I'm motivated by my interest in the use of aero engines in tank development. A 1m wide engine would be really nice for a low-profile tank though the bump on the rear deck of the Hellcat wasn't a major issue for that design. But most radial of the WWII and pre-war era were about 1.35m-1.40m wide, regardless of the number of cylinders, displacement of each cylinder, etc. Does anyone know why this is? And does anyone know what would be the consequences of using shorter connecting rods, especially with regard to power and durability?
My default assumption would be that the diameter was chosen not based on the needs of the engine since that would change with the number and size of cylinders, but on the typical diameter of a typical fuselage of a plane with a single engine and a single pilot.
Higher "side loading" of the cylinder and a shorter dwell time, is the downfall of a shorter connecting rod.
Side loading is the pressure the piston exerts on the cylinder itself when the crank throw is on the upstroke.
Its higher with a short rod because the shorter length makes the rod "lay over" more. As the crank comes around and starts to push the piston upwards, the initial force pushes the piston into the side of the cylinder wall. A longer rod would be at a steeper angle, so the force from the crank would be more upwards and less sideways.
This can also effect rod bearing wear, as the side load pushes against that bearing, on the other end.
The dwell time is shortened because the shorter rod allows the piston less time to sit at top dead center (as measured in crankshaft degrees). The shorter dwell time allows less flame to develop, so it tends to "chase" the piston more, than with longer rod.
This is why little hot rod tricks, like using 6" rods in your 350 Chevy, are so popular.
The upside of a shorter connecting rod is it allows a stiffer and more compact cylinder block to be built.
Hope that helps you understand that aspect of your question.
Elvis