My buddy has a crossfire. He had a custom exhaust put in. It has a x pipe put in after the rear cats with dual pipes to the back with no muffler. His sounds like hell because it's an auto and it drones but on a manual it would be sweet. What does an X pipe do for an exhaust? also people tell me I will hurt my engine with a high flow exhaust because of the reduced back pressure. But i call bs. This car has 4 cats. I believe that is plenty of backpressure. What's better? True dual (with and X pipe or no X pipe) or keeping the stock design with bigger piping?

 

you will not hurt your car even if you have no cats and no mufflers... that's a old school tale that was taken the wrong way... the warning was for people running super short pipes instead of headers and exhaust piping... the result was the valves super cooled when shutoff and slightly while running between cylinder ignitions and cracked the valves... this will not happen on your fully piped car, not even if you just ran half the piping...

 

Your engine is basically a big air pump. The less work it has to do moving air into or out of it (e.g. overcoming restrictions) the better. So, "backpressure" is bad. What a good exhaust system is supposed to do is function as a tuned cavity (much like a trumpet or a trombone) so that the exhaust pulses flow smoothly and efficiently out the back. If the system is not tuned properly the exhaust pulses will ricochet around inside and interfere with each other which will slow down the exiting gas flow.

An X-pipe or an H-pipe provides a crossover path between the two sides of the exhaust system to take advantage of an effect knows as "scavenging". Basically what happens is that since the engine fires cylinders so they alternate between the left and right side, one cylinder is just opening it's exhaust valve as another's exhaust pulse is traveling down the plumbing. If the crossover pipe is placed at the correct length down the exhaust system, engineers can use the relative vacuum behind the exiting pulse to suck the spent gasses from the cylinder on the opposite side.

There's a lot of math involved in calculating where the crossover must go, so a "custom" exhaust builder has a very slim chance of getting it right. I have seen many examples of custom or home-built systems actually making LESS power than the stock one! The only thing they do is make more noise which gives the driver the illusion of going faster than they really are.