If the exhaust gets noisy during compression braking coasting, you need a better design muffler. Most of the low restriction (glass or steel pack) mufflers that the hotrodders like to use on stovebolt engines produce a series of rapid backfires that are extremely annoying, at least to some of us. Coasting with a closed throttle produces an extremely lean condition that promotes backfiring. That's especially common with a "split" exhaust manifold or headers that produce a 120 degree separation between exhaust impulses alternating between dual pipes.

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