High flow cat Q

[feerocknok]

I understand they flow better, but are they significantly louder?

I like my truck quiet.

[Eagle]

Just about any cat you can buy will be a "high flow." They all have a 2-1/2" core and neck down to 2-1/4" after they exit. No, they aren't louder -- the muffler is what controls the noise.

[feerocknok]

Okay. I suppose my understanding was off on the Magnaflow high-flow cats and such being different. What enables them to take the name?

Think my cat's started melting down, so I don't know if it's worth the prices Magnaflow lists.

[jeepthing07]

My friend bought a high flow cat for his 1998 5.9 grand cherokee limited. You could look through it it was a straight shot through. The factory ones you can't it has the ceramic honeycomb/stuff blocking the flow. While the high flow just had it around the walls of the cat. I don't have to get emissions tests so nothing i have has a cat anymore :D

[feerocknok]

The reason I run one is it helps keep the Greenies off the back of the four wheeling community, and noise. After gutting the family Tacoma's cat, and then putting some 3" there instead, there's definitely a noise difference!

[Eagle]

I cannot understand Jeepers' fascination with "high flow" exhaust systems. Headers, high flow cats, 2-1/2" cat back exhausts -- that sort of modification is beneficial in a race vehicle, where you routinely operate the engine at and above 6,000 RPM. The torque peak on the Jeep 4.0L engine is (depending on year) between 2,000 and 3,500 RPM. Highway cruise in my '88 with 5-speed is between 1600 and 1750 RPM. When I ran a 2-1/2" turbo muffler cat back on my XJ I LOST low end torque, which means driveability was worse and gas mileage was worse. I was very happy when it rusted out and I had an excuse to go back to OEM replacement 2-1/4" components.

 

If there's a premium attached to somebody's so-called "high flow" catalytic converter, it isn't worth it unless you run a race truck. It will NOT benefit you on the street.

[aemsee]

I did a straight pipe on my 93 ZJ only because the cat con was rattling badly. I also lost power and MPG. I definitly agree with Eagle on this one.

[jeepthing07]

i gutted the cat on my 94 4.0 YJ and it ran rich got really bad gas mileage and lost power (and no it did not have a O2 sensor after the cat). So i put in a straight pipe my gas mileage returned and i got my power back. I would never gut a cat again unless you sleeve it with straight pipe.

[carnuck]

My '87 Comanche's cat got gutted a long time ago. I pass the E-test easily. Fuel economy is 17 in town and pushing 20 on the highway.

[rocketwheels]

....after owning several "big block" long stroke vehicles, as a rule, I never put anything larger than 2 to 2.5 inch pipes and ever lessen the restriction in the exhaust. I'd rather have that low end torque. I have a 72 Galaxie with a 460PI , I lost a load of low end when I went with 2.5 pipes and Thrush Cannons, It sounds awesome and I really noticed the expanded power band while accellerating around 125mph, but I lost all the light to light launching power.

[pingpong]

I have been saying that for yrs, that the amount of power you gain from a "hi flow" exhaust is lost in fuel economy and in low end power. As eagle pointed out... it is rare that a 4.0 sees over 2500rpm, unless you are like me and believe in if you can't get up it.. put on a good show trying :brows: Seriously I have always ran 2" stuff with a glasspack on my rigs. Cheap and easy to repair if mangled.

[randimal]

I'm curious now. Sounds like several are of the opinion that high flow is not necessarily better. Does this also apply to headers? plus, the stock downtube has a bend in it that is effectively 1.75 diameter and a new header will eliminate this.

 

I need to replace mine because of leaks and I have a pacesetter thats been lying here for years. I was going to put in the header, high flow cat and a few feet of 2.5 inch. However, I may change my plan now depending on how this discussion goes.

[Eagle]

I'm curious now. Sounds like several are of the opinion that high flow is not necessarily better. Does this also apply to headers? plus, the stock downtube has a bend in it that is effectively 1.75 diameter and a new header will eliminate this.

