What changes in performance,power noise can I expect when I go to an H or X pipe, Car is 70 SS396 now 408 with agressive cam 1 7/8 headers,flowmasters 2 1/2 "exhaust, mild ported heads 3.91, 4 speed.
Is there much difference between H pipe and X pipe.

IMHO x-pipes deliver far more power than an h pipe, but x-pipes change the sound/tone of the exhaust and in my personal experience made the vehicle very very quiet for a 550 horse motor. Going from true duals that you could hear a mile away to doing nothing but adding an x-pipe and being barely able to hear it start in the garage if you are in the house.

X's also make for tight room, esp if its big pipe. Then again, exhaust/headers and anything obstructed by them is always a pain.
For all out power, go x, for tone, stay h, for loud stay h or go x plus a smaller muffler.
Idle sounds about the same IMHO. Its when you stomp on it that you notice the difference, or when cruising.

Matt

No significant performance difference between either the H or X...with the X you MIGHT gain a tenth in the 1/4 mile.

I was either told or I read it somewhere, the determening factor of how loud a motor is going to be, is the cam. I don't remember which spec, but that's what I remember. I've got a dual 2 1/2" system with a muffler shop made x-pipe, and it's still loud as heck. Actually, when I first had my Spin Tech mufflers installed, it was pretty quite, but as I "broke them in," they've gotten significantly louder. I have been told that my car sounds similar to a Nascar car with the pedal to the metal...I don't know about you, but I think that's pretty kool :D