The problem with small radials is the sound. The smaller they are the less 'radial like' they sound. It has something to do with the piston size and exhaust tone. A 5 cylinder OS or Saito sounds too much like a high winding race car engine vs. that of an aircraft radial. A Saito 450 sounds better, but even at high RPM they lose that deep rumble. The Mokis are probably the best sounding RC radials, and they can sound 'off' if the exhaust isn't configured right. Somebody said that they sound best if the exhaust pipe size is ever expanding from the port to the exit, ie. each pipe gets bigger. The collector ring is bigger than the port pipe, and the final exit pipe is bigger than the collector ring. Of course this doesn't work too good with unpumped glow radials since they need the back pressure.