IMO and experiences with the 340 manifolds, the X-pipe is a risk., espeically with a street car. Use a 2.25 or 2.5" to the muffler, 2.5" good mufflers - anything mentioned by Tom, I've also done OK ith Magnaflow. The over the axle with whatever makes a smooth transition and gets over the the axle - no sharp bends right at the muffler exit. Tailpipe diameter not so critical.
As
@VOETOM typed in bold, the details of the X pipe make a difference. Dr Gas seemed to work well.
@CPDave saw improvement on his (IIRC) Goldfish, but I don't think he had any dyno data. But he did autocross it 10 to 15 times a year.
I put an exhaust system and mufflers with 3" X from Accurate on my 340 with exhaust manifolds. The rest of the system was 2.25 and 2.5" diameter. The car was no more powerful and felt less responsive (than the shop made dual exhaust with 2.5" Dymnomax mufflers). I've posted the dyno pulls from before and after that system went on. BUT there were other changes besides just the X-pipe. So one can not lay the entire blame on the X-pipe - but it was a contributor to the loss of power as well as throttle response.
Here is what I think was happening. The exhaust pulses flow OK until they got to a big space (the 3" X) where they naturally slowed down. Then they try to enter the smaller diameter tubing on the outlet side of the X. But momentum has been lost. The gas sits there until more energy comes along and pushes it into the mufflers and then over the hump and into the 15 psi atmostphere. Great - Not.
Calvin Elston has written
gas flow is king, and this is exactly what he's talking about. For efficiency we have to keep the gasses moving away from the exhaust valve for the entire range this engine will be used for.
Larry Meaux has shown that less efficient engines can benefit from
slightly larger tube sizes than more efficient engines. Using his program "Pipe-max" we can see the
placement of the H-pipe or X pipe is very important. The H-pipe in particular is about tuning the harmonics. Wrong location and it will not help, and may even hurt the rpm range(s) of interest.
