I'd talk to a perfomance exhaust shop...H pipes have been around a long time...then there is the question where to install it..mine (78)was forward of the tranny crossmember...and it was smaller than the main pipe..nice sound but don't have a clue on HP gain..buying a 69 BB and the H pipe is aft of the rear end...headers,3" exhaust...race magnum mufflers..

Here is my old exhaust on the 78:


This is the setup on the 69 BB I'm buying...

 

I put an H-Pipe right under my spring and it really helped the sound. No more popping.

 

H Pipes, X Pipes and Crossovers

I am installing an X-pipe right now (more pics to follow later in the week). It is under the rear end for two reasons. Firstly - space, not a lot on a vette and secondly to squeeze the pipes together down aft to clear the pivot points on the strut rods (mine have heim joints and zerks). I am also keeping the spare tire carrier. If you ditch the spare tire carrier you can almost install a C-5 system back there. (We test fitted one!)



In a ideal world the a true crossover pipe fitted as close to the headers/maifolds as posibble would give the best result.

In David Vizards books he talks about the best muffler being no muffler at all. Less exhaust pressure = more air flow though motor = more HP. Hence if you have mufflers on the car yhe way to minimise back pressure and improve flow is to cross connect a true dual system.

If you think about air flow then you understand how the H's, X's and crossovers really work. Lets say you have 20PSI on each side of the system. When the engine fires the exhaust pressure is 20PSI due to pipe size and the muffler holding back the exhaust gases. Now what happens if you connect the two sides together. You now have two mufflers (twice the area) so the pressure should be halved to 10PSI. It is more complex then this due to heat trasnfer and the actual loaction of the cross connect but you get the idea.

The H-pipe cross-connects the two pipes and equalizes pressure, it doesn't aid in actal flow rate the same as an X or crossover.

The X-pipe provides two alternate paths for the exhasut to pass through. Similar to a H-pipe but unlike the H-pipe it keeps the exhaust heasding towards the back of the car. So it equalizes with minimal scavenging effect. A true X crossover is where the 4 Y joints are used. One side is plit in two and then joins the other side towards teh rear of the car split in two. This is a true venturi type system and allow with equalizing pressure will scavenge (suck) exhaust to pull more exhuast out of the head. Hence the current exhaust pulse is pulling the next exhaust pulse through the pipes.

It is near impossible to fit a true crossover under a vette due to ground clearance. H-pipes are fairly easy. X-pipes are doable. If you fit an X behind the front cross-member it can be tricky to tuck it up but can be done. Under the pumpkin is the easiest place but again it needs to be tucked up a little. Regardless of where you put it should still give you a 3DB reduction in exhaust noise as both mufflers are effectively passing the exhasut gases for each exhaust pulse..

If you have a mild steel system and can weld you can probably install a H-pipe for $5. Mine was stainless so I had to farm out the work.

My guestimates.
You can fit a H-pipe for under $100 (2 TEE bends + pipe + welding or just a pipe)
You can fit an X-pipe for around $200 (X -pipe $100 + bends and welds)
A true crossover will cost around $200-$300 + installation/welding

If you do fit an X or H be sure to consider how you can remove the exhaust at a later date. A joint in the middle of the H allows the two side to still split. Without a joint it will be a real pain to get pipes out and then back in again.