High Flo Cats and CEL Emu's

[Afboy143]

I have a few questions

On the 99 4.0 SOHC, I believe there is 2 pre-cats and 2 cats. What I am wanting to do is cut out the 2 precats and on the reg cats replace them with magnaflow high flows. My question is, what would I do about the 02 sensors and what size piping should I get for the high flows?

 

[Jakee]

I have a few questions

On the 99 4.0 SOHC, I believe there is 2 pre-cats and 2 cats. What I am wanting to do is cut out the 2 precats and on the reg cats replace them with magnaflow high flows. My question is, what would I do about the 02 sensors and what size piping should I get for the high flows?

Keep the two front O2 sensors exactly where the're at.

Now the rear 02 sensor should be after the cat, I'm sure you know that. I've seen people actually move the location for the High flow cat without a problem.

What I planned is this. Take the Stock y-pipe off and brace it in a jig made from anything, just as long as it holds it in place. Maybe screw the top flanges to a 2X4 or something. That will hold the manifold side in place so when you cut out the pre-cats, it won't move and you won't have a problem putting it back in. Now, replace the cats with the same size pipe.

Then, I'm going to cut the flange off the bottom (With it still braced in a jig so It won't move). If you notice, you'll see that at the bottom of the pipes, just before the flange, the pipe actually closes IN to be able to go in the flange. Well, Mine does anyways. I'm going to cut this bottle kneck out of the pipe and put a magnoflow Y-pipe right there and get it welded up. Then add pipe (2-1/2") to clear the transmission/torsion bar crossmember, and add the high flow cat (1) (2-1/2"). Right after the High-Flow cat, add a O2 sensor "bung" and get it welded up. Now, this will move the location of the rear 02 sensor just a little further down the pipe but this should be okay. It's really the front two 02 sensors that you don't want to move as they control air fuel adjustments and such. The rear 02 sensor can actually be turned off with a tune, so that tells me it just reads if the fuel is being cleared by the cat.

 

[Afboy143]

Keep the two front O2 sensors exactly where the're at.

Now the rear 02 sensor should be after the cat, I'm sure you know that. I've seen people actually move the location for the High flow cat without a problem.

What I planned is this. Take the Stock y-pipe off and brace it in a jig made from anything, just as long as it holds it in place. Maybe screw the top flanges to a 2X4 or something. That will hold the manifold side in place so when you cut out the pre-cats, it won't move and you won't have a problem putting it back in. Now, replace the cats with the same size pipe.

Then, I'm going to cut the flange off the bottom (With it still braced in a jig so It won't move). If you notice, you'll see that at the bottom of the pipes, just before the flange, the pipe actually closes IN to be able to go in the flange. Well, Mine does anyways. I'm going to cut this bottle kneck out of the pipe and put a magnoflow Y-pipe right there and get it welded up. Then add pipe (2-1/2") to clear the transmission/torsion bar crossmember, and add the high flow cat (1) (2-1/2"). Right after the High-Flow cat, add a O2 sensor "bung" and get it welded up. Now, this will move the location of the rear 02 sensor just a little further down the pipe but this should be okay. It's really the front two 02 sensors that you don't want to move as they control air fuel adjustments and such. The rear 02 sensor can actually be turned off with a tune, so that tells me it just reads if the fuel is being cleared by the cat.

How many cats does your truck have? You have 4 O2's? Where are you 02's located?

My truck (99 SOHC) does not have a stock Y pipe, it goes two pipes off the headers and came into a dual in muffler stock. I have a magnaflow Y pipe welded up.

If I wanted to weld in an X pipe, what size should I keep it to keep from losing low end torqe?

Would I benefit from putting in high flows at 76k miles?