I removed the catalytic converter about two months ago and just welded a 2.5" pipe to replace it. I saved the Cat just in case I wanted to re-install later. The installation began with cutting the stock pipe just in front of the rear suspension. The passenger side of the exhaust was installed first, and once in place he spliced the stock pipe and made a Y. I would have liked to use a Y pipe, but he did not have one. I would suggest that you find and use a Y pipe if possible. Mine turned out fine but took some time to get it right and exhaust flow seems equal. There is not much room under the Allante as we all know. We had to bend pipe to get to the mufflers and from the mufflers to the cut outs. The Borlas were mounted under the heat shield that was above the existing muffler. The mufflers were installed at a slight angle and fit quite well. The pipes from the muffler to the bumper cut outs had two 90 degree bends to fit right and have room for the tips. We cut about 1.5-2" off the chrome tips so they extended only slightly from the cut out like the stock tips did. I know that this does not paint as good a picture as pictures do, so I will email pictures to anyone that wants them.
As for the idea of using parallel pipes, I think that you may have trouble with space available. You may want to leave the stock converter and start with a Y pipe in front of the rear suspension. My best advice is to find a good muffler shop and babysit the installation.
Mine is just a bit louder than I wanted. It sounds good, but just a bit loud for a Cadillac. I may tone it down by either putting the Cat back in place or installing a resonator. It may take a combination of both to get the sound that I want. I did hear an Allante with the Flowmaster type Camaro Muffler on John Monzo's and mine is comparable to his as I remember it. I am looking for a more exotic sound and am open to suggestion.
Did removing the catalytic converter have any adverse affects on computers? I'd be kinda surprised if it didn't.. I suppose at the least, there was some adjusting time. No? How does it feel with less back pressure? I know Cadillac tries to keep the exhaust somewhat quiet - so freeing it up will help. But, I also hear that removing the catalytic converter could affect torque negatively.. How have things been since the exhaust updates?
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No problems with cat removal or lack of back pressure. I got some advice from a GM engineer prior to removal of the cat. There are no O2 sensors behind the cat so there were no problems with the computer. Exhaust flow is much better, but like I said before "its a bit loud for a Cadillac". I haven't reinstalled the cat yet, but I guess I will to tone it down a bit. A resonator will come next if the cat dosen't give me the sound I want.
Allantes did not have dual exhaust or dual exhaust pipes. I had to use a cut out developed by an Allante owner to adapt to the rear bumper. I let my local Cadillac dealer install the cut out and while the bumper was off, I had them re-paint the bumper. End result was a factory look. I have had several complement on it from friends and guys at the dealership. Cadillac should have made them this way from the factory. Many other Allante owner use a Camaro muffler with a single in, dual out configuration. I wanted two mufflers and the Borlas were the right size and price. True duals on an Allante can be done, but with great difficulity. There is not much room under the Allante for dual pipes and the exhaust manifolds merge into a single pipe that has to cross over the transmission.
You gotta love that Borla Growl. Sounds best at WOT and I just love it when she hits those high notes at 5000-7000 RPM. I really wanted the Corsa exhaust, but they were not too willing to work with Allantes. They do sound awesome on other Northstars. Try there web site for a listen.
