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  #25  
Old 03-09-2012, 04:43 PM
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Re: Removing catalytic converter

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Originally Posted by PeterB16 View Post
I hate to revive an old thread, but after searching, this is what I found.

I have an 04 4.7HO Overland.
I recently had the front two cats replaced.
Same old O2 sensors put back in.

The other day the exhaust down from the "Y"pipe came off. The techs did not securely fasten the rest of the exhaust which contains a straight run of pipe, the 3rd cat and the muffler.
When the "Y" pipe fell off, I lost most of my low end torque immediately.
I was planning on replacing my 3rd cat with a straight run of pipe to free up some flow. (No emissions testing in my state). But once I realized how much torque I lost, I have changed my mind.
My question is, why did I lose this much torque? I thought that freeing up flow would increase performance. Does the 4.7 HO really need that much back pressure to operate or do I have something wrong?
Without a datalog it is hard to say exactly.

I suspect one of two things: either the removal of piping, cat, muffler, etc increased flow and scavenging so much that it skewed the fuel trims/WOT AFRs.

Exhaust length is also an important factor in making power- typically, making the exhaust shorter more often than not will increase power (tq included). It's possible that the removal of the the downstream exhaust fell into that range where power can fall. It has alot to do with pulses.

Backpressure is mostly a myth used to sell smaller diameter pipes and more restrictive mufflers. Its better to think of it all in terms of flow, and velocity. Vellocity (and good scavenging) are the keys to producing tq. Torque x rpm = hp.
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  #26  
Old 03-10-2012, 01:25 AM
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Re: Removing catalytic converter

Most engines now are designed to run with a certain amount of backpressure and releasing too much of the stock back pressure accultly leads to a loss in low end power...
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  #27  
Old 03-12-2012, 08:19 AM
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Re: Removing catalytic converter

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Originally Posted by FlyinRyan View Post
Without a datalog it is hard to say exactly.

I suspect one of two things: either the removal of piping, cat, muffler, etc increased flow and scavenging so much that it skewed the fuel trims/WOT AFRs.

Exhaust length is also an important factor in making power- typically, making the exhaust shorter more often than not will increase power (tq included). It's possible that the removal of the the downstream exhaust fell into that range where power can fall. It has alot to do with pulses.

Backpressure is mostly a myth used to sell smaller diameter pipes and more restrictive mufflers. Its better to think of it all in terms of flow, and velocity. Vellocity (and good scavenging) are the keys to producing tq. Torque x rpm = hp.
I know one thing, myth or not, I lost drastic power. Nothing imaginary going on there.
My 1975 CJ5 with a built 304 LOVES the open header design. No cats, very high flow muffler and headers. Add anything else to it and it is being restricted.
My 2004 4.7HO, apparently does not work that way.
UNLESS the air intake is now the restriction. I didnt take off any air intake parts. So maybe being that I freed up some exhaust, I would have to free up some intake as well to create HP. But, regardless, I want to remove my 3rd cat and am now a little apprehensive in doing that now that I have noticed a dramatic decrease in power with my pre-cats back exhaust missing.
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  #28  
Old 03-12-2012, 08:29 AM
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Re: Removing catalytic converter

Quote:
Originally Posted by PeterB16 View Post
I know one thing, myth or not, I lost drastic power. Nothing imaginary going on there.
My 1975 CJ5 with a built 304 LOVES the open header design. No cats, very high flow muffler and headers. Add anything else to it and it is being restricted.
My 2004 4.7HO, apparently does not work that way.
UNLESS the air intake is now the restriction. I didnt take off any air intake parts. So maybe being that I freed up some exhaust, I would have to free up some intake as well to create HP. But, regardless, I want to remove my 3rd cat and am now a little apprehensive in doing that now that I have noticed a dramatic decrease in power with my pre-cats back exhaust missing.
I had an '02 non-HO WJ, and I can tell you from seat of the pants to track passes that my best times (from the 60' to the back end) were with free-flowing exhaust. The less cats I had, etc etc the faster I went.

I ran no 3rd cat on my '02s 3" exhaust, it was a good setup. Again, if the exhaust isn't the right length, power will drop off noticeably.
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  #29  
Old 03-12-2012, 09:25 AM
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Re: Removing catalytic converter

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Originally Posted by FlyinRyan View Post
I had an '02 non-HO WJ, and I can tell you from seat of the pants to track passes that my best times (from the 60' to the back end) were with free-flowing exhaust. The less cats I had, etc etc the faster I went.

I ran no 3rd cat on my '02s 3" exhaust, it was a good setup. Again, if the exhaust isn't the right length, power will drop off noticeably.
Ok, I can buy that. When you removed 3rd cat, did you keep your muffler in same stock location?
Did you use different muffler?
My y-pipe from the precats joins into a 2.25 or 2.5 inch pipe. It wouldnt matter at that point if I had 3" pipe or not from there since the Y-pipe is the restraint, but I could run a straight pipe to a muffler and keep it in its stock location if I need to.
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  #30  
Old 03-12-2012, 09:46 AM
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Re: Removing catalytic converter

Quote:
Originally Posted by PeterB16 View Post
Ok, I can buy that. When you removed 3rd cat, did you keep your muffler in same stock location?
Did you use different muffler?
My y-pipe from the precats joins into a 2.25 or 2.5 inch pipe. It wouldnt matter at that point if I had 3" pipe or not from there since the Y-pipe is the restraint, but I could run a straight pipe to a muffler and keep it in its stock location if I need to.
Yes, pretty much. I used a bullet type muffler.
You want to open up the Merge to 3".
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  #31  
Old 06-17-2012, 10:55 PM
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Re: Removing catalytic converter

did you guys have any codes come up from taking the cat off
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  #32  
Old 06-17-2012, 11:21 PM
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Re: Removing catalytic converter

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Originally Posted by stephanwj View Post
did you guys have any codes come up from taking the cat off

you dont have a o2 sensor on the 3rd cat only on the pre cat and if you remove the pre cats then you will get a cel unless you do a tune to remove it
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  #33  
Old 06-18-2012, 09:37 PM
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if u have extra $$$ remove the cats install w.e exhaust you like. and get tuned you will love it.... or guy.with lap top and have him turn off emissions controls

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