$970 Borla Paper Weight

M1ck3y · October 9, 2020

Do you have any information stating the gm borla dual side exit is supposed to be 3 1/2? It splits after the muffler right, so maybe it doesn't need it. As far as the gap at y-pipe connection goes, just loosen up your clamps and pull some pipe out of the flanges on the muffler. Your supposed to leave it loose and start from the y-pipe back when installing.

I had the same issue getting my corsa to line up with my kooks y. Its a flex pipe pull on it, force it tight and clamp it up.

Edited October 9, 2020 by M1ck3y

reardiff · October 9, 2020

19 hours ago, M1ck3y said:

Do you have any information stating the gm borla dual side exit is supposed to be 3 1/2? It splits after the muffler right, so maybe it doesn't need it. As far as the gap at y-pipe connection goes, just loosen up your clamps and pull some pipe out of the flanges on the muffler. Your supposed to leave it loose and start from the y-pipe back when installing.

I had the same issue getting my corsa to line up with my kooks y. Its a flex pipe pull on it, force it tight and clamp it up.

No that's what I'm looking for is something that says if it's supposed to be 3 or 3 1/2. I've searched all over and can't find anything, dealer parts dept. didn't know either. When I go to Boral's site directly and search for their catback systems (non GM) it does say 3.5 inch. It does split after the muffler so maybe that's the reason I believe it's 2.5 dual out. The dealer did get it to bolt up and I specifically asked them to verify the size is right first, so right size or not it's on now.

On 10/8/2020 at 4:09 PM, LDM said:

I can't speak to the GM performance exhaust but I just put a Borla Touring with dual rear exit on my truck. The pipe was 3.5" on my Borla for the flange that mates with then end of the catalytic converter. The 5.3 is suppose to only have the 3" flange, which I found out when trying to buy a new flange clamp since mine broken when trying to get it off. I got the 3" clamp first not knowing there was a difference and then had to go back and get the correct 3.5" clamp for my truck. So my guess is you were send the 5.3 version of the exhaust since it has the 3" flange.

The OE clamp was reused, not sure what size it was but didn't have to get a new one. Not sure about the flange size but the OD of the pipe after the flange all the way up to muffler is 3 inch. The stock piece that came off was 3.5 inch which I still don't get how making it 15% smaller would be of any benefit. One would think the Catback would be same size if not larger for increase flow, but maybe because it's mandrel bent and has duel outlets on the muffler it doesn't need to be as big?

Beamie · October 11, 2020

There are very talented welders who could make up an adapter without a second thought. I know one such fellow who has even made body panels but he's not near IL, but they do exist. Hot rod/restoration shops have some talented people.

Edited October 11, 2020 by Beamie

reardiff · October 13, 2020

Completely agree there's lots of very talented exhaust shops and welders out there. But the whole point was to not be going from stock 3.5 inch down to 3 inch. That is quite backwards from the 'upgrade' to cat back system.

Also for what it's worth, last day or two since getting it on mpg has gone done substantially. Down from 15 - 16 in town to 12ish. Now I know most will say this is normal because your using your on the gas more and have different driving style to hear it, but I have been very careful to make sure my 'before' and 'after' driving style is the same as much as possible. Same routes, same idle time, RPM's at take off, coasting to stop signs, etc... Maybe it needs more time to break in and the computer to learn?

Again making me wonder if the 3 inch isn't' really to small and should have been 3.5. It's really messing my mind now.. It's messing with my mind now just not knowing and now seeing the MPG drop but maybe I should just forget about it, drive it and enjoy the nice sound.

M1ck3y · October 14, 2020

For what its worth, I can tell you that the exhaust in the pictures is definitely 3" up to the muffler. 3.5" exhaust looks fat, 4" exhaust just looks ridiculous.

https://accessories.chevrolet.com/product/62l-cat-back-dual-side-exit-exhaust-upgrade-system-by-borla-associated-accessories-19303345?year=2017&make=Chevrolet&model=Silverado 1500&modelId=491&body=Crew Cab Pickup&bodyId=22&wheel=Short Box&wheelId=132&trim=LTZ&trimId=478&drive=4WD&driveId=8&engine=6.2L V8 GAS&engineId=5920&bodyNumDoors=5&categoryId=98022

But... it could be a picture of a 5.3's exhaust, lol. Your new exhaust has less restriction then stock regardless. Stock exhaust isnt 3.5 the whole way anyways, or at least on mine. It goes from 3.5" to 3" (muffler/valve), back to 3.5" and then down to 3" after the resonator. And that with a restrictive muffler and a single pipe.

Edited October 14, 2020 by M1ck3y