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TL;DR - side mufflers and center mufflers are all basically the same.
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Some minor differences internally from what I've seen online, but I've now ran 991 GT3 center muffler with TPC side deletes, and as of today run 991 GT3 side mufflers with a Sharkwerks center muffler delete. Valves even seem to work better, although that's probably because the mufflers I replaced with have far less miles/use on them. The only difference is the bracket that mounts the muffler to the car, but the bracket is attached to the mufflers in the same exact way, so just take the brackets off of your old muffler and voila.
Not terribly important information, but for anyone that experiences a failure of some sort (maybe the flaps, or a flex coupler crack like on my side mufflers) on their 997 exhausts and sees a 991 one for sale, just go ahead and grab those instead. Probably pay a little less for the mufflers themselves that way too, since its a newer car.
No comparison pics of the center mufflers, but just speaking from experience.
Are the exhaust tip lengths and mounting brackets different?
Yes. 991 muffler tips protrude a little bit further past the bumper than 997 ones, but not to the point that it seems unnatural. I am currently running 991 muffler tips on my 997 right now since to me it looks more "normal" with the larger 991 exhaust tip diameter.
Mounting bracket differences are show in the last picture of my multi-image IG post linked in original post (there is a little circular right arrow you can click to swipe through images). Everything is identical except the bracket that connects the muffler to the car, which are different on 997.1, 997.2, and 991. The brackets all use the same mounting point on the body of the muffler though. I simply transferred over my 997.1 brackets on to the 991 mufflers, which is why in my posts the 997 and 991 mufflers are using the same brackets (I had already transferred the bracket over from a spare set of trashed 997 mufflers I had).
Here's the aforementioned image re-posted though. Same exact bolt through design, same exact dimensions, same exact valve placement and function, everything other than those brackets are the same. Furthermore, from my looking through Porsche diagrams, I believe that half of the 991 bracket is interchangeable with the 997.2 (so on the bracket shown on the left that is not attached to the muffler anymore, the half with the long stud running through on the right should be 997.2 interchangeable, but not the left half that kind of looks like a Motorsports Cup wing upright. Probably saves someone from having to obtain half a bracket.)
Yes. 991 muffler tips protrude a little bit further past the bumper than 997 ones, but not to the point that it seems unnatural. I am currently running 991 muffler tips on my 997 right now since to me it looks more "normal" with the larger 991 exhaust tip diameter.
Mounting bracket differences are show in the last picture of my multi-image IG post linked in original post (there is a little circular right arrow you can click to swipe through images). Everything is identical except the bracket that connects the muffler to the car, which are different on 997.1, 997.2, and 991. The brackets all use the same mounting point on the body of the muffler though. I simply transferred over my 997.1 brackets on to the 991 mufflers, which is why in my posts the 997 and 991 mufflers are using the same brackets (I had already transferred the bracket over from a spare set of trashed 997 mufflers I had).
Here's the aforementioned image re-posted though. Same exact bolt through design, same exact dimensions, same exact valve placement and function, everything other than those brackets are the same. Furthermore, from my looking through Porsche diagrams, I believe that half of the 991 bracket is interchangeable with the 997.2 (so on the bracket shown on the left that is not attached to the muffler anymore, the half with the long stud running through on the right should be 997.2 interchangeable, but not the left half that kind of looks like a Motorsports Cup wing upright. Probably saves someone from having to obtain half a bracket.)
Great info. My apologies, but I meant the exhaust tip backing plate, not the mounting bracket. Is that the same across all models as well? Also, what diameter tips are you running? The outlet on the 7.2 RS is definitely smaller than the 991. Don't want to melt the surround piece.
Great info. My apologies, but I meant the exhaust tip backing plate, not the mounting bracket. Is that the same across all models as well? Also, what diameter tips are you running? The outlet on the 7.2 RS is definitely smaller than the 991. Don't want to melt the surround piece.
Ahhh, you mean on the tips themselves where it gets bolted to the center muffler. I have not compared 997 and 991 tips side by side but they have fit under my "stock" bumper (OEM Euro bumper actually, but they're the same thing sans rear end crash bumperettes) with zero issues. I believe stock 991 tips are 88mm-ish.
Edit: Found an older picture taken by a friend (pre-clearbra), showing the tips. t
Last edited by LateBraking; 04-11-2019 at 02:11 AM.
Thanks for sharing. What about headers? Do you feel aftermarket solutions are close enough to cross shop among 997/991 platforms?
I’ve never had the opportunity to compare both so even though they look similar I’d want to compare those side by side rather than take a leap of faith like I did with the side mufflers, haha. I do have my stock headers sitting around so if there’s any 991 fellas with stock headers lying around....visually they do look pretty similar though.
Thanks for sharing. What about headers? Do you feel aftermarket solutions are close enough to cross shop among 997/991 platforms?
The 997 Metzger engine in all 997 GT3's use a different header than the 991's (9A1 engine block) - headers are the only piece that don't interchange as the side mufflers/center muffler/tips do.
Good day
got a question for central muffler of 991 Gt3/Gt3rs part number 99111102790 Titanium ,will it fit my 997.2Gt3RS (2011 model) ? as i have read that mostly all the part in exhaust system are interchangeable in 991 and 997.2
feed back will be helpful
Was advised today my mechanic that central muffler from 991 gt3/rs ca not be fitted to 997.2gt3rs
still has doubt on his ability ))
anybody has done such installation ?
the main concern that mechanic does not have original 991 Gt3 center muffler ( part number 99111102790 ) , so he is comparing to Dundon exhaust that he has and Dundon and mine from 997.2 Gt3RS bolts are in the very different place
i have share the photo and you help will be useful
He is skeptical on 991 Gt3 exhaust center muffler will fit to 997.2
photo below 997.2 Gt3RS left side and Dundon from 991GT3 right
04-10-2019, 10:47 PM
PSA: 991 and 997 GT3 muffler interchangeability.
does not have original 991 Gt3 center muffler ( part number 99111102790 ) , so he is comparing to Dundon exhaust that he has and Dundon and mine from 997.2 Gt3RS bolts are in the very different place 
