Originally Posted by Larry DeCamp
1-I have read calculations that a 3" dia. intake is all the air a 180HP 0360 can use and the SJ cowl has a 3" hole.
Assume a very high Vi/Vo inlet...small diameter, with a good internal diffuser if you expect any manifold pressure increase due to captured dynamic pressure. Air requirement for your 360 is:
[(360/2)x2700]/60 = 8100 cubic inches per second, assuming 100% volumetric efficiency.
The RV-4's VNE is (IIRC) 200 MPH, which would be 3520 inches per second.
8100/3520 = 2.3 sq inches required, or a diameter of roughly 1.75".
Note in this calc the air is passing through the inlet at freestream velocity, thus any conversion of dynamic pressure to increased static pressure must happen internally...in a really well shaped diffuser airbox. Prettiest one I've seen is on Paulo Iscold's recent speed record airplane.
This tiny pitot inlet offers the best chance of maximized external
streamlining. However, at any speed less than VNE, the 1.75"D inlet size would be a choke, one reason we usually only see 'em on bespoke racers.
The 3" inlet works out to match about 65 MPH, which means it would not be a choke in slow speed full power climb, and it would pick up some external diffusion at higher speeds. There is no one perfect answer. Personally I'd go with 2.5"D for my RV-4 if committed to internal diffusion and low external drag.
A low VI/Vo inlet works better across the speed range, and doesn't require nearly so much internal airbox space. More later.