'00 xj performance questions...

[robsjeepxj]

If I had a 2000-01 w/ the cali package, I'd do exactly what you planned to do and ditch them underhood bakers and sim out the precat sensors(you can y the output of one), but leave the main upstream and downstream sensors.

Dumb azz. My gf's 97 TJ ran like crap,had zero power and smelled horrible when we first got it. It turned out the cat had been hollowed out and 02's were freaking out. Long story short replaced the cat and it's like a new jeep, runs great, big improvement in power and MPG, no more codes and no more burning your nose smell too.

 

[Talyn]

Even if the down stream O2 sensor was missing, or reading exhaust without a cat it wouldn't cause the engine to run any different then with a properly working cat and DS O2 sensor. You obviously had some other issues.

 

[srmitchell]

Talyn, you know better than I do, I need to replace my 02 sensors (I have 4), are the 2 upstream sensors the ones that affect mpg?

 

[Johnnie Walker]

Talyn, you know better than I do, I need to replace my 02 sensors (I have 4), are the 2 upstream sensors the ones that affect mpg?

Yes, the upper O2's are the ones that the computer registeres for performance/mileage.

 

[Talyn]

Correct, the upsteam ones(closest to the head) are the ones that the engine uses to determine fuel trim.

 

[CherBear]

Correct, the upsteam ones(closest to the head) are the ones that the engine uses to determine fuel trim.

Correct me if I am wrong, but the ones AFTER the cat are only their to check if the emissions system is working properly. Has nothing to do with performance or fuel injection.

 

[Talyn]

Correct me if I am wrong, but the ones AFTER the cat are only their to check if the emissions system is working properly. Has nothing to do with performance or fuel injection.

:doh:

 

[dieselenthusiast]

Cherroarkee, which header did you go with? Have you installed it yet?

 

[88JeepXJ]

Proof of this?

Proof that freeing up the flow in an exhaust system will increase horsepower? Yeah, I think proof has been shown before...once or twice.

 

[88JeepXJ]

:doh:

What? That's a true statement. Post O2 sensors are there to read how well the cats are doing at removing hydrocarbons.

Listen I don't recommend removing emissions equipment for an on-road vehicle, or any vehicle if you can help it, but the truth of the matter is that the more stuff you stick in the flow path the less peak power the engine can make. Simple proof of this is that in '99 when Jeep had to start sticking more cats on the 4.0L they revised the intake manifold to make up for the power decrease.

 

[CherBear]

:doh:

Confused as to why you posted this. Maybe an explanation would be more helpful?

 

[hubs97xj]

The down stream O2 sensors contribute no information to performance or the air-fuel ratio. The PCM gets data from them just to verify that the two precats are working. Siming them out will not do as you described.

However, the 4.0L PCM just looks at them to see if the cat is working or not.

Even if the down stream O2 sensor was missing, or reading exhaust without a cat it wouldn't cause the engine to run any different then with a properly working cat and DS O2 sensor.

Yes, the upper O2's are the ones that the computer registeres for performance/mileage.

Correct, the upsteam ones(closest to the head) are the ones that the engine uses to determine fuel trim.

Correct me if I am wrong, but the ones AFTER the cat are only their to check if the emissions system is working properly. Has nothing to do with performance or fuel injection.

:dunno: Maybe 5 times wasn't enough...

 

[Milford Cubicle II]

...but the truth of the matter is that the more stuff you stick in the flow path the less peak power the engine can make. Simple proof of this is that in '99 when Jeep had to start sticking more cats on the 4.0L they revised the intake manifold to make up for the power decrease.

They only added cats to XJ's with CALI emissions. And the reason they revised the intake manifold was to make up power from the necked down exhaust ports in the 00-01 heads. They restricted the exhaust ports to try to clean up NOX emissions.

But you're right, the less obstacles in the exhaust the better as far as performance goes.

 

[casm]

:dunno: Maybe 5 times wasn't enough...

