[Ecky]

This is a really interesting article:

http://www.veryuseful.com/mustang/te...Scavenging.pdf

If you think that leaving some exhaust gas behind in the cylinder before the next intake stroke is not important (in a bad way), look again at Figure 5 below. This is, once again, Jim McFarland's classic graph comparing the volumetric efficiency curve versus the torque curve.

The tests came about because Kevin has developed a patented variable-flow exhaust that uses a butterfly within the exhaust pipe. He initially expected to use the system to cause some backpressure at low loads 'to help torque.

However, he soon changed his mind when any increase in back pressure proved to decrease torque on a properly tuned engine. What increasing the backpressure does do is dramatically quiet the exhaust.

and

If, in fact, power does increase with increased exhaust back pressure, it is most likely the air/fuel ratio and/or ignition timing that are no longer optimal for the altered state of engine tune.

Larry Widmer comments on the above textbook quote:

At less than WOT and peak power rpm, the diameter of the tubing should change in ID. Just as with intake ports (unless we're just running off port volume), cross sectional area should be only sufficient to supply the flow rate necessary to feed the engine.

and

The exhaust system is much the same. Just changing backpressure is a bogus way of trying to create the "ideal" pressure in the system. The exhaust system should work like a correctly conceived header. It should extract the exhaust from the header, to minimize pumping pressures.

The only way to create a system that will serve as an extractor is to properly size the tubing to allow the flow velocity to create a sort of "vacuum" behind it.

Just as with headers, creating a system that will provide the best of all worlds at all throttle positions and rpm ranges is impossible. It's all going to be a trade-off. You can tune for the throttle positions and rpm ranges where you desire the greatest performance, but you'll sacrifice performance at the other
end of the rpm range.

I'm running an LS engine in my Del Sol, which does not have VTEC.