c63addix

Anyone know the proper sizes high-flow cats we need to replace the stock secondary cats?

will these sizes fit?

2.5" inlet-outlet
4" body diameter
11" length or 12" length?

Also is 200CEL a big difference compare to the 300CEL? I kinda want the 300CEL for less fuel smell unless the 200CEL has a much noticeable gain.

Thanks for your helps

BO Knows

Most people just remove the 2nd cats, kept the primary cats and replaced it straight or x-pipe. When I removed my 2nd cats, it only had a minor fuel smell. You also will gain about 5-10 hp by removing the 2nd cats.

CoolBDPhenom03

c63addix

Oh, I am really not sure how bad the smell is, just heard from others. But is it bad enough to smell it inside the cabin? Morning cold starts, will the whole garage smell like fuel? Thanks

I don't really mind the fuel smell, but gf is sensitive to it and hate it . It would also be much less hassle to just put an xpipe on.

nrgy

I think I'm one of the few running high flow cats (300cel if I remember right) with the long tube headers as most delete them, but even with the cats I noticed a bit more gas smell after the mods. No, it is not noticeable inside the cabin, and outside the car it doesn't bother me but there is a noticeable increase.

If you still have your primary cats in place, then I would say without hesitation just delete the secondary cats, because all of the above I referenced was without primary cats.

And I would say there is a minor hp loss with the cats, 5-10hp sounds about right as BO Knows pointed out. But at the track and in real world conditions as you can imagine, it's almost immeasurable on a 450-550hp car depending on mods.

c63addix

Oh thanks for the info and yeah I got my primaries on, do you know which set up would be better sounding, i don't really care for performance as I only drive around the city.

-Put my resonators back on and straight-pipe the 2ndary cats.
or
-Keep straight pipe in place of resonator and xpipe the 2ndary cats.
or
-Keep straight pipe in place of resonator and straight pipe the 2ndary cats. (I suppose this option will lose power, but will be be bad for the engine?)

Thanks

Matt47FL

Do you have a sound clip or a video of your car running? I am curious as to how much the 300 cell cats quietened the exhaust.

BO Knows

I had my 2nd cats removed and no smell in the cabin or with windows down. If you stand right behind the car, then you smell it a little. I recommend to remove your 2nd cats and replace it with the bolt on MBH x-pipe:
http://www.mbhmotorsports.com/C63-OEM-X-pipe.html

It will give you a bit more performance and better sound. If it's still not loud enough for you, then remove the resonator and straight pipe it. None of this will lose power, in fact it will only increase power and does no harm to your engine. Also no Check Engine Light.

DuaneC63

You should not get any gas smell if you remove your secondary cats. The primary cats do all the work needed to pass smog regulations. The secondary cats are there adhere to the 100,000 emissions warranty required in the US should a primary cat start to deteriorate but not to the point of throwing a CEL. If you're thinking of replacing you primary cats with after market a 100 cell cat of the same diameter will flow better but may throw a CEL. Europipe (Porsche exhausts) focuses on total catalyst surface area to produce a 100 cell cat which has the same catalytic surface area as a 400 cell cat. Hence you get the higher flow while still meeting the emissions standard. BTW If you want to know what real exhaust gas smells like, stand behind something with an LS7 crate motor in it, Holley dominator, headers, and straight pipes and remember not to smoke.