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  #1  
Old March 9th, 2007, 08:08 PM
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Mike
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Another Exhaust Question

I need new cats and O2 sensors. I went to a local muffler shop to quiz them about fab'ing this and they tell me I need OBDII cats, one of which needs to be offset just like the OEM one. I also seem to have some leaking b/t the exhaust manifold and the down tube. I was wondering if I should replace the whole thing by fab'ing a new Y-pipe or use the original which is solid and without rust. They also told me not to get O2 sensors that I have to splice together. However, I have had good luck with this in the past. What are your thoughts? Do I need a cat with offset input and output? If not then I assume I need to fab a new y-pipe. Is that correct?
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  #2  
Old March 9th, 2007, 10:32 PM
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The spliced together sensors should be fine if the connection is good. I'd highly recommend soldering and sealing with a good heatshrink tape, when you just crimp them together is when you get problems.

The offset input/output situations when it comes to MOST exhausts has more to do with just getting the things to physically fit under the car, and has very little to do with how well they perform. In this case I'd recommend just getting a new Y-pipe with the cats pre-installed, as it will just bolt right in and you don't have to worry about fitting issues and all the work. It's been a while since I bought mine off E-bay, but I think I payed around $500 or so for the whole thing, which has the correct holes for the pre and post-cat sensors.

-Hans
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  #3  
Old March 10th, 2007, 12:53 AM
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Thanks Hans. i was trying to go with some high flow cats while I was changing them. What about them being OBDII vs OBDI? I didn't know cats were made to match the computer. or am i way off on this?
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Old March 10th, 2007, 05:58 PM
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I've also never heard of that either, I know that an OBD-II exhaust requires the extra post-cat sensors to monitor the condition of the cats themselves, but I wasn't aware that the convertors had anything different to them. First I've heard of it. But then again, I don't play around with many OBD-II vehicles on a regular basis.

-Hans
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  #5  
Old March 10th, 2007, 09:09 PM
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Most OBD-II systems have air pipes running to the Cats themselves. This increase in O2 inside the Cat allows more thorough burning of the excess fuel vapor and decreases emmissions further. To my knowledge, there is no other difference in newer Cats.
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  #6  
Old March 10th, 2007, 09:58 PM
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My buddy cut in and installed high-flow cats from Summit Racing, on his 98 Disco, and they were pretty cheap and functional.
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Old March 10th, 2007, 10:06 PM
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The air injection has been around for quite a while actually, since the late 70's at least, in some applications. Sometimes it's done at the cat itself, but I've also seen it done in the exhaust manifold too. It's one of the things you need to match with the cat you're taking out, but I don't believe any NAS defenders ever had them.

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  #8  
Old March 10th, 2007, 11:28 PM
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Rover calls it "Secondary Air Injection" and it was on some of the Bosch 4.0/4.6 Range Rovers and Some DiscoII's. Rover's design didn't inject it into the cats themselves though...the pipes attached to the heads. You would see two U shaped pipes, one on each side, and there is a big air pump on the top front of the engine. It is a huge PITA to work on too b/c the heads are aluminum so the pipes don't generally want to come out of the heads very easily. I have never seen or heard of a system that actually forced air directly into the cats though.
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