Inspired by
this posting (last paragraph)
First some technical details:
If a plane flies higher the air get less dense, so it can fly faster. However internal combustion engines need air to work or they will loose 50% power at 20,000 feet. So, in order to keep them working at higher altitudes they need something that pumps additional air into the carburettor – a supercharger. A simple supercharger with one compressor stage running at a fixed speed ensures an engine provides 100% sea level power up to 15,000 feet. More advanced ones can keep the engine power at 80% in 20,000 to 30,000 feet. However they have one sort-of drawback, they are connected to the engine by a gear and thus take need a few horsepower to work. In case of an early Merlin it was 150hp, but the net gain compared to an unsupercharged engine was still 250hp.
The US Military was a bit prone to over-engineering. Since 1918 they were dreaming about and working on a supercharger that would work without taking any power from the engine. That is the exhaust driven turbo-supercharger, or turbocharger. Exhaust gases make a turbine running and that pumps the air into the engine. Sounds great, doesn´t it? But it isn´t!
After more than 20 years of research General electric and the USAAC/USAAF still didn´t have reliable turbochargers and when they finally had some, the beasts were huge and very expensive. But the real problem was a high-tech induced tunnel vision ignoring the potential of cheaper and proven superchargers.
Result: P-40 and P-39. Planes restricted to low altitudes which didn´t bother anybody enough to do something about it.
WI feedback from the UK and the USAAF´s urgent need for high altitude fighters makes Curtiss and Bell start installing advanced British superchargers in their planes beginning in 1942?