From what I have learned over the past 54 years around race tracks and engine builders is this:
That means no X or H and are not joined together at all will sound the absolute best but will be lowest in torque and horsepower at the lower RPM's With no mufflers it is worse at the lower RPM's ( Lost Torque) OK at upper RPM's.
will give the best performance of them all at any RPM but does not sound as good due to the very smooth exhaust flow.
Increases performance over the Straight pipe
and is inexpensive for the manufactures to use. It does have a better sound then the X-pipe but does lose performance at higher RPM's
I have seen race cars with a box that the headers run the pipes into the front and the tailpipes out the back that would create equal pressure and worked great but never seen one on a street car. ( That was before they had to use muffles at most tracks.)I can't tell you if they were good or not but the cars engine builders swore by them.
With today's use of computers and converters in the cars and everything from the good old days being changed it would be nice to know if any of this is still good information.
I'm always up to learn something new. I'm still having a hard time believing that just installing my Airaid CAI on my car that I already had a K&N drop in filter in the stock airbox which was also a CAI would produce so much response improvement on my 3.7 V6 engine.