Originally Posted by ian16th
Rather than why not replace a Merlin with a Griffon in the P-51, why not replace the Allison's with Merlin's in the P-38?
After all, the P-51 experience had shown that it worked!
This was proposed, but the Allison-powered P-51 had no turbo-supercharger, which is why the Merlin improved its mid-high-altitude performance,.
The P-38 had turbo-superchargers for its Allisons, which gave excellent high-altitude performance. Thus, any gains would be much smaller, and as the change-over would cause a significant disruption on production rate, the idea was dropped.
This article has a discussion of the proposal, phrased in terms of the benefits... but glossing over the problems (it only discusses performance
to 30,000 feet).
The P-38 Lightning
In 1940 Packard Motors of Detroit began building the two-speed Merlin V-1650-1 (Merlin 28) under license from Rolls Royce. This engine had 1170 horsepower in high blower with a critical altitude of 21,000 feet. Lockheed ran a study comparing a Merlin XX powered Lightning with a standard V-1710 powered variant. The reported speed difference was over 25 mph, favoring the Merlin powered airplane. Climb performance was similar to the Allison powered machine.
Another Merlin vs. Allison comparison in 1942 involved the V-1710-89/91 Allisons (engines used in standard P-38J) and the Packard V-1650-3 two-speed, two-stage Merlin used in the P-51B/C. Utilizing Military Power speed was almost identical.
Yet another study in 1944 compared V-1710s producing 1725 bhp and "advanced" Merlins using "special" fuel and producing 2000 bhp (no altitude specified). The Merlin powered version could supposedly attain 468 mph at 30,000 feet, which was considerably better than the Allison powered version.
These studies were all conducted by Lockheed and exhibit a certain amount of optimism in regard to maximum speed for both types, but the consensus clearly shows better performance with the Merlin powered Lightning.
Note however, that the same article earlier says that the factory-set maximum manifold pressure ratings for the Allison engines were rarely followed by front-line units. They used significantly-higher manifold pressure settings (up to 60" vs the factory 40"-45"!
As a result, combat units regularly achieved higher climb rates and maximum speeds than the factory did. Thus the above Lockheed factory comparison short-changes the performance the Allison was capable of.
This article mentions one of the areas where performance would have dropped with the Merlin (it discusses performance
above 30,000 feet):
Whatever Happened To The P-38K ?
There were some performance areas that would suffer. While a gain in speed at medium altitudes was expected, the rate of climb would be reduced by as much as 400 feet per minute. Service ceiling would also be reduced as the Packard Merlin XX made considerably less power above 30,000 feet than did the Allison V1710.