There was CONSIDERABLE debate about that flat spot on the NAXJA forum back maybe four or five years ago. It became quite ... "agitated" ... at times, because people on both sides of the argument were so sure they were right that they became unable to look at the question objectively.

 

The key issue is, none of us know why the dent is there, but it WAS designed in from the factory, so it must have a purpose. Keep in mind that the pipe that dent is in is a 2-1/2" down pipe. The amount of metal making up the pipe wasn't changed, so flattening it slightly certainly doesn't reduce the cross-sectional area down to the equivalent of a 1-3/4" pipe.

 

The other thing to consider is a concept from Physics that my grandfather used to talk about: the Bernoulli Theorem. Basically, it says that in a long pipe/tube/duct of a given diameter/area, if a short length is reduced in cross-section, the fluid (or air column) flowing through the pipe or tube will not be affected because at the area of localized reduction, it will simply flow faster.

 

This lends credibility to the notion that the dent is there to provide physical clearance rather than performance. On the other hand, we know that the 4.0L needs some back pressure to develop low-end torque. Many of the people in the great dent debate on NAXJA swore that running a header or custom down pipe without the flat spot resulted in less torque. I have never tried it, so I don't know.

 

What I DO know is that a 2-1/2" cat back system with a turbo muffler very much cost me torque and gas mileage. I have no reason to believe that a header wouldn't have the same effect. Like I said -- all that high flow stuff is intended to handle ... high flow. The stock engine operating at less than half the stock redline RPM doesn't produce high flow and does NOT need additional exhaust capacity. You would need high flow components if you built a drag race truck, like Bob Salemi in Rhode Island, that routinely sees 6,000 RPM and up. To cruise down the Interstate at 1750 RPM, or to crawl over rocks at barely more than idle -- tell me again why the Hell you *NEED* a high flow exhaust system.

 

Randimal: If your leak is the stock down pipe, I would just replace with another stock down pipe. If your leak is the exhaust manifold, since you already have the header by all means use it. But skip the 2-1/2" stuff down stream. Stay with stock replacement components and you should be okay.

[randimal]

Thanks, Eagle. I think I will adjust my plan. My leak is in the manifold, so the Pacesetter will go in there. I'll stay with a stock cat and pipe from there back. I'll probably even throw in a muffler since I have one thats not too beat up. Thanks for the info.

 

Twould be nice if someone with dyno/computer type stuff would do some testing. Install some sort of valve in the downpipe that could be easily adjusted. Then test it at different settings. Well, actually, I don't know anything about this kinda testing, but it seems like it should be possible.

[feerocknok]

I've never driven a 4.0L more than 15 miles, but my 2.5L cruises the highway at about 3 grand (taller tires, stock gears, 3 speed auto), and for passing traffic's sake, the truck seems to be revved up to about 4500 before shifting in town.

I don't necessarily consider this high RPMs, but it's certainly not the down-low grunt work in which many of you are referring too.

 

Right now, I'm running the stock exhaust system with a 2.5" Magnaflow for a muff, so maybe the Magnaflow is the issue and reason I have to shift at such high engine speeds, but I don't believe a muffler alone will make that huge of a difference.

[87manche]

that's the nature of the beast, the little 4 banger needs to rev. The long crank and extra cylinders of the 6 make it a torquey motor.

I cruise all day long at 70 MPH, with gearing close to stock @ 2K RPMs, and I only shift down for really steep hills.

When wheeling I never make it past 3K RPMs, I just don't like the sound of the motor up that high. No need, torque peak on the 4.0 is around 2500, with 90% of that available from 1200 to 3K.

[Eagle]

I've never driven a 4.0L more than 15 miles, but my 2.5L cruises the highway at about 3 grand (taller tires, stock gears, 3 speed auto), and for passing traffic's sake, the truck seems to be revved up to about 4500 before shifting in town.

I don't necessarily consider this high RPMs, but it's certainly not the down-low grunt work in which many of you are referring too.

