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Well, I'll be the first to admit I don't know all the theory behind exhaust systems, nor would I pretend to even understand it all. No doubt there are countless volumes dedicated to the subject. So much to know, so little time.
As you mentioned the idea of a vacuum in a cannon barrel is not easy to see, unless you picture a very long cannon barrel. At some point the ball is no longer being pushed but instead travelling under its own momentum, and as long as the ball is still within the barrel it will leave a negative pressure behind it.
Continuing this analogy to its absurd conclusion, at some point the cannon ball would come to a stop and then reverse(!). For a tuned exhaust, my guess is the point where the "exhaust ball" stops is where the pipe should be cut for maximum performance.
Something else that wasn't touched on was the effect of the cooling of the gases and how they contract as they travel down the pipe.
The electronic muffler sounds like a variation of the anti-noise generator or active noise control. I wouldn't scoff at the electronic muffler just yet. I'm sure someone somewhere is working on a DSP for that at this very moment, probably the guy with a Harley rider for a neighbor. We'll just have to wait a bit longer.
Besides, who knew we would be using scan tools and DSOs to diagnose our cars today?
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