2 speed 2 stage supercharging, like the Merlin 60 series or the R-1830-76/-86 in the F4F-3 and 4. Merlin 20 were 2 speed 1 stage, as was the R-1830-90 in the F4F-3A. The V-1710 1 speed 1 stage, relying on turbo supercharging for altitude performance, the design decisions made early made it hard to change the engine driven supercharging.
War Production Board report,
V-1650 1 Stage August 1941 to December 1944 (no production in 1945), 49, 7,246, 9,503, 7,255.
V-1650 2-stage, November 1942 to December 1945, by year 5, 2,792, 15,736, 13,346
V-1710 2-stage, December 1942 to December 1945, by year 1, 514, 2,867, 1,645, main user the P-63, which developed its best speeds above 20,000 feet.
After building 112 engines May to July 1943 V-1710 2-stage production dropped to 2 then 1 in August and September, rebuilt to over 300 engines a month in Q1/44 then back to 11 then 12 in August and September 1944, another rebuild to over 300 a month mid 1945, when V-1710 production was phased out in favour of the jets, mostly the J-33.
The USN reports 10 V-1710 built in 1946, then 603 in 1947 and 145 in 1948.
The US built Merlin 28, 29 and 31 in 1941/43, switching to Merlin 33 and 38 in 1943, with the first 2 speed 2 stage engines for Britain in November 1943. All single stage engine production was for Britain apart from 2,813 engines August 1941 to March 1943. Almost all 2 stage production was for the US, the main exception was the Merlin 266 for the Spitfire XVI. There was nothing stopping the US making more Merlins if the requirement was there. To end 1943 Packard Merlin for Britain were 5,200 Merlin 28, 480 Merlin 29, 560 Merlin 31, 880 Merlin 33 and 6,862 Merlin 38. Merlin 31 production ended in February 1943, Merlin 33 mostly April to July 1943. Packard had built Merlin production to around or over 800 a month from July 1942, dropped it to around 600 a month in March and April 1943 as the M28, M31, and V-1650-1 went out of production and the M33, M38 and 2 stage versions began, production in May was 1,222, production in the second half of 1943 was about 50% more that of the second half of 1942. By end 1943, ignoring the need for spares, there were enough M31 and M33 for actual Canadian and Australian Mosquito production until into 1945.
Canadian built Hurricanes arriving in Britain had any US Merlins removed, Canadian built Lancaster and Mosquito kept their US Merlins. The RAF restricted use of British and Canadian built aircraft with US Merlins to Britain and North West Europe.
The P-40 was the USAAF interim design, the best way to obtain a better performing fighter than the P-35 and P-36 in numbers quickly. To be replaced by the turbo P-38/39/43/47 "soon".
The RAF spent most of 1940 to 1942 trying to plug gaps, firstly at home, then in the Middle East, then USSR then Far East, as well as greater needs for anti submarine operations. That meant a dilution in quality of aircraft and aircrew to make up numbers. On top of that it had little ability to force Luftwaffe to fight by day. The amount of time the armies in the western desert fought divisional or larger sized operations is days in the 3 years of the campaign. As over France, the Luftwaffe could usually operate to the strengths of its fighters in good tactical situations while limiting exposure of other aircraft types.
As of end 1941 the USAAF was not expecting to keep large numbers of aircraft operational in the tropics, nor until late 1942 large numbers operating in North Africa, nor then the campaign to clear Tunisia would last as long and initially would have the axis air forces better able to mount operations and force the allied air forces into fights. Then after about mid 1943 having the need for the latest fighters in Italy substantially decrease as the Luftwaffe withdrew to defend Germany.
As of early 1943 the USAAF was looking at phasing out the P-39, using the P-40 as a fighter bomber and the P-47 as the air superiority fighter. The P-38 as twin engine air superiority and escort, the Merlin P-51, even if it lived up to the prototype results, was still more listed as headed for units like the 9th Air Force. As the 8th Air Force in particular proved the need for more long range fighters things changed, it also meant more theatres around the world would have to wait for the latest types until the 8th had enough, and the 9th for that matter given the expected timing and importance of Operation Overlord.
The P-51 airframe was obviously superior to the P-40 one, making the P-51 the better candidate for the better engines. While the Red Air Force preferred the P-39 to the P-40 and kept asking for them. When it comes to numbers 4,454 P-40 in 1942 and 4,258 in 1943, P-47 532 and 4,428, P-51 634 and 1,710, it made the P-47 a second half of 1943 and the P-51 a 1944 fighter in terms of arrival in numbers.
A problem for the British was ramping up 2 stage Merlin production, the Spitfire VIII was meant as the overseas service version, 795 in 1943 compared with 1,769 mark IX and 1,328 mark V. Fighter Command found itself taking a back seat for IX allocations in the first half of 1943 at least, the Australians received their first VIII in October 1943. Until around mid to end 1943 every front was seeking the latest Spitfires in numbers as an urgent requirement. The P-51 took over that position in 1944.
Remember the starting positions, the aviation industry was small in the 1930's, then had to compete with all the other priorities in the 1940's Canada increased airframe output about 100 times from mid/late 1930's to 1944 peak, the US 30 times. To help do this Canada largely avoided building engines and propellers.
First order for Canadian made Hurricanes in 1938, first production in February 1940. There was no combat worthy P-40 design in production until the Tomahawk II in October 1940, factor that into a Canadian P-40 time line, more sensible to create a second US line. In mid 1941 came an export order for 400 Hurricanes, to be sent across Pacific and having US engines, became what most references call the RCAF order, 50 more built as Sea Hurricanes. Also a mid 1941 idea was importing US engines for Hurricanes retained in Canada to equip fighter OTU. Canadian Hurricanes built for the RAF were incomplete airframes, requiring brakes, instruments etc. before issue.
In 1942/43 the 8th Air Force lost 737 heavy bombers to enemy a/c, which meant they probably shot down around 500 Luftwaffe fighters, the US fighters in theatre made 458 claims. It was 1944 when the US fighters took over the role of shooting down Luftwaffe aircraft.
The Hurricane and P-39 and P-40 and other types remained in production for so long thanks to the laws of mass production, the cost of creating and tearing down production lines, the forecasts of future needs, their usefulness in secondary roles and so on. In the Hurricane case add the problems with the Sabre engine and Typhoon airframe.
