In '42 and '43 the possibility of replacing the F-5 and F-10 in the P-38E, F and G with a XX series 2-speed/single-stage Merlin, ie the V-1650-1, would have been attractive. The Allisons were not cleared to run at emergency powers of 1325 hp, and in the AFDU tests they weren't even being run at 1150 hp mil power. Eventually the Allisons were cleared on sort of a sliding scale of power ratings that put them ahead of the Merlin XX series depending on altitude. But for quite some time the Merlin XX would have offered better performance at lower altitudes, and roughly equivalent performance at higher (25,000 ft) altitudes. Except, the Allison was roughly 30-35% more fuel efficient than contemporary Merlins. 11% higher compression ratio and the exhaust driven turbos compressing the charge made a big difference in fuel economy. So you would have seen a substantial decrease in range compared to Allison engined Lightnings.
By the time the 2-speed/2-stage V-1650-3/7 would have been available the F-17/B-33 combo was also available for the P-38J. And there wouldn't have been much if any advantage for the Merlin over that Allison/turbo combination. The F-17/B-33 put out more power at ALL altitudes than the -3 could manage, and the -7 only had a very slight 100-120 hp or so advantage in the 5000-7000 ft range, other than that the F-17/B-33 outperformed it at all altitudes. Lockheed had some performance estimates showing the Merlin giving slight speed improvements at altitude, but that is probably more due to the higher reduction gear ratio and lower prop tip speed than any superiority of the Merlin. And Lockheed based their calculations on using the original engine cowlings without the draggier beard radiators the -J/L used, I don't know if they appreciated the fact that even the aftercooled Merlins would have required more heat exchanger capacity than the B-33 intercooler used. I suspect in practice they would have had to go with the beard radiator as well, and there wouldn't have been any drag advantage for the Merlin engined aircraft. And again, the Merlin is going to decrease the aircraft range by 30%+ due to the Allison's better fuel economy. All in all not a real attractive change, especially compared to the K which could have been available at the same time.
Greg Shaw
