Andy is correct about the heat and given enough volume and surface area for heat transfer that is a possible solution. I am reminded though of a school bus that I used to ride as a kid that had a 40 foot long exhaust pipe that sounded like a machine gun. It is the size of the pipe that matters isn’t it?
Seriously another big aspect of the noise for an IC engine is pulsing of the exhaust gas. It is this pulsing and the pressure waves that cause much of the racket that annoys the neighbors. This noise is generated by the exhaust valve opening and the corresponding pulsing of the exhaust gas expanding as part of the blow-down process. It is this pulsing that a good silencer (muffler) system dampens and thereby reduces noise. For example, the exhaust on a small Honda generator is not cooling the exhaust gas to any great extent simply because of space and layout constraints. They are cleverly removing the pulsing of the exhaust through an elaborate exhaust/silencing system. On a well-made system such as the Hondas and I assume the Yamahas, you will find they are able package a silencer in a very small space and therefore the cooling of the gas is likely quite marginal. But with their designs, they are able to dampen and smooth the pulses thereby reducing noise.
The trick used by Honda is to dampen this pulsing blow-down is through a very elaborate silencer. I once saw photos of an exhaust silencer from a Honda generator that had been purposely cut open for evaluation of its cross section and it was more elaborate than one might expect. They are obviously quite cleaver as to how and the extent they are able to dampen, isolate, and smooth the pulsing thereby eliminating noise. Again, no significant heat transfer but pulse dampening and smoothing. They are trying to achieve constant flow from the exhaust tip vs. something that pulses like a beloved Harley exhaust!
By the way, an even more efficient silencer is a turbo charger which reduces heat by creating work from the flow and smoothing the flow throw the vanes of the rotor. So another solution would be to turbocharge that Briggs to take some of the noise out.
These muffler/silencer designs, employed by Honda and others, drive the cost up. You gets what you pay for, sometimes!
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