Shortround6
Lieutenant General
Difference is that the engine driven supercharger on some of those aircraft engines was delivering 7-9lbs of boost several thousand feet above sea level. They could deliver more boost than the fuel would support without knocking or detonating.
That was the Army's goal. Please remember that the US Army had been working with turbo-chargers since the late 20s. Fuel may have been 70-77 octane. The early engines had no engine driven supercharger. AS fuel got better and the engine makers added or improved the superchargers on their engines (on some 1920s radials they were actual called mixing fans to improve the distribution of fuel and increase atomization rather than really increase boost). The US went to 100 octane fuel before anybody else but due to an insistence not to use aromatic compounds it was pretty much 100/100 fuel and not 100/130. This limited the amount of boost that could be used (still better than 87 octane) even running rich.
The Early P-38s used 6.44 supercharger gears instead of the 8.80 gears to get more take-off power and used the turbo to get altitude performance even in the teens that they would have lost compared to the "altitude rated" engines with their 8.80 gears. The P-38F switched to 7.48 gears and the P-38H went to 8.10 gears. But by that time the P-40s and P-39s were using 9.60 gears. This is one reason the "military power" of the P-38 engines was always higher than the non-turbo engines of the same time period. They were using the turbo to make for the lower pressure of the engine powered supercharger. By spinning the supercharger slower it took less power leaving more power available to drive the propeller.
The radial engines were rarely geared (supercharger wise) for even medium altitude performance. The R-1830 and R-1820 during 1939-1942 could only hold 1200hp to around 3-5000ft.
By changing gears you could get more power in the low teens but on a single speed engine that meant giving up 100hp or more for take-off/low altitude to prevent detonation.
The turbo R-1830, R-1820, R-2800 engines all used the lowest the supercharger gear the company offered in order to get the best take-off/base line performance.
10/130 fuel changed things.
The turbos got "unleashed" once;
A) better materials were used in the basic engines.
B) 100/130 fuel became wide spread (for the US this meant sometime in 1942, plans for "hot rod" aircraft have to take into account that the fuel development was faster than either engine development or airframes).
C.) WEP ratings were approved (trading increased maintenance and shorter engine life for increase performance)
D) Water injection was introduced to allow even higher boosts than 100/130 fuel would allow on the air cooled engines.
E) 1944, the introduction of 100/150 fuel along with water injection on the P-47s.
trying to used turbo on an R-1830 in 1940-41 to bump up the pressure at low altitude fails at least 3 of those conditions. Perhaps they could have OKed a higher power setting and gotten less engine life. but the increase would not have been as large with the poorer fuel and the maintenance and blown engines would have been higher. Water injection might have seen a quicker service introduction (jimmy Doolittle was fooling around with it in the early 30s) but I don't know how far the experiments were from service introduction.