I've been kicking this idea around with some mechanical engineers for
a couple of years.

Motorcycle in-line fours are state of the art high performance engines
unencumbered by smog controls. They get more than 100HP per liter
with some at 140 HP/liter. Of course, they rev like a dentist's
drill.

More importantly, the cylinder/head assembly just bolts to the
crankcase. Now, think of 7 or 9 of these in-line water cooled 4-cyl
blocks arranged radially around a common crankcase.

To make it work, you have to make the case, crank and new master/
articulating connecting rods. You'd also need a gear case to drive
the cam chains. Finally, you'd need some serious gear reduction
(~7:1) planetary gear set in the nose case. I'd also use an aircraft-
type fuel injection system instead of carbs.

If it worked, in the case of 9 cylinder blocks, you'd have a 36
cylinder, 500 pound, 30" diameter, 1000HP+ liquid cooled radial
reminiscent of a half scale Lycoming XR7755.

Now build a slick little carbon composite airplane around the monster
and take it to Reno. I'll bet the sound alone would wow the crowds.