Originally Posted by
JDNSW
The major problem with the turbos was that they required special alloys that were simply not available in the necessary quantities. Allison was not the only company stressed by wartime production problems - at least they didn't have bombs coming through the factory roof like RR did. Everyone was pushed to the limit, and many companies were doing things they had never done before. To take a local example, during the war, there were about ten thousand Gipsy Major engines built in Australia - by GMH and Tasmanian Railways! Neither organisation had built aeroplane engines before, even small ones like these.
The US did, as you say, largely use aircooled engines in WW2. This was mainly due to experience with the Liberty engine of WW1, which seems to have had major issues keeping water in. Both aircooled and liquid cooled engines were widely used by all the combatants in Europe, with top performers such as fighter aircraft using liquid cooled engines mainly because of their lower frontal area until fairly late in the war when the generally larger size of aircraft meant engine frontal area became less of an issue.
The US had an advantage in the aircooled engine in that a lot of development had continued in these in the two years before the country entered the war - once the war is on there is a strong pressure to stick with what you already have and develop that. A prime example is the Merlin. Developing 890hp in the Fairey Battle in 1936, in 1945, in the Hornet, it was producing over 2,000hp.
Aircooled engines were less able to be increased in power, a good example of a comparable power is the P&W R1830, which produced 800hp initially, but never exceeded 1350hp. It is a lot easier to increase the cooling in a liquid cooled engine!