When the US sent out the actual requirement that resulted in the Allison V-1710, it specified a single-stage supercharged engine and, if a higher altitude capability was desired, the aircraft could use the newly USAAC-developed turbo-supercharger, or turbocharger as we call it today.
Actually the turbocharger was developed by General Electric. This was initiated by NACA and then funded by the USAAC.
Not many aircraft engines had a supercharger with more than one stage when the V-1710 program was initiated in 1929. Some didn't even have a supercharger at all.
The USAAC overestimated the state of development of its own turbocharger. General Electric was the sole source of American turbochargers during this period and for most of the US involvement in WWII.
GE was the sole source of production turbochargers for the USAAC/F for the duration of the war. Other turbochargers were trialled, but never went into production.
GE was also the source of most of the aero industry's supercharger designs for most of the 1930s. This included Allison.
The Allison Engine Company built engines to order, and the government ordered what they wanted.
All engine manufacturers built engines to order. No pint building 1,000 engines that nobody wants of can use.
Improvements in manufacturing brought the cost down from $25,000 to $8,000 and allowed the installed lifetime to be increased from an initial 300 hours to as much as 1,000 hours for the less-stressed powerplants. Weight increases were minimal with the result that all models were able to produce more than 1 pound per horsepower at takeoff rating.
The cost fell with the increase in production. Not really related to any engineering improvements.
Comparisons with the Merlin are inevitable. What CAN be said is the Allison made more power with less boost, had a longer time between overhauls, and did so with a parts count that was nearly half that of the Merlin (around 7,000 Allison parts versus 11,500+ Merlin parts).
Oh no, not the parts count again! Considering that most of the difference was composed of extra fasteners. And is the comparison like-for-like? That is, are we comparing single stage, single speed version of the Merlin with the V-1710, which, as you noted, was almost always single stage, single speed.
Of course the fact that the Merlin was typically rated at higher altitudes had no bearing on this. If it is true at all.
The early V-1710's were about 1,000 HP and the late P-38L was 1,600 Hp, while the final V-1710-143/145 was rated at 2,300 HP maximum.
That 2,300hp version was at 100inHg manifold pressure. Meanwhile the Packard V-1650-9 was not far behind on a mere 90inHg manifold pressure.
here was a high degree of commonality of parts throughout the series and individual parts were produced to high degree of standardization and reliability.
And Merlins were hand crafted by fitters using files and feeler gauges and drilling holes in-situ?
Hasn't that been debunked enough?
The V-1710 was modular, so that the core engine could be configured in different ways - for extension shafts, as a pusher, regular, single stage or two stage supercharger, etc. That is a blessing in someways, but in others it was a downfall. Much time was spent doing these extra options before the engine was fully sorted.
The original Bell P-39 prototype had a turbosupercharger that was specified by Ben Kelsey and Gordon Saville. Numerous changes were made while Kelsey was busy with P-38 work, and the turbocharger was deleted, making the P-39 a low-altitude fighter that was not suited to Europe's higher altitude requirements. The P-39 was rejected by the British but was used by the U.S.A. in the Mediterranean and Pacific Theaters, and particularly by the Soviet Union in large numbers. In the P-39, Soviet fighter pilots scored the highest number of individual kills made in ANY US or British fighter type, primarily because the Soviet war was a tactical, short-range, low-altitude war.
The XP-39 with turbocharger was a turd. It never had the performance promised, and likely not even the performance of the modified XP-39 without turbo.
The Allison V-1710 initially had some issues in Europe. Some of the issues were: 1) Poor intake design (Allison), 2) poor regulation of the turbocharger temperature (GE), different fuel formulation from that used for development (not sure it was anyone's fault). The P-38 also had a very poor cockpit heater, which was a major problem in Europe's higher-altitude environment. Last, one of the biggest issues was poor or almost complete lack of pilot training on good flying practices when entering combat areas. The intake design, fuel issues, cockpit heater, and pilot training were "fixed" within a year, but the temperature regulation of the turbochargers was never really fixed by General Electric.
The problem wasn't really the regulation of turbocharger temperature, but rather regulation of the turbocharger speed, which was controlled using the wastegate. The problem was the wastegate would get stuck closed, sending too much exhaust through the turbine and causing the unit to overspeed.
Fun fact, for an engine designed to be used with the turbocharger, it was a long time before a V-1710 was tested on an engine stand with a turbocharger (1939 or 1938, after the XFM-1 and XP-37 flew with turbos). The aircraft manufacturers just slapped them on.