If Twice as Big is Twice as Good, Then Five Times As Big Has to be Five Times As Good-the Chrysler A57 Multibank Engine.
One of the difficulties that tank designers faced in the frantic rearmament of the late 1930s was coming up with a suitable powerplant. When the reality sunk in that a shooting war with Fascism was a distinct likelihood, designers in the U.S. and Britain turned to nearly anything that they could get their hands on to power their creations. Nothing was safe and no design was too goofy.
Consider that what was needed was something that had the right power output and plenty of torque right where it was needed. If it was reliable, durable and economic, so much the better-but the need was for torque, and lots of it.
The British had a selection of tandem engines, but the best that they had early on was the Nuffield V-12 that was a water cooled Liberty aero engine of World War I. In the American made M3 Grant and M4 Sherman, a variety of powerplants were used that included a collection of adapted radial aircraft engines, the Ford GAA V-8 (what else?), GM diesels, and that most unlikely of creations, the Chrysler A57 multibank engine.
The A57 was five Chrysler flathead six cylinder engines mounted on a common crankcase. The engine that was thus created was capable of about 450 shaft horsepower. In its initial iterations it was unreliable, valve and seat life was short, synchronization must have been a problem, and it was decided that all the M4A4s that used this configuration would be sent to the British.
Well, the British were used to making do with goofy stuff, so they set to, and between the engineers at Chrysler and the input of the British Purchasing Commission, the A57 was turned into a reliable and robust power plant. All it needed, it seemed, was stellite valves and seats.
I've found an interesting website where a bunch of fellers over in Europe have restored an ex-Belgian Army Sherman Firefly, and it's got some great photos of the A57. I recommend it highly.