The theory behind the crossover pipe is that it balances the pressure between the two sides of the exhaust banks (extemely sorry that is v-8 speak). Further it helps cancel the resonance of the exhaust sound and gas flow. That means it takes a high frequency wave (short distance between peak and trough) and lengthens it into a low frequency long wave. Thus the sound and gas charge is not continually banging on the inside diameter of the exhaust pipe trying to escape. Theory the gas flow is smoother and faster. How this translates to I6 I haven't figured out yet. The function of any exhaust system is to maintain the velocity of gas flow as high as possible. Remember as exhaust gasses cool they slow down. So really a step system should be best 1 3/4 to 1 7/8 to 2 to 2 1/4. As gasses cool they expand so your fooling the exhaust into a larger area. Yeah right I tried something like that on my 76 Peterbilt w/ Detroit 12. Besides if theory was correct than no manifold/header is needed right. Well I'm a farmer and if I lived in the theory world where everthing works 100% of the time correctly--I wouldn't be farming I would be rich. I6 exhaust still has a head lock on me, good luck and hopefully this doesn't muddy the water.
