speed cowls:
the compromise is to get enough exit flow with out adding excessive drag-
ideally you have a cavity which just clears the cylinder
the inlet size and shape must provide flow around the cylinder -all around with no hot spot left at the rear of the cylinder
So---- the streamlining of the "exit becomes the real task
unless pressure is low enough - you get no real flow thru the cowl
The cowl shapes on old models such as th DeBolt Speedwagons - is pretty good.
a rather short cowl with a tall exit.
Box shaped cowls do nothing to accomplish the real task: make the air hug the engine and have a constant flow
On gasoline industrial engines such as leaf blowers weed cutters - same engineering is used by forcing a flow from the inlet thru the fins .