[curtis73]

cam timing events have a lot to do with it. When an exhaust lobe ramp is timed so that it opens early, it opens while there is still lots of pressure and some combustion still happening. This effect is also simulated by fast exhaust opening ramps on the cam.

Having headers allows some of the exhaust noise to come right out in the open. The exhaust pulses hit the inside of the manifold/header just the same, but manifolds being made of cast iron deaden the sound a lot more.

Mufflers and exhaust diameter are what I would say causes the biggest change. For a given engine configuration, the most change that can be affected is with the exhaust. Consider it like a big trombone. The engine is like the vibrating mouthpiece. That is unchanged. The exhaust tubing does the exact same thing with vibrations that a trombone does; shapes, alters, and modifies the sound. Of course in a car, the point is to cancel some sound waves along the way. Bigger diameters allow flow and more sound to make it out. Smaller diameters (in exhaust AND trombones) make it sound more treble-y and tinny. Go too small with either and it will sound like a trumpet; blatty and harsh.

Altering the length of either will alter its natural resonance frequency. Hence why trombones have longer tubing than trumpets.

You can't make a V6 sound like a V8 any more than you can make a Waltz in 3/4 time sound like a March in 4/4 time. The pulses are just not divided over time the same way.