Originally Posted by Mark Powell

Actually I think that up to about '120' size (20 cc) a four-stroke single sounds more realistic. Because the number of firing strokes per second, thus the frequency, over its operational RPM range is closer (but slightly lower) to the frequency of a Merlin/Griffon over its operational RPM range.

But as the RPM decreases with size a four-stroke flat twin, which always has evenly spaced firing strokes is better than a single at larger sizes. Or a '360 degree' in-line twin (think 1960's British motorcycles) where the two pistons go up and down together but fire alternately, so again give evenly spaced firing strokes.

Four-stroke triples can't have even firing strokes so their sound frequency varies in an unrealistic 'rhythmic' manner, which explains their uneven (and thus unrealistic sounding) tickover/low speed operation and overall their sound frequency is too high.

A guy at our model site used to fly a Hawker Sea Fury with a 7 cylinder 250 cc Moki radial. That sounded very realistic in flight, but was too 'clattery' (mechanical noise) at low speed and tickover.

The Kolms are lovely engines but I would not put 'realistic sound' as one of their merits.