "backup" engine that did not get proceeded with after the Allison had success ... but COULD have worked out just fine.

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They gave up on the IV-1430 in 1944. They started in 1932.

Basically because it did not perform as advertised.

GregP said:

I guarantee it would have been SOME U.S. engine, not the Merlin, just due to attitudes at the time ... not due to the Merlin, which was and is a great engine among successful WWII engines. If we could go back with hindsight, we probably WOULD build the Merlin into our fighters ... but not at the time in any great numbers.

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As stated previously, it depends on timing.

If the Allison is lost late in the piece, a replacement would probably be needed quickly. The quickest way was to build the Merlin in its stead. Pre war it is needed to produce P-40s for the British and French (and the Mustang I). The IV-1430 wasn't ready around that stage, so couldn't step into the breech.

GregP said:

Yes, you ARE missing something. The Allison didn't fail and WAS ordered. If it HAD failed, another alternate engine would have been developed and proceeded with. It NEVER happened, so I don't know which one would have been developed and neither do you ... but one would have beeen developed.

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I don't believe I am missing anything. IF the Army had chose to pursue either of the hyper engines it would have been pouring money, time and effort down a rat hole.

Look again at the O-1230 Lycoming and lets assume you can bend it into a 60 degree V with little trouble. Now compare it to the R-R Peregrine. The Peregrine was 1296 cu in instead of 1234 or 5% larger. The Peregrine was 200lbs lighter and a whole lot shorter. Peregrine could make 885hp at 15,000ft. without turbos or extra supercharger stages.
The Lycoming would turn 3400rpm which helped but basically the USAAC would have trying to turn a Peregrine into a Merlin. It was too small to do the job. No amount of flag waving can change that.

The Army screwed up several times in the initial development of these engines. That is with the benefit of hindsight.
One was not funding a 12 cylinder version for way too long. One and two cylinder test rigs can tell you a lot about the combustion and breathing a of a cylinder design, they tell you nothing about vibration problems. No Allison may mean the Army builds a 12 Cylinder version sooner, depending on when the Allison fails in this scenario.
Two was insisting on a 300 degree coolant temperature for too long. Good in theory, it was supposed to keep radiator size down but in practice it brought very diminishing returns over 250 degree coolant temp in actual heat transfer and shifted some of the cooling load to the oil system requiring larger oil coolers so there was NO net gain.
Three, and probably the most important was the Army's insistence on separate cylinders like a WW I Mercedes or Liberty engine. Granted they did use a one piece head across the cylinders but this form of construction makes for wider cylinder spacing which means the engine is longer than the equivalent enbloc engine. The extra length means more weight. The extra length of the crankshaft means more problems with torsional vibration ( the cylinders further from the load/prop twist the crankshaft more when they fire and then when the cylinder/s go from power to exhaust the stress is removed) than a short crank. The assembly is weaker than the cylinder block style engine which limits the increase in power without more beefing up (weight).

Now there is nothing to prevent the Army and Continental from taking the cylinder dimensions, piston, con rods, valves/valve train and spark plug placement and designing an enbloc engine that is shorter and lighter using those features except time and money.
That and somebody slapping the Army project officers up side the head to STOP all the silly extra features until they get the basic engine sorted out. 1350-1600 hp engines don't need two speed propeller drives, or the choice of two different gear ratios without extra parts depending on how they are assembled, or some of the other "stuff" they played with while the basic engine didn't work.

The engine construction was a fundamental flaw that would have bitten them in the butt even if they had gotten the original engines to work as advertised. It is harder to increase the power when the better fuels become available because you are dealing with a weaker engine/crankshaft assembly.

And as Wuzak has noted, the I-1430 was NOT dropped and the Army was signing contracts for prototype fighters using the I-1430 in the summer of 1941.

Your faith is touching but let us also remember that P W thought they could get the R-4360 into production quicker than ANY of the liquid cooled engines they were working on. A mistake in judgment or was it actually true?

