When I put my ported and polished heads and a Crane PM260 cam in my 305, the entire exhaust tone changed. The mid-range rap grew louder.

Does anyone know the exact reason for this? What is the theory of it all?

 

Well, with the ported heads, more air is flowing out the engine, which will increase the sound/tone.

As far as the cam, I'm not too sure why exactly that changes it.

I guess, by the way it opens the valves faster it will affect the air flow, thus again changing the tone.

Just a guess though.

 

Im guessing a larger cam makes the valves open faster and more so that = more airflow. More airflow the more exhaust. You can also change the tone of the exhaust by messing with the timing.

 

thats right for the heads, Mark, as for the cam,

there is one basic reason why changing the cam makes more power. (assuming the efficiency of the actual burn isn't changed by stronger spark etc..)

Cams make more power because they allow valves to open sooner / more. (lift/ duration) and alow the outgoing gasses to draw new ones in by briefly keeping both valves off the seat (overlap.) open a door quickly, and the air rushes after it, same principle. also the reason hot cams can cause a drop in inlet vacumn.

The net result of this is more fuel / air in the combustion chamber, meaning more to burn. More to burn means more to expand inside the chamber, then more to exit it. this burn / exit is what produces exhaust sound (before the muffler). so more burn = more noise.

incidently, with a blower, you dont need any overlap and low compression (you would blow the gas right out of the chamber instead of cramming it in, Guess what? Smog cams have almost no overlap and a typical smog engine has low compression, so that 78 Z28 350 with four bolt mains 8:1 compression and smog cam, will take boost better than most!

Overlap is bad for emmisions because you are throwing un-burned petrol / air out of the exhaust, not good for emissions.

well, that's my understanding of it anyway..........

 

more air in = more air out