Yeah, the 2.5L is a somewhat different animal. They mostly came with 3.73 or 4.10 gears, they have a narrower torque band, and the torque peak is higher than with the 4.0L. For a direct comparison, if my 4.0L 5-speed with 3.07 gears turns 1750 RPM at highway cruise, a 2.5L 5-speed with 4.10 gears would turn 2337 RPM. 1750 is below the torque peak for the 4.0L, and 2337 is below the torque peak for the 2.5L. I would expect a 2.5L to run at higher RPM.

 

BUT ... it's a smaller engine. 37.5% smaller, to be exact. So at the same RPM, it's pushing 37.5% less gas through the system.

 

I'm not sure what came stock on the later 2.5Ls with multi-port injection, but on my '86 XJ and MJ 2.5L and the '88 MJ 2.5L, the stock exhaust is either 2" or 1-3/4" -- it's definitely smaller than the 2-1/4" on the 4.0L vehicles.

 

2-1/2" exhaust on a 4-banger is massively oversized and isn't helping you at all. You're giving away torque, and the 2.5L needs the torque a lot more than the 4.0L does.

[87manche]

I have heard though that a 4.0 throttle body is one helluva an upgrade for the 2.5.

cheap way to make more power.

[feerocknok]

Well, the 2.5" muff fits the stock piping as if the stock piping was 2.25" (a bit of gap to fill, but not a perfect fit).

I should be able to get a used cat from a junkyard for free, right? My reasoning on the thought is since they are used, they're illegal to sell.

[BIGMASS]

I should be able to get a used cat from a junkyard for free, right?

I wouldn't bank on that. Cat con's contain precious metals. My buddy's pappy owns a small yard and they pull em all before crushing. The guy who comes for them pays by weight.

 

I've got a 2.5" Magnaflow high flow cat. I got it cause it was cheeper than any direct replacement or universal cat I could find anywhere else, and I chose 2.5" cause I knew the muffler I wanted to run wouldn't come in a smaller size (I wanted duals in a bad way).

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My MPG is a sickly 12.8, and I don't know bout the torque cause I never drove it with what was left of the stock exhaust before the rebuild.

[DirtyComanche]

FWIW I don't have a cat. And just run a cherry bomb (straight through glasspack).

 

 

It's not loud. My friend with an LT1 in his YJ laughs at it when I start it. :roll:

 

 

 

Anyways, with the stock exhaust I obliderated the cat after it had a few issues. And I had a pry bar and a bit too much spare time. Exhaust volume increased somewhat. It sounded like your typical quiet car at that point. My intake was still about 10X louder than the exhaust. I think with a straight through cat one would have a similar experience.

 

Then I obliderated the muffler on a rock. It made a really loud buzzing that made me want to kill people. So I removed it. It had three obvious effects - a loss of low end, a gain of mid-high, and it was forking loud. Annoying forking loud.

 

After that I picked up the cherry bomb and bent up the new exhaust. It works fine, low end is back and the high end works well enough. It's not over-restricted (stock) or under-restricted (no exhaust).

[Eagle]

I have heard though that a 4.0 throttle body is one helluva an upgrade for the 2.5.

cheap way to make more power.

Not for most MJ 2.5Ls. The 86 - 90 used a throttle body injection. ALL the 4.0Ls were multi-port. Put on a 4.0L throttle body and you have no way to put gasoline into the engine. It won't make a lot of power running on air.

[Eagle]

I should be able to get a used cat from a junkyard for free, right? My reasoning on the thought is since they are used, they're illegal to sell.

If they can't sell it legally, what makes you think they can give it away? You aren't allowed to install a used converter in any event (at least, not legally). I don't think they'll let you out the gate with a cat.

[carnuck]

They make a chunk of money off the old cat from the platinum, stainless and cadmium

[wyk]

hey guys, I'm, uh, new here. Anyways, I am just barely not passing smog in California with my 1986 Cali MJ. It's running fine, otherwise. I reckon a new cat would do it. I want something that will actually WORK, and work for years. Will I have to go with stock? Or is there an OEM-style outside of jeep that will work and not break the bank?

 

Obliged,

 

WYK