Can I Tell If My Cat. Converter Is Good By Sight??

[ParadiseMJ]

Just like the title says...

 

My catalytic converter looks like an old battleship from the outside, but when I installed the new muffler a couple months ago, I looked inside the backside of the the cat. converter.  It was pearly white with nice honeycomb that all looked good.  Problem is I haven't looked at the other end.  The Jeep is an 88 MJ and I know someone in the first twenty years of it's life had changed the cat, by the weld job. I've had it for 4 yrs and only put about 3,000 miles on it since I smogged it 2 yrs ago.

 

So just for curiousity, if the back end looked good, could I tell if it was OK or NOT ?  Or is it more scientifical than that ?

[Minuit]

I don't really imagine you can tell much from looking at the outside (unless something really major happened to it.) Unless your performance starts to drop off because it's restricting exhaust flow I wouldn't worry about it.

[ParadiseMJ]

I was looking IN to the cat from the muffler end.

 

Flow restriction is possible, just wanted to know if I looked INside could I see anything significant.  I already can see that it looks nasty from the OUTside.

[HOrnbrod]

On my rig, I discovered the clogged cat when we were installing a dual exhaust system at my friends muffler shop. My rig still had the original engine then. The outlet looked fine as you describe, but the input was clogged with a black sticky substance nearly blocking flow completely. My friend said he had seen it quite a bit on Jeep I6 engines. So the only way you can really tell is to cut it off and look into the inlet - that's where the clogging will start first.

[ParadiseMJ]

Yeah, I was thinking that clogging or deteriortation would start at the front, logically.  Like I said, from the outside it looks like it's as old as the hills (like me).  The condition of the downpipe is iffy too, I may need to bite the bullet yet again.  Furk!!

 

CA legal catalytic converters are a tad more $$ than the OTHER 49 states...AND Puerto Rico!!  Damn Gold Rush!!

[cruiser54]

Do the downpipe mod while you're at it and pick up some ponies.

[64 Cheyenne]

Do the downpipe mod while you're at it and pick up some ponies.

Downpipe mod ??

[cruiser54]

 

Do the downpipe mod while you're at it and pick up some ponies.

Downpipe mod ??

Top secret. I haven't done a write-up on it yet.

[ParadiseMJ]

 

Do the downpipe mod while you're at it and pick up some ponies.

Downpipe mod ??

Yeah, downpipe mod??  I'm very interested since I have the whole thing apart. 

 

 

 

 

Side note:  At RockAuto there are 4 TPS's said to fit a Renix, ranging in price from $38 to $95...which one??  the $38 one and the $95 one are from the same manufacturer?  I

[cruiser54]

Get one for an auto trans. Cheaper. Get a good quality one, not the cheapie.

[cruiser54]

The factory downpipe has a "crush" in it that appears to be for front driveshaft clearance. That's not why it's there. It was done to slow the exhaust down and heat up the catalytic converter faster. A good muffler shop can make you a custom downpipe using two 45* angles instead of the one 90* angle the stock one has. Wait til you see just how much that pipe is crushed. Probably 30%.

[Eagle]

So just for curiousity, if the back end looked good, could I tell if it was OK or NOT ?

 

Nope.

[airspeed]

Here is a photo of the down pipe from my son's 96 Cherokee 4.0. Even with this pipe in there the vehicle runs fine.  He can tow a dual snowmobile  trailer over a high mountain pass ~11,000ft no problem. His vehicle has about 240k and runs strong.  When you look at it this photo there is a bit of an optical illusion, the dimples in the pipe are above and below each other, but the restriction is at least 30% if not more.

[Ellman605]

So what benefit is it to get rid of that?

[Biotex]

So what benefit is it to get rid of that?

 

Increased backpressure in an exhaust = loss of HP at higher rpms.

 

I always assumed the dimple was for drive shaft clearance. Thanks for the tip cruiser! :cheers:

[cruiser54]

So what benefit is it to get rid of that?

More power. If you have a 3" or more lift, you can route the pipe like it was. I don't though. Unlifted, be sure to have your exhaust guy tuck the pipe out of the way of driveshaft up-travel.

 

I stole your photo airspeed. Thanks.

[Eagle]

Increased backpressure in an exhaust = loss of HP at higher rpms.

