Engine air intake

[Mycool]

Working on air intake, does anyone know minimum air intake hole size? I'm guessing it can't be smaller than the air intake in the carb itself. Of course the bigger you go the more you get. Is there a chart on size vs performance?

[longline]

If you are referring to induction air, I just finished researching that. For the O/IO 360 engine, at 2500 RPM and 180 MPH it seems that 2.2 inches in diameter is the minimum number... rounded up to 3 inches would give about I inch ram increase without inducing much drag. Larger is not necessarily better here, as the drag would increase with a larger duct.
If you are talking about engine cooling air I would refer you to the DanH "shrinking exit" for exhaustive discussion.
Others, more knowledge than I, might chime in here.

[rv6ejguy]

This depends quite a bit on what your duct material is. If you duct with SCAT hose, plan to go up in diameter a fair amount, depending on run length. SCAT has very high losses, especially a few feet of it.

If smooth inner wall, you don't need anything bigger than the throttle plate area and maybe not even that big as the venturi or servo throat is usually quite a bit smaller in area than that.

[longline]

is correct. I had settled on SKEET tube, and did not think to mention the loss that would occur with something not smooth walled. Excellent catch!
My induction intake is designed to do away with the typical folded baffle material seal and reduce losses to the lower cowl volume.

[Mycool]

“Longline” thank you for clarification, I am talking about induction air. Where did you get that “2.2 inch diameter” from? My engine is a O-320.

[Marc Bourget]

I think you can go larger than 2.2" if you design the intake opening correctly, Same as for cooling intake.

With engine intake, a superior installation would have a smooth intake duct with diminishing cross section down to the correct size.