And several projects that "looked" easy took a long time to get into operation, like the R-4360. Just take the cylinders from a R-2800 and put 7 on each crank throw and then use 4 crank throws. Or the Wright R-3350, just take an R-2600 and add two cylinders to each row, from 7 to 9 and Wright was already making 9 cylinder engines. We know how long that took to sort out.

Shortround6 said:

Again, we have a moving "target" or moving "base line".

In the Spring of 1939 the XP-40 demonstrated 22% less drag than the P-36 and since the XP-40 was the 10th production P-36 airframe we can assume the differences aside from the engine were pretty minimal. By late 1942 P&W had gotten the drag difference down to about 8% on the factory test mule pictured earlier. And/or done a better job of using exhaust thrust? Better radial installations were done but they come even later.

What kind of radial installation can you get (reasonably expect) in 1940-41 for the R-1830? somewhere in between 8 and 22% more drag than the Allison? assuming that the engine/radiator installations on the XP-40 and Allison P-51s were equal. If the P-51 installation is better that leaves the radial with further to catch up. An R-1830 powered Mustang may be a much faster plane than a P-36 but that does NOT mean it was up to world standard in 1942.

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The Mustang easily had the lowest zero-lift drag coefficient of any propeller fighter to see service in WW2, with a value of about 0.017 (I've seen values from 0.0163 to 0.0176). Most fighters were between about 0.021 and about 0.025 and, except for the Mustang, there was little to chose between air-cooled and liquid-cooled engines. Cooling drag is a major issue on all piston-engined aircraft, and it's one which is remarkably easy to get wrong, whether the engine is air-cooled directly, like an R-1830, or indirectly, like a Merlin. NAA did a remarkable job on the Mustang's aerodynamics; it had a zero-lift drag coefficient that was no less than 15% better than any other piston-engined fighter to see service in WW2, and most of that was probably due to excellent cooling system design.

Back to the Allison engine. As as been mentioned several times, the timing of the V-1710 failure is crucial. If the engine is a total failure in 1937, there's no chance to get a domestic replacement before about 1941; to be useful, the alternative engine would have to be started no later than 1933. The only way for that to happen would be for the Army or Navy to fund an engine with the same external dimensions, weight, and prop shaft location and rpm in about 1930. If it happens later, say after the UK shops around for US plants to license build Merlins, the answer is to use the Merlin.

swampyankee said:

The Mustang easily had the lowest zero-lift drag coefficient of any propeller fighter to see service in WW2, with a value of about 0.017 (I've seen values from 0.0163 to 0.0176). Most fighters were between about 0.021 and about 0.025 and, except for the Mustang, there was little to chose between air-cooled and liquid-cooled engines. Cooling drag is a major issue on all piston-engined aircraft, and it's one which is remarkably easy to get wrong, whether the engine is air-cooled directly, like an R-1830, or indirectly, like a Merlin. NAA did a remarkable job on the Mustang's aerodynamics; it had a zero-lift drag coefficient that was no less than 15% better than any other piston-engined fighter to see service in WW2, and most of that was probably due to excellent cooling system design.

While the cooling system design was the best or near best of the war, the wing was the dominant Drag reduction contributor..


Back to the Allison engine. As as been mentioned several times, the timing of the V-1710 failure is crucial. If the engine is a total failure in 1937, there's no chance to get a domestic replacement before about 1941; to be useful, the alternative engine would have to be started no later than 1933. The only way for that to happen would be for the Army or Navy to fund an engine with the same external dimensions, weight, and prop shaft location and rpm in about 1930. If it happens later, say after the UK shops around for US plants to license build Merlins, the answer is to use the Merlin.

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Horkey also believed the second order curve of the upper and lower cowl lines were significant.

