A few things:
First, nitpicking over O-320 vs O-360 is silly. The Cessna 172, the most popular aircraft ever built, uses an O-320. On the other hand, every larger Cessna, including the 172XP, uses an O-360 or bigger. A Piper Warrior/Cherokee 140 uses an O-320, but a Piper Archer uses an O-360. All of the Mooney aircraft use O-360 or bigger. The Grumman Tiger uses an O-360, but the Grumman Traveller and Cheetah uses an O-320. Almost all Beechcraft use at least an O-360.
Where does it all come out in the wash? I have no idea, but certainly if you took the AVERAGE engine size, it would be a lot bigger than 320 CI, considering all the light twins, bush planes, etc. that are out there.
Mufflers: I don’t know of a light aircraft certificated since the 1950’s that doesn’t have a muffler. For one thing, cabin heat is usually provided for by having a heat exchanger on the muffler.
However, aircraft mufflers aren’t as quiet as car mufflers, because of size, weight, and certification requirements. Often they’re something like a ‘glass-pack’ muffler, or even little more than an expansion chamber in a tuned exhaust.
Nevertheless, it’s correct that the prop is the major source of noise on most aircraft. But I would dispute that a lot of propellers go supersonic. I think that’s a bit of an aviation myth. The thing is, there is a huge amount of drag created by the shock wave, and I would guess that most engine/prop combinations don’t have the power required to push the tips into truly supersonic range.
Rather, I think the “BRAAP” sound you hear from a C-185 is the sound of the prop tips entering the transonic region (maybe .9-.95 Mach) . That added noise means a lot of power is being converted to noise, and that prevents the tips from going any faster. There is a huge drag rise once you get into the transonic region, but before you actually go supersonic. The props on a Russian TU-95 could apparently go supersonic, but only at high aircraft speeds and at high altitude, and those propellers were both huge and had supersonic airfoils at the tips.
Republic also tried to design an aircraft with a supersonic prop (The X-84?). They gave up because of the tremendous power loss from the supersonic tips.
A C-185 typically uses an 86-inch prop (2 blade). At a max 2700 RPM at takeoff, the tips would be going 690mph. The speed of sound at one standard atmosphere is 761 mph.
Now, the tip speed is also determined by the forward speed of the aircraft, and I have heard that a C-185 in full flat pitch MIGHT run the tips supersonic after takeoff if the pilot doesn’t back off the pitch properly. Sometimes when you hear a 185 take off you’ll hear a loud ‘whine’ right after takeoff. That may be the tips temporarily breaking through the transonic region. But certainly at the start of the takeoff roll and at any low speeds a C-185 can’t push its prop tips supersonic.
Even the T-6 Texans that race at Reno don’t run their prop tips supersonic. In fact, on really hot race days, when the speed of sound comes down enough to put the prop tips high in the transonic region where there is a lot of drag, the pilots swap in smaller diameter propellers to avoid the power loss.
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