Sizing cooling air intake?

[Koncretekid]

This is similar to Stay'Tee's question about radiator size, but with an air-cooled motor.  I only have less than 50 hp now, and have only a 1-3/4" id inlet duct.  I am building an aluminum shroud around the motor and will add an outlet ducted to the side of the fairing, which I hope will pull air thru.  Maybe I'm being optimistic about the small inlet size, but I see NASCAR cars with 600 plus hp and most of the front inlet being covered by duct tape.

It will be a major job to cut out that inlet and increase the size, but if necessary, I can.  20 days til Loring.

[Rex Schimmer]

If you want to have air going past the engine at all times that it is running then run your exhaust header through the center of your cooling exit pipe and end it right at the end of the cooling pipe. Bernoulli will take care of the rest, the high velocity and heated gases of the exhaust will draw air past your engine cooling fins and at WOT it certainly could be enough to keep it cool and it also eliminates the additional aero drag that you would have if you were only depending on the air flow generated by forward movement. 

Rex

[Peter Jack]

Great thinking Rex. 

What an amazing web site.

Pete