I remember when first (too many years ago) I noticed that the speed of the early B-17's were a good bit higher than the later models, the G understandably being the slowest. Even then, it clearly was not considered to be high enough. But i coulkdn't help thinking that increasing speed could be an alternative to increasing armament.
So again I have for a long time wonderd why neither the B-17 nor the B-24 got more poerfull engines, though there certainly were some of considerably higher power available before the US entered the war. So i hope those who understand the technical side better than me can give some input. Would stronger engines be more difficult to boost for high altitude, making a difference so small it was not worth it? Would the increased fuel consumption and weight be unacceptable? In other words, did the engine packages used simply hit a sweet spot, neccessitating a new design to take advantage of the higher power available?
A few things, or more than a few.
the Allison was sort of the next step up from the R-1820 and R-1830 but only with the turbo. But.............
Production of Allisons by year.
1940..........................1149
1941..........................6402
1942......................14,904
1943......................21,068
When did they KNOW how many Allisons would be available and when (and the 1940 engines were the long nose C-15 engines that were 150hp lower than the radials).
The B-17s were built by Three factories, and they needed way more engines than Wright could provide so the Government a few weeks/months after the Packard deal, signed a deal with Studebaker to build the R-1820 radials. This was in late 1940 and due to the government changing their mind (Studebaker was originally supposed to make R-2600s) the first engines don't show up until 1942. Studebaker builds 6091 engines in 1942 and over 23,000 in 1943. Brand new factory, not built in Studebaker car factory/s.
Buick and Chevrolet were both signed up to make R-1830s for B-24s at about the same time.
Now as the pre war engines (counting 1939 as the start of the war) there was only one. The R-2600 was it. And this is not really a small quibble. for the R-2800 P & W only built 17 in all of 1940. Yes it shows up before Pearl Harbor but it is later in timing (Ford gets the Contract for the 2nd R-2800 Factory in Sept 1940, they were planning ahead, but it took time).
SO we are sort of down to the R-2600 and it went through a few problems.
1, the R-2600A (1600hp) did not take to turbocharging very well. A lot of trouble with the Prototype A-20 and after the first one all of that production batch had the turbos deleted, never to be seen again.
2. The R-2600B (1700hp) used a different crankcase (steel instead of aluminum and many other parts changes) so you could not build an R-2600B in a factory designed to build R-2600A's without a lot changes. It also was never turbo charged on a production version.
The R-2600BB (1900hp) doesn't show up until 1943. which is a bit late. It was used post war for converted water bombers.
The R-2600 in the early versions would offer a lot more take-off power than the normal B-17 and B-24 engines (but was a lot heavier) but without the turbo they didn't offer any more power at around 25,000ft. So no advantage at cruising speeds/altitudes.
A big problem with air cooled engines was keeping them cool. It is a matter of the amount of fin area on the cylinders and cylinder heads and the density of the air. The thinner air at 25,000 (or around there) would not allow for the cooling. I am bobbling this a bit. With a liquid engine you can design a bigger radiator to get the cooling you want on the same engine you were using. On the air cooled engine you need to change the engine, It was done but it took new methods of making fins and new ways to machine them to get the depth of fin and spacing they needed. P & W started with an advantage in that they were using smaller cylinders than Wright. 14 instead of 9 and 18 instead of 14 on the big engines and this almost automatically gave them more fin area for cooling. They still needed a lot of refining.
There were a lot of paper bombers designed using the R-2600 and R-2800 but some of them didn't offer much improvement or it was decided to make the jump to the R-3350 bombers. The B-24 program was huge and trying to change it was not easy and would have needed months to even split off parts of it.