An inline 6-cylinder does not need a crossover pipe, because the firing order separates the exhaust pulses from the front to the rear manifold outlet between each cylinder. A 1-5-3-6-2-4 firing order puts 1, 3, and 2 out of the front flange, and 5,6, and 4 from the rear. here's no way two pulses can stack up behind each other and restrict the flow. A Chevy V-8 is different, with two consecutive exhaust pulses, 5 and 7, coming down one pipe on the driver's side right behind each other. The crossover allows both pipes to share these pulses, and back pressure is reduced. The only way to avoid that is to use tuned-length pipes for each cylinder, and have at least the volume of one cylinder contained in each pipe before they dump into the collector. I did that on a 440 Dodge tractor pull engine, and ended up with primary header tubes over 8 feet long. Another way is to route consecutive exhaust pulses to the right and left exhaust pipes respectively, but that makes the underhood or under-frame area look like a snake pit. Smokey Yunick did that with some dyno research engines back in the late 1960's, and got some impressive HP and torque figures, but the plumbing was very impractical for street use.
Jerry
