Hi Joe . . .

I am with Tony P. on this topic. In my experience exhaust design is about compromises. The best low end performance and throttle repsonse comes from the stock exhaust manifold. Because of the close quarters it creates the most 'cross over' and suction from the other 5 cylinders (or 3 in the case of a v thingy). This is great at low RPM. At high RPM these close quarters create back pressure, causing the engine to use HP 'pumping action' to force exhaust gases out. Headers are a compromise on the other end. Long tube headers are tuned to scavenge at a specific higher RPM, but this can create hesitation in throttle response at low RPM. A cross-over pipe is another compromise. It equalizes flow in a dual exhaust system, and allows for more scavenging at low RPM by creating flow 'between' the header banks (2 halves of v8 or the split 3s in an i6). This has the effect of restoring some of the low end throttle responsiveness, and also takes some of the rap and rasp out of the exhaust note.

Tubing diameter choice is another exercise in compromises. Smaller diameter pipes create higher exhaust gas velocity, which is great for low end throttle response. But smaller diameter pipes lead to back pressure at high RPM. Bigger pipe diameters will cut back pressure at high RPM, at the expense of low end throttle response. I found this site on Exhaust Theory instructive.

regards,
stock49

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