Lets set this straight.

[8Mud]

So, from what I'm getting is that it probably helps to remove the cat...but not the muffler. Right? When I put that I straight piped I meant that I removed the cat, and put a turbo muffler generic flowmaster 40. Identical baffling inside. Last night I raced a Durango V8 and was ahead 2 cars at 50 mph. So I don't think I lost torque.

You may loose a little torque, but what you do is move the peak torque higher into the RPM band, with a more open exhaust.
I spent a thousands hours tuning exhausts for specific tracks (before the dyno), plug it up a little and lengthen the collector you accelerate out of the corners faster. Shorten the collector and open the exhaust and you get higher RPM's at the end of the straightaway. I've still got my collection of header baffles, to adjust exit diameter.
Look at a few horsepower and torque graphs.
To get an idea of what is going on, think about cam grinds and which are for torque and which are for horsepower, in the 2000-3000 RPM band.
Even Dodge in the muscle car days, used a smaller exhaust valve in large body applications. I guess they were just dumb.
Somebody mentioned exhaust scavenging, this is actually the low pressure between the pulses. Opening the exhaust slows the flow, the exhaust looses velocity and the pulses become more undefined.
But one thing I learned, that theory is fine, but what actually works works despite the theory. One guy made a square collector that everybody tried to copy, kept the major manufacturers test bench guys busy for a year trying to figure out what and how he made it work so well.
I use my butt dyno and rarely make a large mistake, some motors you just know they need to breath better. I've never felt the need to open a 4.0 exhaust up much, but do install a slightly lower restriction muffler on occasion, in the Renix. I've never felt the need to open an HO up any.

 

[PurpleCherokee]

Bottom line: for PEAK POWER you need as little back-pressure as possible end of story. If you just want some low end torque then get ya a cat-back system or better yet some long-tube headers. Headers/free-flowing exhaust systems do not kill power! There's a reason drag cars and other race cars run open headers, sometimes with a "glasspack" or just a collector tuner on the end of them... cause they make more peak torque!!! You can move that torque around via tuning your exhaust to put it where you want it but you're gunna make more power with a better flowing exhaust with less backpressure than you will with an exhaust with more backpressure! I'll put a vid up so you can see how much my dual outlet SHORTY with dual 2 1/4 pipes, straight through muffler, x-pipe, and NO CATS has killed my low end torque LOL. Then whoever wants to can argue with the video till they're blue in the face

 

[wolfpackjeeper]

as long as the video is on a dyno, I am sure you will get no arguments.

But untill someone ponies up the dough and time for multiple dyno pulls and the time to change out exhausts then there will still be an argument.

 

[BillBraski]

Bottom line: for PEAK POWER you need as little back-pressure as possible end of story. If you just want some low end torque then get ya a cat-back system or better yet some long-tube headers. Headers/free-flowing exhaust systems do not kill power! There's a reason drag cars and other race cars run open headers, sometimes with a "glasspack" or just a collector tuner on the end of them... cause they make more peak torque!!! You can move that torque around via tuning your exhaust to put it where you want it but you're gunna make more power with a better flowing exhaust with less backpressure than you will with an exhaust with more backpressure! I'll put a vid up so you can see how much my dual outlet SHORTY with dual 2 1/4 pipes, straight through muffler, x-pipe, and NO CATS has killed my low end torque LOL. Then whoever wants to can argue with the video till they're blue in the face

Very well said. Less restriction for higher RPM power. Long tube headers improve low RPM torque and if low restriction mufflers are added, it adds to upper power. Tuned headers with the right exhaust pipes/muffler(s) improve economy and power. On dual exhaust, a H pipe will improve low torque and get bypassed at higher engine speed . A X pipe adds velocity and high RPM hosepower. Its all about velocity and scavenging.

But on our Jeeps, if the engines stock from the throttle body to the exhaust ports, a cat delete/flowmaster will not help low torque much. It may improve the top end power, but why would that matter? Its not like these 4.0's were intended to be motor cross or road race engines. They were designed for 4x4ing and daily driving, with a 5k red line. They are more like diesel engines, but with low compression and no turbo.
Lets be happy they make good torque and can get the $hit kicked out of them and ask seconds. I love the 4.0, its on my short list of greatest engines produced.

 

[BillBraski]

as long as the video is on a dyno, I am sure you will get no arguments.

But untill someone ponies up the dough and time for multiple dyno pulls and the time to change out exhausts then there will still be an argument.

