H-pipe Hell

I am building a 68 Bonneville and i just rebuilt the 400, bored .030 over, mild comp cam, nothin crazy, a nice street motor. I had a dual exhaust system installed on the car a few months after when i had the motor broken in. 2.5" pipes the whole way, and Magnaflow Performance XL muffs. I went back because of some payment issues and talked about an h-pipe with the installers. They told me an h-pipe would "even out the sound and increase performance a little" I wasn't complaining about the sound or performance, i was more than appy with what i got. He talked me into it and he quick put an H-pipe in, awesome job, did it in about 20 min. But now it feels less powerful. I thought it was all in my head but no, my buddies noticed a difference too. I don't know what to do, I'm sure i'll end up having it pulled and put some flanges in the place where the holes are so i can unbolt the exhaust but i wanted to know if anybody had any input on this. I still can't belive that an h-pipe would reduce power, i am at a loss. I also noticed after all this happened that my tranny won't downshift. I thought maybe the evening of backpressure on the pipes might have affected the engine vacuum and maybe not enough to down shift, i can manualy downshift. And under heavy acceleration it shifts way too early. like 3500. No idea what to think.

Does that have the TH400??? I think all the big cars came with that trans, if so... I'd check that you didn't bump or remove the kickdown wire that's on the driver side of the trans.. that'll cause the early shifts and no down shift, not to mention the appearance of a lack in power because of that.

never heard of an H pipe causing a drop in performance.... have heard of them not giving any or as much as they've been "hyped" to do. Most racers prefer an X pipe as they flow better, but... I wouldn't even bother with either if you're not racing it at the strip.... that's the only place you'd see the difference.

I checked that connection already, it would make sense if that were it but its firmly attached. Maybe its got a fault somewhere on the line. Also i was told that x-pipes were an unneeded expense if you have less than 500 horsepower. Oh well, they put it in for free.

I checked that connection already, it would make sense if that were it but its firmly attached. Maybe its got a fault somewhere on the line. Also i was told that x-pipes were an unneeded expense if you have less than 500 horsepower. Oh well, they put it in for free.

Yeah.. I'd get a test light and make sure you have voltage to the trans... Also, you can have a friend work the switch and see if you can hear the solonoid clicking inside the trans.. they go out all the time. I replaced the one in my 72 Th400 twice in one year...lol.

Engine backpressure and the like willnot keep it from down shifting or cause the early upshifts.... the downshifts are only controlled by the Kickdown switch... the early upshifts are controlled by the Vac. Modulator and Governor... If either one of these are bad, or you've got more vacum at the trans then before, that could cause the early upshifts (but the Kickdown switch should still operate).

yea, turns out that solenoid was bad. I had another th400 in the back so i pulled it off that one and put it in. All good now, forgot how much downshifting to 4000 rpm felt like, thats nice. thanks man