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Bob, the further back you put the muffler, the quieter it'll be, and will also impede the flow the least. I wouldn't say mine is exactly quiet with a 3" pipe, but it's not offensively loud; just sorta deep sounding. Smaller pipes are quieter yet.
Brief discussion on pipe diameter, which is subject to a fair amount of mythology:
Restriction (a.k.a. backpressure) is always detrimental to performance and economy, but flow velocity in the exhaust tends to increase efficiency / power. The column of gas flowing out the pipe hs mass and momentum -- it will leave a low-pressure condition behind between exhaust cycles, and that helps the intake air get moving. The first of the intake air helps flush out the last of the exhaust. This is why cams have overlap -- the exhaust valve is still open for a bit while the intake is opening up. The more overlap, the higher the RPM at which this is efficient.
If we use a header that's perfectly matched to the cam and everything else, that header contributes greatly to the efficiency of that process, and the engine runs best with no exhaust pipe at all. That's why I'm using a huge pipe: to get as little effect on the header as possible. With the stock manifold and downpipe(s), or with just any old header, the exhaust pull / intake flush is not going to be optimised without bringing the rest of the exhaust system into play.
So the trade-off becomes apparent:
Small pipe = high velocity at low RPM (good low-end torque) = restriction at high RPM (poor peak horsepower -- you basically hit a maximum velocity in the pipe at which faster pumping rates don't increase the flow).
Large pipe = low velocity at low RPM (poor low-end torque) = good flow at high RPM (good peak HP).
With the double-downpipe manifold, a 2-1/4" pipe seems to be the best compromise. If low end is more important than power above 4000 RPM, a 2" pipe might be better. The more free-flowing the muffler is, the better, in all circumstances.
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