 

I always assumed the dimple was for drive shaft clearance. Thanks for the tip cruiser! :cheers:

 

And reduced backpressure in an exhaust = loss of torque at low and mid-range RPMs. Since I never operate a 4.0 liter higher than 2500 RPM (and rarely even that high) I'll take the torque that I use every day over the 5000 RPM horsepower that I'll never use -- ever.

[Biotex]

 

Increased backpressure in an exhaust = loss of HP at higher rpms.

 

I always assumed the dimple was for drive shaft clearance. Thanks for the tip cruiser! :cheers:

 

And reduced backpressure in an exhaust = loss of torque at low and mid-range RPMs. Since I never operate a 4.0 liter higher than 2500 RPM (and rarely even that high) I'll take the torque that I use every day over the 5000 RPM horsepower that I'll never use -- ever.

Absolutely! That is why I don't waste my money on larger or "free flowing" exhaust systems. Replacing defective exhaust components is one thing, but adding headers and 3" pipes are just a waste of money to me. I am a low rpm jeep driver. A certain amount of back pressure is required. (or should I say desired)

 

Now, if that dimple was put there solely to cause more back pressure than the optimum, as cruiser is suggesting, so it will heat up the converter, then I would do that mod.

Not many dyno experiment results available to go by, so much of this talk doesn't really mean squat. We can only generalize. If anyone knows of some dyno results, please share.

[cruiser54]

Here's where "old wive's tales" come into play and are usually propogated by those who have never done the mod. . This lower back pressure deal is an absolute crock. That's something that applies to 2 stroke engines mostly anyway.

 

 I will now add that I was advised to do this mod by a Jeep engineer who worked on the 4.0 project. He's the one who told me about "why" the crush is there.

He told me I would feel a "seat of the pants" difference in throttle response and power. He was right.

 

Don't let the "old wives tales" keep you from enjoying the benefits of this mod.

[HOrnbrod]

One of the first things I did when I bought my MJ originally was to install a Mike Leach header, junked the dimple bend, ran 2.5" pipes all the way back from the muffler, and a K&N intake. It made a difference, not great, in performance and actually provided a bit better mileage. Unfortunately I had no before and after dyno readings for these mods. About a year later, the stroker went in, along with a 2001 intake manifold and Unichip. Along with the new stroker the same exhaust and K&N went back in. For all this I do have dyno readings here:

http://www.jeepstrokers.com/forum/viewtopic.php?f=5&t=387

Pretty good results. My conclusion from all this is that exhaust mods by themselves really don't do much to justify the expense, especially on a N/A Renix, w/o other enhancements. When these other things are done, that's when the free-flow exhaust, along with better intake capacity and tuning to balance it all out makes it worthwhile. And I agree with Cruiser: there's no way a crush like that restricting exhaust flow on any engine can do any good. Except on two-smokers.

[cruiser54]

One of the first things I did when I bought my MJ originally was to install a Mike Leach header, junked the dimple bend, ran 2.5" pipes all the way back from the muffler, and a K&N intake. It made a difference, not great, in performance and actually provided a bit better mileage. Unfortunately I had no before and after dyno readings for these mods. About a year later, the stroker went in, along with a 2001 intake manifold and Unichip. Along with the new stroker the same exhaust and K&N went back in. For all this I do have dyno readings here:

 

http://www.jeepstrokers.com/forum/viewtopic.php?f=5&t=387

 

Pretty good results. My conclusion from all this is that exhaust mods by themselves really don't do much to justify the expense, especially on a N/A Renix, w/o other enhancements. When these other things are done, that's when the free-flow exhaust, along with better intake capacity and tuning to balance it all out makes it worthwhile. And I agree with Cruiser: there's no way a crush like that restricting exhaust flow on any engine can do any good. Except on two-smokers.

Yes. Elimination of the crush works great. I have done that mod twice all on it's own with no other mods at the same time and experienced better throttle response and acceleration, just like my buddy at JeepTech told me I could expect.

[Biotex]

I'm obviously not stating my thoughts well. I'll say it more clearly this time.

 

Cruisers mod of the removal of the dimple is advisable IMO also, here is why. The stock exhaust diameter is tuned for the stock 4.0L engines rpm range. The dimple appears to have been installed for emissions purposes, by increasing back pressure to slow down the exhaust gas so the catalytic converter can heat up more. I'm convinced that this is robbing HP.

