Hi!
On modern pylonracing airplanes (.40 size) an opening of about 6-8mm and 20mm in height is all it takes. Then we talk high performance engines like IR and Nelson. Quadrouple the outlet area and everything would work out nicely . It's very important that the area surounding the engine cylinder and head is really tight, 1/4 inch is nearly too much distance, better to keep it as tight as possible