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Interesting. Thank you! I was hoping it would be more a performance improvement and possibly some weight loss at the back as well. I deleted the sound symposer on my 991.1S to avoid harsh sounds. So, your advice is well considered.
FWIW, @997 tt/rs I am running cat-delete Fabspeed X-Pipe and RSR modified stock mufflers. I am not nearly engineering savvy enough to understand all the dynamics that Bill explains, and I never view exhaust choices as doing anything materially more than: - improving sound, enjoyment, and modest performance gains, usually when coupled with other "breathing" mods.
My observations with this setup:
1) Sound is greatly improved over stock center section - and improving more over time with more cracks / burbles;
2) Absolutely an improvement in power over stock center section;
3) Not nearly as loud as I expected for a cat-delete setup;
4) Not nearly as loud as a friend's C4S on which I installed only Fabspeed muffler delete (Supercup) - that resulted in a much deeper / growly tone;
5) At a standstill, yes, the cat delete smell can be unpleasant, but just kick on the cabin fans / AC and it's almost unnoticeable;
6) I am debating adding Fister II because I could tolerate an even more aggressive sound, but may not do that . . .
I love and have had Cargraphic systems on other cars, but that is absolutely not how I would spend $8,000+ for this car. Dramatic overkill. If you haven't already refreshed all your suspension, spend that $$ there first and just try some cheap FVD cross pipes.
Of note - I have a full Akra titanium system on my 997RS, and it's glorious, but I would never have paid $15,000 new for that; I got a lucky deal on it. Simply no way to justify that $ when you can achieve the same end results with various other setups.
My 95 came with LPMM modded mufflers and a Fabspeed x pipe. It was v loud and sounded amazing but lacked low end grunt. I added the fabspeed 200 cell cats which helped with additional back pressure/torque, muted the sound a bit and eliminated the stink. Still sounds amazing but liveable.
My 95 came with LPMM modded mufflers and a Fabspeed x pipe. It was v loud and sounded amazing but lacked low end grunt. I added the fabspeed 200 cell cats which helped with additional back pressure/torque, muted the sound a bit and eliminated the stink. Still sounds amazing but liveable.
I want to try cats on my X Pipe + Fister IIs. I think that'll be my next combo. Fortunately the X Pipe is modular so it's dead simple to swap the cats in and out.
I have a set of cat delete pipes just like this - brand new unused and I'd sell for $300 + shipping - seems a little less than the going price from retailers.
They came with my supercharger kit (I got the kit 2nd hand but never used, along with a whole bunch of other brand-new parts - ignition cables, distributor, ignition coils, these cat delete pipes, etc). Because I didn't buy them from a retailer, I'm not sure about what brand they are, but they look exactly like these - polished stainless steel, and each pipe a single O2 bung with plug.
Last edited by CosmosMoon; May 25, 2025 at 12:54 PM.
Id probably go for the 54mm to keep gas velocity up, and they will be easier to fit.
If you were racing a 3.8 o4.0 w/ extended rpm the bigger ones might be more appropriate, but I doubt it.
If the system is to be used with a muffler, I think the ~ 10 mm transition from the bigger to the smaller diameter stock pipe in the muffler would not be great for flow ether.
If the system is to be used with a muffler, I think the ~ 10 mm transition from the bigger to the smaller diameter stock pipe in the muffler would not be great for flow ether.
true, 54mm is still on the large size for <~1900cc and <7krpm
Of the 993 RS with modified exhaust systems I have seen over the years in the UK, the swetspot seems to be fitting 200 cell catalyst convertors. This obviously improves the flow, but still gives you a chance of passing the annual exhaust emission part of our "Mot" test. No catalyst fitted is an instant fail, where as 100 cell cats fitted can be very marginal at passing the emissions test, particularly as they age. Everything is a compromise?
On a fully rebuild to 0 hours engine equipped with the so called "common mod" (muffler mod - with the turbo shaped upper perforated duct between the 90° curves) but also with larger diameter bypass duct inside the muffler as for the soundkit, with the addiction of HJS Hi-flow 100 cells motorsport cat inserts, the car passed the tests here in Italy with a large margin, being the limit 150(but I don't remember in what unit of measurement) and the measured value being 110.
I am now dealing with the complation of the running-in, but a thing I would like to do afterwords is the map of the engine, but I don't think is easy to find someone really expert in such "hybrid" engines, in the real sense, with a partially electronically and partially mechanically controlled engine management, hence I don't know how much of an improvement to expect, even if performed by someone really skilled.
Muddy inner side because of the river mud - but here it is the original shape of a n/a 993 without the kit sound
the river-mess, once the muffler were opened
here the final set-up, with larger diameter bypass and the turbo style mod to enhance the gas flow
rock wool layer
additional insulating element as originally present, the name of which I don't remember
comparison between original and Hiflow 100 cells cat insert
On a fully rebuild to 0 hours engine equipped with the so called "common mod" (muffler mod - with the turbo shaped upper perforated duct between the 90° curves) but also with larger diameter bypass duct inside the muffler as for the soundkit, with the addiction of HJS Hi-flow 100 cells motorsport cat inserts, the car passed the tests here in Italy with a large margin, being the limit 150(but I don't remember in what unit of measurement) and the measured value being 110.
I am now dealing with the complation of the running-in, but a thing I would like to do afterwords is the map of the engine, but I don't think is easy to find someone really expert in such "hybrid" engines, in the real sense, with a partially electronically and partially mechanically controlled engine management, hence I don't know how much of an improvement to expect, even if performed by someone really skilled.
Muddy inner side because of the river mud - but here it is the original shape of a n/a 993 without the kit sound
the river-mess, once the muffler were opened
here the final set-up, with larger diameter bypass and the turbo style mod to enhance the gas flow
rock wool layer
additional insulating element as originally present, the name of which I don't remember
comparison between original and Hiflow 100 cells cat insert
very very nicely done!!!
that lower crossover mostly makes it louder the bigger it gets, and quieter the smaller it gets