Agreed. I would be hesitant to drop $230.00 a dyno run and a few hundered on exhaust parts. And for what? To prove exhaust work can unlock 8 horsepower and 15 pounds of torque? I think the guys that own the dyno near my work would fall on the ground laughing if I took either of my Jeeps in to be put through the paces. I might as well bring in a massey ferguson tractor.

 

[PurpleCherokee]

My point about the video is to show that the difference in low end power when put to the road is negligible. If I get a good speedo shot you'll see And contrary to what I thought about having a "high-reving" motor... wow is it fun to drive now! I mean, it's still a 4-oh so there's still plenty of torque it's just that now, the top end matches the low end!!! I couldn't believe how much of a difference it made! And for those of you that think that the 4.0 just good for low end torque and can't be a high horsepower motor are dead wrong. Yes, it's an inline of course but YES you can get some real ponies out of it! I was grinnin ear to ear when I first beat it from 50 to 75... it was actually kinda relieving to know that you can indeed get some horsepower out of an inline! And please people, I've been through the off-road scene and it was an absolute BLAST but it got a little pricy for me (my jeep being a DD) so now my jeep's a total street jeep. In my perfect world (a world where I have money) I'd have like to have fifteen jeeps all built for different purposes and definately a hardcore trail-rig! So those of you w/street-jeeps I HIGHLY reccommend my set up, it's so much fun

 

[wolfpackjeeper]

Agreed. I would be hesitant to drop $230.00 a dyno run and a few hundered on exhaust parts. And for what? To prove exhaust work can unlock 8 horsepower and 15 pounds of torque? I think the guys that own the dyno near my work would fall on the ground laughing if I took either of my Jeeps in to be put through the paces. I might as well bring in a massey ferguson tractor.

damn dyno runs are expensive where you are at. Around here it is 50 bucks for 3 pulls. I am about to take a bone stock 91 I just got up there to have it done.

PC, the only thing I have against a high revving cherokee motor is that most people are still sitting on a stock bottom end with at least 60k on it. I rev my stroker quite a bit, but I built the bottom end to be tough, and after I megasquirt it(working on that now) it will be getting a turbo. The turbo will be getting a mince smooth exhaust, I have just never seen the need to open it up more than it is

 

[Lowdown]

You have to define better.
Does your XJ, pull a trailer better with a slightly restricted exhaust at 55 MPH, likely.
Does it accelerate through peak torque quicker on a flat street into the upper RPM band and maybe loose some torque in the process with a more open exhaust, likely.

horsepower = torque * RPM divided by 5252

You can lower one and raise the other, move them around in the RPM band some and mess with peak RPM. But in the end you have three variables and one constant. Messing with the exhaust changes the curve some.

Most XJ's are heavy, have a motor designed more for torque than peak horsepower, often turn big tires and have mechanical limitations on the highest RPM's that the motor will survive.

I do a little street racing from time to time. I shift early and try to keep the motor in the peak torque band. I regularly beat lighter cars with a better horsepower to weight ratio. How? Because my peak torque band is flatter than their peak horsepower band. My steady acceleration can beat there peaky motor and quick shifts. If I drive it much out of the the torque curve, which falls off fast after mid RPM, they win. I also try to pick my road, and try to stage the race from 15-20 MPH through around 75 MPH (lower if possible). HO motors have moved peak torque a little higher into the RPM band and seem to have narrowed the torque curve a little. Butt dyno.
My HO is marginally a little faster than my Renix, but really not much. They accelerate a little differently, but the end results are largely the same.

I imagine there is some room for tuning the exhaust. But it would likely be easier IMO, to tune it out of the envelope, than to get it much better.

Pretty accurate except you left out leverage. The reason the 4.0 (and even more so a 4.7 stroker) is a good low rpm torque motor is that the stroke to bore ratio is higher than say 5.0L ford.

To answer the question of whether a flow through exhaust would make a big difference, I think not. However, in many cherokees there is a "dent" in the exhaust right after the manifold. It comes from the factory with it. Although a 2.5 inch exhaust and a glass-removed cherry bomb are not really gonna help llow-end performance on a motor with such a long stroke, getting rid of this dent does make a sizeable difference (and it is only about $50 to have a muffler shop replace it with non-dented pipe).

 

[rapom]

I just heated up the dent so it was nice and orange and used a pry bar to work it out.