On my Thunderbolts the cowl looks to have a large opening but it is only half open. So the exit hole which is not very big is still twice a big as the entrance hole. Its an illusion.

Don,

Cooling air inlets and outlets have been a topic of discussion in CL Speed forever.  Here are some guidelines:

It is critical that air flow through your cowl be generally in the direction of flight (inlet ahead of outlet) and that flow passes over the cylinder and head. Any flow thru the cowl which does not pass over the engine is wasted in drag production. With a rear exhaust engine, flow should also be directed to the exhaust port area.

Its true that air heated by cooling the engine expands, and for this reason the outlet should be somewhat larger than the inlet (maybe 25% or so-) a more important consideration is whete the outlet is located.

The outlet should be aimed downstream, and not in front of any projection which would increase local pressure.

Will Stewart

Don,

Keep in mind that over the long history of CL Speed,  racing engine design and construction has changed substantially, as have fuels and restrictions on fuel formulation.

Early engines (Hornet,  McCoy and Dooling) used a steel liner and cast iron ringed aluminum pistons. The ring friction required both lots of lubrication and good heat rejection to cooling air from the cylinder barrel as well as the head.  The exhaust was open and lots of heat was dumped this way.  Since the piston expanded more than the liner, cylinder cooling was needed to prevent piston sticking.

The later lapped iron-steel pistons in finned steel cylinders also needed cylinder cooling but were less critical when properly broken in (or lapped).

ABC design and construction changed the relationships and the brass liner and aluminum piston expanded together, making cylinder cooling less necessary.  Head cooling was necessary to protect the plug and was generally enough for the entire engine.

Use of tuned pipes, with exhaust reentry under some conditions, resulted in heat buildup at the rear exhaust port. Under worst-case conditions the crankcase would expand enough in the rear to bend the cylinder forward increasing friction.  Note that serious F2A ships have exposed heads, and air intakes directing air to the exhaust port area.

Will Stewart