Valid point as it applies to the CA-emissions models - but how does it apply to Federal-emissions XJs which only have two O2 sensors?

I've got an idea of what the answer likely is, but would be interested to hear feedback before taking that jump.

 

[scoobyxj]

Later model vehicles have free flowing cats on them from the factory. The only reason I can see removing it is if it was damaged in some sort of way. Every vehicle I have removed the cat on has run worse than what it did with it. The best way to increase performance if your 2000 is to replace the after cat system with a quality piece. Even just replacing the muffler with a free flowing design will improve the get up and go of it. 2000 have the LP Dana 30 front axle so the down pipe isn't crimped like the the pre models. I don't see an atvantage to a header unless the original is cracked either.

 

[badron]

don't see an atvantage to a header unless the original is cracked either.
==================================================
TRUE: If your running a real muffer and cats there is not to be gaind with headers. The stock cast flow just fine (cali spec 98 to 01) and LONG tube headers are even more of a joke in front of a quit poop pipe. With long tube headers you would need a main pipe or runer of 42 to 48 inches inorder to gain anything at or near stock RPM. But remember that is ONLY ture with an OPEN HEADER start tacking much stuff on behind them and tuned header just don't work.
Headers did help in the OLD days with a stock or near stock system. When stock casting sucked MAGORLY. The old "M" type Chevy small block manafold is a prime one to look at.
LONG gone are them days. Factory casting (including the Jeeps) have come a LONG way and not or very little is to be gained with headers on an near stock system.

 

[DoMiNiC]

Correct me if I am wrong, but the ones AFTER the cat are only their to check if the emissions system is working properly. Has nothing to do with performance or fuel injection.

This was the case on my SCCA turbo race only car. No cats, 3in trubo back, front were fine, the rear had a little metel ring (o2 delete) around the sensor that kept it out of the exhaust flow. NEVER through a code and picked up 22hp at the wheels going from high flow to no cat. I was running a tdo620g turbo, 25psi, 660 injectors, S-afc tunned, on a 2.0 4g63 in a 97 gsx eclipse. Not sure if this helps... Just my .02. Car was obdII BTW

 

[CherBear]

This was the case on my SCCA turbo race only car. No cats, 3in trubo back, front were fine, the rear had a little metel ring (o2 delete) around the sensor that kept it out of the exhaust flow. NEVER through a code and picked up 22hp at the wheels going from high flow to no cat. I was running a tdo620g turbo, 25psi, 660 injectors, S-afc tunned, on a 2.0 4g63 in a 97 gsx eclipse. Not sure if this helps... Just my .02. Car was obdII BTW

I assume there was some sort of computer tuning or O2 senser fooler type deal?

 

[DoMiNiC]

I assume there was some sort of computer tuning or O2 senser fooler type deal?

I was tuning with a S-AFC (Super air flow converter) Made by apex-i. How ever the "fooler" was a shroud that went around the rear o2. I also had a wide-band 18 inches down stream to keep my tune in check. But the tuner I used was simple, it was a piggy-back that controled the air flow measurement that the computer saw coming from the MAF. Thus making it richer or leaner depending on your settings. But the point is that the rear o2 "fooler" was just a simple ring that went around the o2, it worked well, and for the most part or at least as it relates to the rear o2, was on stock ecu.

 

[wheeling365]

I've seen these around from back in my 5.0 mustang days. http://www.o2simulator.com/?gclid=CLGJiMGqgKYCFYnd4Aodqn_Dmg


On my 97TJ Chrysler had a ton of drivability issues when they first came out due to the post cat o2. They had to reflash my ECM at the dealer with a new program to "fix" it back then. The major change was that they rewrote the post cat o2 sensor out of the system. I made a huge difference at the time. Years later my converter started breaking apart so in an effort to fix it until I could affort the right parts I gutted it completely. Never set a code at any point before, during or after the cat issues I had.