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Tallboy
05-19-2006, 07:10 AM
So, let's say I have a car with a bone-stock exhaust system, but the tune it's running has both rear O2s shut off. Let's say, for the sake of argument, I was to torch off both rear cats. Would I trip a check engine light? What should I do with the O2s? Will the exhaust note change?

Mind you, I would never do this. Purely theoretical.

TAF, you out there?

greggash
05-19-2006, 07:26 AM
Hi Tallboy;

If the O2's are off, I am sure they are. Then you can rip out the Cats all together......
there are 4 cats on our cars, taking out 2 will enhance the flow dramatically
ask magindat
The note stays the same, its just a bit fuller

Gregg

Smokie
05-19-2006, 07:27 AM
So, let's say I have a car with a bone-stock exhaust system, but the tune it's running has both rear O2s shut off. Let's say, for the sake of argument, I was to torch off both rear cats. Would I trip a check engine light? What should I do with the O2s? Will the exhaust note change?

Mind you, I would never do this. Purely theoretical.

TAF, you out there?

Lets, say in theory I have 2 cats (front ones) only and my tune has the rear sensors shut off, I would not trip an engine light at all, there would be a little less back pressure.:D

If all else in the system remains stock, I would not expect a significant difference in sound or power output...the biggest bottleneck are the exhaust manifolds.

Hotrauder
05-19-2006, 07:33 AM
I don't know Chuck but the Cobra exhaust system on my car from DR uses only one Cat on each side as you know. All 4 02 sensors are connected but I have had no check engine light even tho the guts of the left cat broke directly in half, 2 pieces, one in the right muffler entrance and the other in the right pipe just before the muffler. The X pipe sent the pieces to the other side. There is no carbon, no burning or melting and no damage, not even a scratch; to the casing. I have no idea how long this condition has existed and no indication that there was a problem. So, in effect there was no cat on the left side. I don't know if this info helps with your question but I appreciate the chance to :hijack: jump on board. This is what I pay you for, right. I am guessing that with the tune the rear sensors are off and so the magic box doesn't know the difference. Thanks. Dennis:)

TAF
05-19-2006, 07:39 AM
TAF, you out there?

Yep....


So, let's say I have a car with a bone-stock exhaust system, but the tune it's running has both rear O2s shut off. Let's say, for the sake of argument, I was to torch off both rear cats. Would I trip a check engine light? What should I do with the O2s? Will the exhaust note change?

Mind you, I would never do this. Purely theoretical.

No...it wouldn't throw a code if the rears (O2s)are disabled. However, I'm not a big fan of "hacking/torching" of main CATS to run through pre-CATS. Just get you someone to replace the 4 with 2 Maganaflow High-Flow CATS. (Preferrably 2 1/2" to increase flow).


Will the exhaust note change?

Yeah...your car will have the tendancy to want to pull into every Government Housing Project on Nebraska Avenue in Tampa... :P :D

^^^AKA "Ghetto-Fabulous"

Zack
05-19-2006, 07:53 AM
If you ever run your car with open headers and just the front 02's, you will have to lean the fuel tables out 50 % to keep the car running.
The car runs pig rich cause the o2's cant heat up so it thinks the car is lean and dumps tons of fuel into the engine.
Ask me how I know!

magindat
05-19-2006, 09:34 AM
I hacked the rear cats. Unfortunately, I can't report on just that. At the same time I had a custom 2.5" mid section with a 2.25" H pipe. I couldn't break the 20's loose when I went in. I scared myself when they smoked as I left!

Dropping the second cats works. It's cheap. Maybe it's ghetto. But, I love my exhaust note now. It makes all the honeys look! It makes all the machos scared to run!

Oh yeah, no codes on a DR base tune.

Rider90
05-19-2006, 09:46 AM
If you ever run your car with open headers and just the front 02's, you will have to lean the fuel tables out 50 % to keep the car running.
The car runs pig rich cause the o2's cant heat up so it thinks the car is lean and dumps tons of fuel into the engine.
Ask me how I know!
Will removing two rear cats alone cause a problem for the tune? should it get re-tuned? will there be a performance gain or just sound?

magindat
05-19-2006, 09:59 AM
Will removing two rear cats alone cause a problem for the tune? should it get re-tuned? will there be a performance gain or just sound?

I didn't get re-tuned.

I think you'll get a tiny bit more power.

Had me res-del's done at the same time, so I couldn't tell ya about sound with just hacking the 2nd cats.

BTW, it's not legal.

ctrlraven
05-19-2006, 10:03 AM
What about emissions testing? Im not sure how they do it in other states but here if you have an OBDII system they just plug in, check to make sure everything is connected and print you a pass letter if you pass. Now with the xcal units and going in and turning off the 2nd set of 02 sensors, wouldnt the emissions ppl pick up on that or would the xcal unit just mask them being turned off and they just get over looked? I've been waiting to find this out before I go to PA (no local MD shops will do it unless someone knows of one) cause of the fines that the motor vehicles ppl will charge them for doing it.

mrjones
05-19-2006, 10:07 AM
There was a member here that I believe is gone now, Muslhed, that had done that to his car. He'd lopped off the back two cats and replaced his muffler's with 18" Magnaflows. I raced him a few times at Hallsville when my car only had about 2K on it. He trounced me by a few car lengths, and was much stronger out of the hole than mine was. His only other modifications were SilverStars and the peanut butter mod. His car did sound GREAT though.

FordNut
05-19-2006, 05:50 PM
Be careful. Some states with emissions testing are getting more picky lately. OBDII cars get emissions testing done via the diagnostic port. They may be able to determine if rear sensors have been turned off and fail a car because of it.

MENINBLK
05-19-2006, 06:11 PM
OBD-II testing is just checking for Diagnostic codes in memory
and that all OBD-II emissions test monitors have passed.
If disabling the rear O2 sensors disables a monitor test,
then yes, it will be picked up.
If it doesn't, then you should still pass.