Cat Removal

[DC0046]

I have a 2004 4.0. Will the removal of the rear catalytic converter result in an increase in performance?

I have maybe, possibly, perhaps, theoretically done this in prior vehicles with possibly, perhaps, and theoretical positive results. Could the removal of the rear CAT in a 4.0 resul with an increase in performance? (Theoretically speaking).

 

[Jerry Bransford]

No, its restriction is insignificant. Despite some companies pushing "low restriction" cats that make some think the OE cat must be restrictive.

What size tires are you running & what transmission?

 

[DC0046]

No, its restriction is insignificant. Despite some companies pushing "low restriction" cats that make some think the OE cat must be restrictive.

Thank you. I know better sound is a matter of opinion, but do you think the removal will improve the exhaust sound from a flowmaster or magnaflow muffler?

 

[Jerry Bransford]

Thank you. I know better sound is a matter of opinion, but do you think the removal will improve the exhaust sound from a flowmaster or magnaflow muffler?

No. What size tires & transmission are you running? Are the axles still running the factory gearing?

 

[DC0046]

Tires are 285/75/16. It is a 2004 Rubicon with an automatic transmission with what I beleive are the factory axles. I am pretty certain gearing is still 4:10 since it lugs a bit at 60 on the highway.

 

[Jerry Bransford]

That tire size is roughly 33" and your engine is lugging with the factory 4.10 gear ratio due to that larger diameter tire.

It'd be very easy to pick up a dramatic performance improvement by regearing the axles to 4.88 which is the appropriate ratio for that size of tire. That would produce both much better highway RPMs and make it perform better offroad too.

 

[DC0046]

That tire size is roughly 33" and your engine is lugging with the factory 4.10 gear ratio due to that larger diameter tire.

It'd be very easy to pick up a dramatic performance improvement by regearing the axles to 4.88 which is the appropriate ratio for that size of tire. That would produce both much better highway RPMs and make it perform better offroad too.

Thanks Jerry. I will look into that. What would be an appropriate price to pay for parts and labor? Thanks again.

 

[Jerry Bransford]

I negotiated my last regearing job down to roughly $1087, my first was $1300. But it probably averages more like $1400-1500 now. Some shops will quote as much as $2000 and hope the customer will be inexperienced or naive enough to go for it. These costs are total parts and labor for both axles.

 

[DC0046]

I negotiated my last regearing job down to roughly $1087, my first was $1300. But it probably averages more like $1400-1500 now. Some shops will quote as much as $2000 and hope the customer will be inexperienced or naive enough to go for it. These costs are total parts and labor for both axles.

Excellent. Thanks for the info.......

 

[Chris]

You stand nothing to gain from removing the cat on a 4.0. Not to mention you'll throw off check engine lights which won't go away. You'll also probably slightly hurt the fuel economy, and cause the engine to not run optimally.

While it is possible on some high performance engines (i.e. not the 4.0) to gain a bit of power from removing the cats, the thing you have to consider there is that they're using updated software (i.e. an ECU reflash) to adjust the ECU for the fact that the cats aren't there anymore.

My BMW M3 was a good example of this. Upon removing the cats and getting an new ECU re-flash (which compensated for the removed cats), I gained a considerable amount of horsepower (though a good chunk of that was just the re-flash in general).

Unfortunately people don't re-flash our Jeep ECUs, so you'd be out of luck there anyways.

 

[Jeepaholic]

Not recommending removal of it, but there are no o2 sensors checking on the rear cats (only the front small cats are monitored). So no check engine light will occur. However, you will fail smog, visual and probably the sniffer as well...
I personally don't think the 4.0 doesn't spin up fast enough to benefit from exhaust modifications. Now days, mostly only turbo engine benefit.

 

[glwood]

There are three cats next door I'd like to remove...:buitre:

 

[DC0046]

Not recommending removal of it, but there are no o2 sensors checking on the rear cats (only the front small cats are monitored). So no check engine light will occur. However, you will fail smog, visual and probably the sniffer as well...
I personally don't think the 4.0 doesn't spin up fast enough to benefit from exhaust modifications. Now days, mostly only turbo engine benefit.

Thanks. Fortunately no smog tests in Oklahoma. Although, I am pretty certain this string is now on the CIA and NSA watch list.

 

[DC0046]

There are three cats next door I'd like to remove...:buitre:

Gotcha. Not a cat person myself either. Love Dogs......

 

[Nick Schnepp]

Thank you. I know better sound is a matter of opinion, but do you think the removal will improve the exhaust sound from a flowmaster or magnaflow muffler?

Contrary to others, yes they can sound pretty decent, not like a V-8, but like a hopped up tractor yes, less hesitation opened up too, I did cat knockout (all 3) Mbrp 2.5" stainless for $100 on e-bay, open air filter & smog delete. That was December & no codes.
Don't attempt it in California though strange advice & air there.

 

[Aquaman]

I'm currently removing my rear cat since it's confirmed clogged, I'm going to clean it out for a week and reinstall. How do you remove those headless bolts from the rear side of the converter??

 

[Gilaguy23]

Pull the cat and you'll choke on the fumes coming in the cab if you have leaky gate seals, a softop or the top off. Been there done that on a few. 0 and I do mean 0 power gains by doing it. An utter waste of time.