I do agree your 'range' of CDo as long as we recognize that when calculating actual CDo based on high speed flight tests that Sea Level speed range is less influenced by compressibility drag contribution than at 25-29000 feet. It is interesting that the P-51B-1 flight tests noted the probability that the prop tips were operating near or at supersonic velocities adding another component skewing CDo calcs.

The other complications (calculating CDo) include the constant for propeller efficiency, the percent you choose for added contribution of exhaust thrust to total thrust.. IIRC, NAA used 12% for Thrust contribution and .85 efficiency for the four blade 11ft-6" diameter prop and .479:1 Gear Reduction ratio. When using that as the base set of factors, I get .0168 for CDo without racks and .0173 with racks...

Some surprisingly small changes can also have significant effect on drag. I've read that the matte black paint used on some Mosquito night fighters cost close to 20 mph in top speed.

Romulan cloaking device much lighter.

When I started the Merlin thread, it occurred to me the main difference between Allison and Rolls-Royce was the number of products they were developing.

In the mid to late 1930s Allison was developing the V-1710 and the X-3420/V-3420, which was heavily based on the V-1710.

Rolls-Royce, on the other hand, were developing the Kestrel (late in its development), Peregrine, Exe, Vulture, Merlin, Griffon and Crecy. None of which shared much of anything with the others.

Allison was a much smaller company, While they had some work converting Liberties to air cooled engines, they specialized in bearings and gear sets/remote drives. They had built ALL the reduction gears for Packard aircraft engines in the 20s for one example. The also built supercharger parts under sub-contract to General Electric. Until 1939 the V-1710 was a sideline.

Shortround6 said:

Allison was a much smaller company, While they had some work converting Liberties to air cooled engines, they specialized in bearings and gear sets/remote drives. They had built ALL the reduction gears for Packard aircraft engines in the 20s for one example. The also built supercharger parts under sub-contract to General Electric. Until 1939 the V-1710 was a sideline.

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Very good point SR.

I have been taken to task for stating in this thread that an alternative American engine would be produced if the Allison had failed. People have said that using the Merlin would be the choice that would have been made.

Let's take a look at that from a factual rather than emotional or logical basis. In the lists below, which are incomplete, I show 187 fighters that either tried out for service or were acutally procured, running form 1920 into WWII. I didn't even include the US Navy monoplane fighters or the Bell P-63 series.

Of these 187 real planes, not including the P-51 for reasons stated above, only 3 had foreign engines and only 2 were bought ... the P-40F and L.

ALL the rest had US-designed engines. Let's see, I have a bowl with 187 white pills in it and 3 red pills. Reach in and draw one out blind. How many would bet they had a red pill?

Facts like this are why I said the US would develop its own alternate engine if the Allison V-1710 had failed. It hasn't got naything to do with whether or not the Merlinw as a good engine, it has to do with FACTS. If I went ahead and added the Bell P-63 and the US Navy monop[lanes, the list would likey be 250 planes or more with 3 have foreign engines.

So, you guys who think we would design planes with Merlins, on what basis in historical fact would you believe that? If you answer, you might include all the foreign engine piston fighters other than the P-51 series. The list will be VERY short I think.



US Army Pursuits from 1920 – 1932:

