Originally Posted by rebel2thegrave
I know the general purpose of X/H-pipes in true duals, but what are the differences in putting an X/H-pipe before the Cats and after the Cats?
The gases are too hot in front of the cat to begin to pulse or "cross over".
An old racer's trick is to take some black paint and paint the inside of the pipes downstream of the headers. They also metal score a line in the pipe where the paint begins and ends for reference.
Then they go out and get the pipes good and hot. The paint burns off the pipe, from the headers back, and where it starts to cool off (the crossover point - the gas has slowed and cooled here, the gas pulses slow down and equalize) the paint is still there. That's the spot they cut in for the crossover pipe.
An X configuration is more efficient than the old H pipe arrangement as the gases follow the gentler angle of the X vs. the hard right angle of the H.
Obviously, where the X meets in the middle of the two pipes, it's an open chamber.