I'm running split headers on my 292 with Cherry Bomb glasspacks and 2" pipes from the header bell to the mufflers and then the same size out the sides. Runs fine. Much better than with the little bitty strangulation pipe that was stock on it.
I agree about 2-strokes (and rotary motors) needing backpressure, but on conventional 4-stroke, normally aspirated motors, I think it's pretty much academic until you get to things like tuned exhaust with equal length tubes and measuring flow rates to evaluate for maximum efficiency, especially with valve timing that has a lot of overlap where scavenging becomes critical. I'm sure the F1 guys have all sorts of computer models to design this, but on our trucks, I think it's a little over the top.
Granted, I wouldn't put 4" duals on my 292, but as long as I have a relatively mild motor, anywhere from 1.75 to 2.5 should function about the same.
Now if anyone has done extensive research or knows of any that refutes this, I'd love to investigate it.