1. Vermille VCP-1, Wright-Hispano H in prototype, re-engined with a Packard 1A-2025 V-12.
2. Dayton-Wright XPS-1, Lawrence -1 radial.
3. Curtiss PN-1, Liberty 8, L-825.
4. Curtiss PN-1A, Wright R-1 (R-1454).
5. Engineering Division PW-1, Packard 1A-1237.
6. Engineering Division PW-1A, Packard 1A-1237.
7. Loening PW-2, Wright H.
8. Loening PW-2A, Wright H.
9. Loening PW-2B, Packard 1A-1237.
10. Orenco PW-3, Wright H.
11. Gallaudet PW-4, Packard 1A-1237.
12. Fokker V-40, Wright H.
13. Fokker PW-6, Wright H-2.
14. Fokker PW-5, Wright H-2.
15. Curtiss XPW-8, CurtissD-12.
16. Curtiss PW-8, -8A, -8B, Curtiss D-12.
17. Boeing XPW-9, Curtiss D-12.
18. Boeing PW-9, Curtiss D-12.
19. Engineering Division TP-1, Liberty 12.
20. Thomas-Morse TM-23, Curtiss D-12.
21. Thomas-Morse TM-24, Curtiss D-12.
22. Curtiss P-1, Curtiss V-1150.
23. Curtiss P-1A, Curtiss V-1150.
24. Curtiss P-1B, Curtiss V-1150.
25. Curtiss P-1C, Curtiss V-1150.
26. Curtiss P-2, Curtiss V-1400.
27. Curtiss XP-3A, Pratt Whitney R-1340.
28. Curtiss P-3A, Pratt Whitney R-1340.
29. Curtiss XP-4, Packard 1A-1500.
30. Curtiss P-5, Curtiss V-1150.
31. Curtiss XP-6, Curtiss V-1570.
32. Curtiss P-6, Curtiss V-1570.
33. Curtiss P-6A, Curtiss V-1570.
34. Curtiss P-6B, Curtiss V-1570.
35. Curtiss P-6D, Curtiss V-1570.
36. Curtiss Hawk I, Curtiss V-1570.
37. Curtiss Hawk P-6S (Cuba), Pratt Whitney R-1340.
38. Curtiss XP-7, Curtiss V-1570.
39. Boeing XP-8, Packard 2A-1530.
40. Boeing XP-9, Curtiss V-1570.
41. Boeing P-12, Pratt Whitney R-1340.
42. Boeing P-12A, Pratt Whitney R-1340.
43. Boeing P-12B, Pratt Whitney R-1340.
44. Boeing Model 100, Pratt Whitney SR-1340D.
45. Boeing P-12C, Pratt Whitney R-1340.
46. Boeing P-12D, Pratt Whitney R-1340.
47. Boeing XP-15, Pratt Whitney SR-1340D.
48. Boeing 218, Pratt Whitney SR-1340C.
49. Boeing P-12E, Pratt Whitney R-1340.
50. Boeing P-12F, Pratt Whitney R-1340.
51. Boeing XP-925A, Pratt Whitney SR-1340F.
52. Thomas-Morse XP-13, Curtiss H-1640.
53. Thomas-Morse XP-13A, Pratt Whitney SR-1340C.
54. Curtiss XP-17, Wright V-1460.
55. Curtiss YP-20, Wright R-1820.
56. Curtiss XP-22, Curtiss V-1570.
57. Curtiss XP-6E, Curtiss V-1570.
58. Curtiss P-6E, Curtiss V-1570.
59. Curtiss P-6F, Curtiss V-1570.
60. Curtiss XP-6H, Curtiss V-1570.
61. Curtiss XP-23, Curtiss V-1570.
62. Berliner-Joyce XP-16, Curtiss V-1570.
63. Berliner-Joyce Y1P-16, Curtiss V-1570.

So far, from 1920 – 1932, we have 63 airplanes trying out for the US Army and 63 US-designed engines. What about the US Navy?

