Go Catless!

[Kevin O'Connell]

Has anybody gone Cat converter free? Is it possible to run all new 2.5" pipe back from the stock manifolds? I've got a shop that will do it- Kansas doesn't require annual inspection, so as long as I give them a waiver of liability, they'll do it. Do I need to include an X pipe? More than one?
What mufflers to use? Can this be engineered to sound good but not be a problem at steady state highway cruise speeds? Any idea on performance gains? Any negative efeects- gas mileage, fuel/air mixture?
Has anybody done a ram air set up for the MK VIII?

 

[whiplash15]

First things first. Going catless is unlikely because the left cat is part of the manifold. Second, unless you install a chip, burned to eliminate the cats, the car will be unlikely to run due to the position of the O2 sensors. Third, if you disconnect or eliminate the O2 sensors, that must also be included in the chip burn. Fourth, if you are running a stock engine, no cats and 2.5" pipe, that will kill the torque curve and make your car a high dollar dog. Nuff said?
Many people have eliminated the 3rd cat. Some have replaced it with a dual in dual out mufffler. Others have just used a crossover. Others have not used anything. Most have used a crossover to eliminate drone and their choice of mufflers to replace the stock ones. Magnaflows are recommended.
Rather than eliminate the cats, some guys prefer to take probes and break out the innards of the cats and start their mods at the "X" pipe or dual in/out muffler.
I suggest (and this is by no means the only way) that you leave the 2 front cats alone, leave the O2 sensors alone, replace the 3rd cat with the longest 2.5" dual in/out magnaflow you can get and then run 2.5" pipe back to the tips. If it is too loud, you can then add a couple of cheap bullet mufflers.

 

[unity]

Huh?

First yes you can use stock manifolds, all you have to due is cut the pipe in from of the driver's side cat, there is plenty to weld to and room to do it.

Second you will need to have just the primary, first, O2 sensors installed. And exhaust shop can put a bung in for those to install, not a problem, but you need them since the primary O2s aid in fuel/air mixture.. The secondary O2 just see if the cats are doing there job and are not needed. You will get a CE light and a chip can take care of that or there is a cheap mod you can do to trick the car into thinking all is well.

The only thing is the 2.5" pipe, its a little bigger than needed and you might lose a LITTLE torque. You will need an x-pipe, h-pipe, something to balance it out.

 

[turborich]

I agree with whiplash15, leave the front 2 cats alone. I might replace the 3rd cat with a high flow 2 in-2 out but I would not ditch it all together. You won't gain much if any power by dumping your cats plus your exhaust will stink.

 

[Kevin O'Connell]

replies

OK, so we pull the third cat- does this leave a place for the 3rd O2 sensor and is it effective? Do we install the X-pipe right behind the first two cats? The x-pipe is a crossover and balances pulses-does this ensure no droning at highway speeds? Do we run individual pipes to two mufflers and then over the axle and out? Or is the best location behind the axle?
Will the exhaust mods force a chip change?
What size pipe should be used for the primary?
Thanks for all the replies!

 

[unity]

There is no third O2 on this system. The third cat is "unmonitored".