I think he might have meant that a direct
drive unmuffled flat-four sounds rough
at idle compared to most cars. And I'd
agree.

It's that "unmuffled" (or partially muffled, at best) part that gives
it that characteristic quality.

Our friend has an airplane powered by an unmuffled Chevy V-8, and it
sounds just as sputtery at idle...
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"