1. Boeing FB-1, Curtiss D-12.
2. Boeing FB-3. Packard 1A-1500.
3. Boeing FB-4, Wright P-2.
4. Boeing FB-5, Packard 2A-1500.
5. Curtiss F6C-1. Curtiss D-12.
6. Curtiss F6C-2, Curtiss D-12.
7. Curtiss F6C-3, Curtiss D-12.
8. Bought FU-1, Wright R-790.
9. Wright XF3W-1, Pratt Whitney R-1340B.
10. Curtiss F6C-4, Pratt Whitney R-1340.
11. Curtiss XF6C-5, Pratt Whitney R-1690.
12. Curtiss F7C-1, Pratt Whitney R-1340B.
13. Eberhart FG-1, Pratt Whitney R-1340C.
14. Hall XFH-1, Pratt Whitney R-1340B.
15. Boeing XF2B-1, Pratt Whitney R-1340.
16. Boeing F2B-1, Pratt Whitney R-1340.
17. Curtiss F8C-1, Pratt Whitney R-1340.
18. Curtiss XF8C-2, Pratt Whitney R-1340.
19. Curtiss F8C-4, Pratt Whitney R-1340.
20. Curtiss XF9C-7, Wright R-1820.
21. Vought XF2U-1, Pratt Whitney R-1340C.
22. Bristol Bulldog, Bristol Jupiter VII (US Navy tested 1 aircraft).
23. Berliner-Joyce XFJ-1, Pratt Whitney R-1340C.
24. Berliner-Joyce XFJ-2, Pratt Whitney R-1340.
25. Fokker XFA-1, Pratt Whitney R-1340C.
26. Curtiss XF9C-1, Wright R-975C.
27. Curtiss F9C-2, Wright R-975.
28. Boeing XF4B-1, Pratt Whitney R-1340.
29. Boeing F4B-1, Pratt Whitney R-1340.
30. Boeing F4B-2, Pratt Whitney R-1340.
31. Boeing F4B-3, Pratt Whitney R-1340.
32. Boeing F4B-4, Pratt Whitney R-1340.

That's 32 Navy planes with one foreign test. So far, we have 95 airplanes with 1 foreign engine that was a demo test from Bristol. What about WWII monoplane fighters? Exclusive of the P-51 series … OK …

