Guys. I just installed an h-pipe in my true dual side exit exhaust. I have long tube headers, with 2.25" pipes, and flowtech afterburner muffs. The sound was too raspy and filled the cab with a high frequency noise. I installed a single h-pipe home made from a piece of 2.25" exhaust pipe from Oreillys. I used a Lowes Bi-Metal hole saw to cut the appropriate holes so that the H would hug the pipes nicely.

Results:
Performance.....Uhh I can't tell anything if at all for performance.
Sound.....this was the major difference. The high frequency noise is now much less and the exhaust has a nice rumble now that wasn't present before. The interior is more comfortable to ride in now. Its pretty amazing what one little H-Pipe does.

Questions:
Has anyone ever thought "well, if one h-pipe is good, would two h-pipes be better?" Would anyone care to make a Hypothesis on the results of two h-pipes installed? Make it quieter still? No difference? I'm curious mostly.

 

I doubt two would make any difference. The reason the sound changes is that most V8's have two cylinders that fire next to each other on the same bank at some point. This causes an imbalance and odd interval on that pipe. The H equalizes the two pipes so when one side has two back to back pulses, the other side has two back to back nothings and it all blends and settles out. Assuming the H pipe is big enough then there should be no further benefit to more.

X crossovers on the other hand by their nature help further blend the two sides together and have their unique sound and supposedly better performance.

If you are interested in further noise reduction you could experiment with some short glasspacks to act as resonators. For my supercab, because I actually want a quiet vehicle, I plan on running a glasspack then a large oval muffler. I'm doing a single 3" system and plan to add a cutout after the glasspack so that when I want it can be loud but most times I want it to be quiet. Silent to a low rumble is the goal.

As a side note, my cousin had a dual 2" system with extra long glasspacks on his 327 67 Chevelle. He installed an H pipe and almost immediately took it back out because of the sound change. With that exhaust and a duntov 30-30 cam it simply was the epitome of the "classic hot rod" sound. The long glasspacks don't have the same harsh raspyness to them the short ones sometimes do.

 

Duntov 30-30. Now that takes me back!

 

Yeah, solid lifter small journal 327, Holley double pumper, close ration muncie 4spd, 3.73 posi rear, duals with headers and glasspacks...

It just sounded... Right...

Young and old, car enthusiast or not, as soon as you heard that car there's only one thought that came to mind - There's a hot rod...

It's got a boat anchor, er 305 in it now... A little too much enthusiasm one night resulted in a spun bearing. Motor's been done sitting in his garage for years now but just never made it back in... Yet...

I poke him about his projects, he pokes me about mine... You know how it is.

 

Lol! Boat anchor. I had a boat w/a 305 in it once. Why, oh WHY put a 305 in a boat?!?! Why not a 350??? Just like a truck, a boat needs all the torque you can get. Hijack over.