 

What I'm also saying, is that bigger pipes are not always better. There is a trade off.

 

As you increase the pipe size, you decrease velocity. Yes you decrease back pressure, and that is a good thing to a point, because

as you decrease velocity, you also decrease scavenging. So there is a trade off, and you want a pipe diameter that has the least amount of back pressure, but enough velocity so you get the effects of scavenging. This is what's considered a tuned exhaust.

 

Exhaust pipe diameters are best suited to a particular RPM range. A smaller pipe diameter will produce higher exhaust velocities at a lower RPM but create unacceptably high amounts of backpressure at high rpms. So if your power band is located 2-3000 RPM, you'd want a smaller pipe than if your power band is located at 4-5000RPM range.

 

This is fluid dynamics, not an old wives tale. Without a dyno, who's to say what that optimum pipe diameter is, so I tend to go with the Jeep engineers recommendations (minus emissions workarounds.)

[cruiser54]

Well stated. We're talking about a dimple in the system right at the exit of the exhaust manifold. Just removing that. We're not talking about putting a 4" free-flow system on full length.

[Biotex]

 

 

Increased backpressure in an exhaust = loss of HP at higher rpms.

 

I always assumed the dimple was for drive shaft clearance. Thanks for the tip cruiser! :cheers:

 

And reduced backpressure in an exhaust = loss of torque at low and mid-range RPMs. Since I never operate a 4.0 liter higher than 2500 RPM (and rarely even that high) I'll take the torque that I use every day over the 5000 RPM horsepower that I'll never use -- ever.

Absolutely! That is why I don't waste my money on larger or "free flowing" exhaust systems. Replacing defective exhaust components is one thing, but adding headers and 3" pipes are just a waste of money to me. I am a low rpm jeep driver. A certain amount of back pressure is required. (or should I say desired)

 

Now, if that dimple was put there solely to cause more back pressure than the optimum, as cruiser is suggesting, so it will heat up the converter, then I would do that mod.

Not many dyno experiment results available to go by, so much of this talk doesn't really mean squat. We can only generalize. If anyone knows of some dyno results, please share.

 

 

Well stated. We're talking about a dimple in the system right at the exit of the exhaust manifold. Just removing that. We're not talking about putting a 4" free-flow system on full length.

 

Agreed, was just trying to point out that there is a point of diminishing return when opening up an exhaust system. I was clarifying (or attempting to) my earlier statement.

Again, thanks for the tip that I am certainly going to use. I also notice another dimple where the pipe crosses over the cross member.  Do you know of any aftermarket down pipes for sale without those dimples? Muffler shops are 300 mile away round trip for me.

[cruiser54]

 

 

 

Increased backpressure in an exhaust = loss of HP at higher rpms.

 

I always assumed the dimple was for drive shaft clearance. Thanks for the tip cruiser! :cheers:

 

And reduced backpressure in an exhaust = loss of torque at low and mid-range RPMs. Since I never operate a 4.0 liter higher than 2500 RPM (and rarely even that high) I'll take the torque that I use every day over the 5000 RPM horsepower that I'll never use -- ever.

Absolutely! That is why I don't waste my money on larger or "free flowing" exhaust systems. Replacing defective exhaust components is one thing, but adding headers and 3" pipes are just a waste of money to me. I am a low rpm jeep driver. A certain amount of back pressure is required. (or should I say desired)

 

Now, if that dimple was put there solely to cause more back pressure than the optimum, as cruiser is suggesting, so it will heat up the converter, then I would do that mod.

Not many dyno experiment results available to go by, so much of this talk doesn't really mean squat. We can only generalize. If anyone knows of some dyno results, please share.

 

 

>Well stated. We're talking about a dimple in the system right at the exit of the exhaust manifold. Just removing that. We're not talking about putting a 4" free-flow system on full length.

 

Agreed, was just trying to point out that there is a point of diminishing return when opening up an exhaust system. I was clarifying (or attempting to) my earlier statement.

Again, thanks for the tip that I am certainly going to use. I also notice another dimple where the pipe crosses over the cross member.  Do you know of any aftermarket down pipes for sale without those dimples? Muffler shops are 300 mile away round trip for me.

 

I don't know of any off the shelf pieces but I heard they exist.

 

I appreciate your clarification on the "point of diminishing return".