JaGa

As some of you know I had sluggish performance with my 99 e55 so I reset the fuel trims and disconnected the pre-cat O2 sensors and that seemed to have solved the crappy performance and I've gained like 4-6 MPG, now my MPG is running in the low 20's. Poking around with this set up I've seen the gambit from people predicting doom and gloom to those saying no problem.

For those of you predicting doom, why?
As far as too much fuel in the cylinder goes that makes no sense to me as I'm getting better fuel mileage which leads me to believe, jump of logic here, I'm using less gas than before so I have less fuel intrusion into the cylinder to affect oil and what have you.
As far A CEL goes, don't really care.

One thing I do wonder about is that as I've been diggin around the previous owners dealer maintenance invoices I have found a recommendation to replace CATs and the guy decided not to. Now would a bad primary, or secondary for that matter, CAT create a false rich reading for the upstream o2 sensor, or in any way affect the ECU fuel trim and contribute to the engine compensating with the lean trim setting.

I would honestly just keep the car as emissions legal as possible but have no problem running it like it is with just x-pipe conversion and maybe high flow cats, if there bad anyway.

JaGa

I'm trying to price out the replacement cats. what is the diameter of the inlet of the primary cats. I know its actually like 60 mm so 2.4 or something like that. So would it be better to buy 2.25" or 2.5" pipes.

snooozie

I took my secondary cats out and the car runs fine. I only left the two primary cats in the car. I just got 2.5" pipes to replaces the second cats. I replace the resonator which weights a ton with a x-pipe from ebay also 2.5"