1. Lockheed YP-24, Curtiss V-1570.
2. Consolidated Y1P-25, Curtiss V-1570.
3. Boeing Y1P-26, Pratt Whitney R-1340.
4. Boeing P-26A, Pratt Whitney R-1340.
5. Boeing 281, Pratt Whitney R-1340.
6. Curtiss XP-934, Pratt Whitney R-1340.
7. Curtiss XP-31, Curtiss V-1570.
8. Boeing YP-29, Pratt Whitney R-1340.
9. Boeing YP-29A, Pratt Whitney R-1340.
10. Consolidated P-30, Curtiss V-1570.
11. Consolidated PB-2A, Curtiss V-1570.
12. Northrop 3A, Pratt Whitney SR-1535.
13. Vought V-141, Pratt Whitney R-1535.
14. Vought V-143, Pratt Whitney R-1535.
15. Seversky SEV-3M-WW, Wright R-975.
16. Seversky SEV-1, Wright R-1820.
17. Seversky P-35, Pratt Whitney R-1830.
18. Seversky 2PA-A, Wright R-1820.
19. Seversky 2PA-L, Wright R-1820.
20. Republic P-35A, Pratt Whitney R-1830.
21. Republ;ic Guardsman (AT-12), Pratt Whitney R-1830.
22. Curtiss design 75, Pratt Whitney R-1535.
23. Curtiss design 75B, Wright XR-1820.
24. Curtiss YP-36, Pratt Whitney R-1830.
25. Curtiss P-3A, Pratt Whitney R-1830.
26. Curtiss P-36C, Pratt Whitney R-1830.
27. Curtiss P-36F, Pratt Whitney R-1830.
28. Curtiss XP-37, Allison V-1710.
29. Curtiss YP-37, Allison V-1710.
30. Curtiss 75R, Pratt Whitney R-1830.
31. Curtiss XP-42, Pratt Whitney R-1830.
32. Curtiss Hawk 75, Wright GR-1820.
33. Curtiss H-75Q, Wright CR-1820.
34. Curtiss Hawk 75A-4, Wright R-1820.
35. Bell XFM-1, Allison V-1710.
36. Bell YFM-1, Allison V-1710.
37. Bell YFM-1A, Allison V-1710.
38. Bell YFM-1B, Allison V-1710.
39. Curtiss-Wright CW-21, Wright R-1820.
40. Curtiss-Wright CW-21B, Wright R-1820.
41. North American P-64, Wright R-1820.
42. Vultee Vanguard prototype, Pratt Whitney R-1830.
43. Vultee P-66, Pratt Whitney R-1830.
44. Curtiss XP-40, Allison V-1710.
45. Curtiss P-40, Allison V-1710.
46. Curtiss P-40B, Allison V-1710.
47. Curtiss P-40C, Allison V-1710.
48. Curtiss XP-46, Allison V-1710.
49. Curtiss P-40D, Allison V-1710.
50. Curtiss P-40E, Allison V-1710.
51. Curtiss P-40F, Packard Merlin V-1650.
52. Curtiss P-40K, Allison V-1710.
53. Curtiss P-40L, Packard Merlin V-1650.
54. Curtiss P-40M, Allison V-1710.
55. Curtiss P-40N, Allison V-1710.
56. Curtiss XP-40Q. Allison V-1710.
57. Lockheed XP-38, Allison V-1710.
58. Lockheed YP-38, Allison V-1710.
59. Lockheed P-38, Allison V-1710.
60. Lockheed P-38D, Allison V-1710.
61. Lockheed P-38F, Allison V-1710.
62. Lockheed P-38G, Allison V-1710.
63. Lockheed 322, Allison V-1710.
64. Lockheed P-38H, Allison V-1710.
65. Lockheed P-38J, Allison V-1710.
66. Lockheed P-38L, Allison V-1710.
67. Lockheed P-38M, Allison V-1710.
68. Bell XP-39, Allison V-1710.
69. Bell XP-39B, Allison V-1710.
70. Bell YP-39, Allison V-1710.
71. Bell P-39C, Allison V-1710.
72. Bell P-39D, Allison V-1710.
73. Bell P-39F, Allison V-1710.
74. Bell P-39J, Allison V-1710.
75. Bell P-400, Allison V-1710.
76. Bell XP-39E, Allison V-1710.
77. Bell P-39K, Allison V-1710.
78. Bell P-39M, Allison V-1710.
79. Bell P-39N, Allison V-1710.
80. Bell P-39Q, Allison V-1710.
81. Seversky XP-41, Pratt Whitney R-1830.
82. Republic P-43, Pratt Whitney R-1830.
83. Republic P-43A, Pratt Whitney R-1830.
84. Republic P-47B, Pratt Whitney XR-2800.
85. Republic P-47B, Pratt Whitney R-2800.
86. Republic P-47C, Pratt Whitney R-2800.
87. Republic P-47D, Pratt Whitney R-2800.
88. Republic XP-47H, Chrysler XI-2220.
89. Republic XP-47J, Pratt Whitney R-2800.
90. Republic XP-47K, Pratt Whitney R-2800.
91. Republic P-47M, Pratt Whitney R-2800.
92. Republic P-47N, Pratt Whitney R-2800.

So, here is a list of 92 monoplane fighters of which 2 had Merlins, the P-40F and L. So far, we have 187 planes that either were bought by the USA or tried out for service. 3 had foreign engines and one of those was a Bristol demo unit. I haven't added the P-63 series or the Navy monoplane fighters, but they run much the same.

GregP said:

Yes, you ARE missing something. The Allison didn't fail and WAS ordered. If it HAD failed, another alternate engine would have been developed and proceeded with. It NEVER happened, so I don't know which one would have been developed and neither do you ... but one would have beeen developed.

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The OP was positing the hypothetical where the V-1710 failed. While I think there would have been hints of its failure long before 1937 or 1938 -- the development started as early as 1929 -- it's an equally legitimate question as "what would have happened had the USAAC given priority to development of mechanically, vs exhaust gas, driven superchargers?"

Had the Allison been seen as a failure early enough, there would be an alternative funded; if it's too late, the USAAC doesn't have a domestic V-12 alternative. I think the USAAC would have had to made this sort of decision no later than late 1933 to get an engine developed in time to be available for aircraft such as the P-40.