And yet that's how they sound on cockpit recordings, so . . .
A buddy of mine who flies the P-51B and D. An excerpt from our conversation a few months back.
Me: How would you describe the sound of a P-51 from inside the cockpit?
Him: Hmm...a cross between a continental 550 and a Merlin.
Me: So it sounds like a Merlin does from the outside a bit? And not like a lawn mower?
I figured a Cessna 414 or 421 would be closer (to a Mustang) than a Briggs and Stratton. Lol. Aces High 3 makes the Pony sound like a lawn mower I kid you not.
Him: Ha ha ha
Yea...you can hear the Merlin influence when inside. What's really cool is when you hit the starter and it feels like a truck slamming in to your airplane. It startled me the first time.
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(The 550 is derived from a 520 which is what the 414 and 421 use. So my instincts are right.)
His opinion: AH3's P-51 sound is inaccurate.
YMMV.
I will let you know myself at some point. I'm going to get a ride in the D with him next year. (Not for free either.)