Hollowing out Cat Conv. vs. Test Pipe Method

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Dec 31, 2008 | 10:52 AM

WillEE

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From: Albuquerque New Mexico

Hollowing out Cat Conv. vs. Test Pipe Method

I'm wanting to do one or the other method:

1. Hollow out my original catalyitic converter by busting out all the innards.

2. Cut the ends off of the existing converter and weld them on to a piece of schedule 40 - 2 1/2" pipe.

I ask that we please not focus on if this is technically legal or not, etc. (like the threads I've reserached on the topic) cause that debate/discussion is not really necessary (let's just pretend it's for a "race car" and call it good).

What I'm really interested to know through the experience of othersis if the hollowing out method sounds tinny when compared to the test pipe method. I don't want a tinny sounding exhaust note but would prefer to bang/drill/etc out the material since it would be the most easiest method and wouldn't cost a dime or require additional fab work (which I'm not opposed to if would result in the exhaust note sounding less tinny). The car currently has stock exhaust elswhere w/excetion of Magnaflow mufflers.

Has anyone done either of these methods and been satisfied or dissatisfied?

Dec 31, 2008 | 11:19 AM

noonie

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From: Florida

Didn't notice much change in sound at all, not even louder.
I did it for possible flow issues.

Dec 31, 2008 | 12:00 PM

jb78L-82

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From: Tennessee/Rhode Island

First off, I appreciate you asking no comments on the whole legality issue (jay walking is illegal too)! I put in a test pipe in 1983 on my y pipe system with aftermarket OEM type mufflers and the change in idle rumble was very noticeable and not tinny at all while running although I think those mufflers were very restrictive. My friend (ESU on the forum) has an 82 and just put in a test pipe and tells me that it is MUCH louder on his 82 with the test pipe. He has the OEM original 82 mufflers which I think were much more free flowing than the 78's. The 82's cat I think is much more free flowing than the 75-79's which were VERY restrictive. Hope that helps!

Dec 31, 2008 | 12:25 PM

Reggie Dunlop

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I have done both. On a basically stock '77 L48 auto (re-worked q-jet, recurved dist. no air pump, 1.6 rocker arms, flowtech afterburner muffs) I punched out the factory cat and epmtied it. The exhaust note did not change and 1/4 times and MPH were unchanged. Next I cut the flanges off the cat and welded them to a straight pipe, making a "test pipe". The exhaust note actually sounded healthy and there was a noticable increase in throttle response. At the track the car went 14.85 @ 92 mph compared to 15.01 @ 91. If you do the math on a 3550# car that's about 8 to 10 horsepower - which is confirmed by the one and a half tenth decrease in E.T. Plus I picked up 2 mpg on the freeway!

Dec 31, 2008 | 12:35 PM

Rich N.

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From: Upper Darby PA

Hollowing out cat vs test pipe

did not hollow out cats, but put test pipes on two ' 81s before de-smogging both. they had a slightly deeper sound. only the ZZ4 had better speed. the other with a semi-stock motor seemed to be about the same.

Dec 31, 2008 | 04:02 PM

WillEE

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From: Albuquerque New Mexico

Quote:

Originally Posted by Reggie Dunlop

Wow - There you have it! This tells me I'd be much better off fabbing up a test pipe. On top of this data I suppose it wouldn't hurt to preserve the Cat and toss it in the original parts pile for my Grandson to sift through in about 45 - 50 years from now (God willing).

Thanks everyone - I appreciate all the feedback.

Dec 31, 2008 | 07:39 PM

MN-Brent

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From: Minneapolis, Mn USA

I have the test pipe and have to say as compared to the untouched 82 cat, is much louder both at idle and on acceleration. Cruising is unchanged though.

I have the Monza exhaust muffflers = 2 tips coming out of each muffler.

Brent.

Jan 2, 2009 | 11:14 AM

1981Z06Vette

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From: Woodstock GA

I have gutted the cat, and reinstalled it, and shoved an appropriate diameter pipe through the center of a hollowed out cat and welded it up. While I can't say that I noticed a real performance difference either way (granted I have only done this on later models than your Vette, which had better convertors to begin with), both were noticably louder. If you just gut the cat, you may get a strange resonance/reverberation, so I would put the pipe through it, or fab a test pipe if you aren't worried about your "race car" looking legal. Or, just use a newer high flo cat and a cutout.