Well depends on the high flow cats and how much restriction is removed. I will agree with you in part Mutley as I know guy who have put on some extremely expensive cats for Neon SRT4s, Renault Clio Sports and Mustangs, some of whom go to my track and they will on some occasion pop something like a P0420 low cat efficiency code.

In addition, to really benefit from such cat, you need something going in, turbocharger, supercharger, nitrous, otherwise just a waste of money.

Most smart guys I know will just ditch the cat if they are running high hps and go straight test pipe, especially where I live, and then for emissions will just put the other pipe back in. The police aren't like they are in California checking for illegal modifications, because for engines there really aren't any, you're exhaust can be as loud as it wants.

I don't have precats anymore on my RX300, just a Magnaflow and when I tested, I always passed what were akin to California emissions standards here. I claim that not having those precats (as does exist on most 1MZFE Toyota Solaras) for example has probably extended the life of my A/F sensors, instead of creating a 'heating room' between the engine block and the top of my precats. And yeah, no CELs.

 

are you sure about that? I thought the random cats were obd2 compliant. or at least some of them....

 

What they say is one thing, what they do is another. Added to a stock vehicle, they should do their work, but sometimes with different engine modificiations, CELs can pop. However, this might go for any cat.

 

is thermal a better choice then magnaflow? is thermal louder then magnaflow? also would you leave resonator?