what size exhaust pipe?

[87mjdriver]

I have a borla header already installed, and I am going to be replacing the whole rusted exhaust system from cat back. free flow cat and muffler. Should I bump the dia. up to 2.5. I haven't decided how far back to run the tail pipe yet, would that make any diferance. either straight out the tail or between the frame and rear spring exiting aft of the tire but tuvked out of the way.

[comanche87]

i would go 2.5 . that be my opinion.

[A-man930]

I'm wondering as well... do I spend the money and get a mandrel-bent 2.25" or a do a cheap non-mandrel 2.5" or it does it matter at all? I know there will be a slight torque drop off with less backpressure, but I'm not really worried about it. I will say that I'm all but certian that I'll go with putting the exit between the spring and frame rail on the pass. side... nice and out of the way :D

[Eagle]

What are you looking for -- torque and gas mileage, or noise and high RPM?

 

My brother talked me into putting a 2-1/2" cat back on my '88 XJ the first time the exhaust rusted out. It sounded nice, my gas mileage went down the toilet, and I could feel the loss of torque off the line and throughout my driving range (it's a 5-speed, so it never sees more than 3,000 RPM and only *very* rarely sees higher than 2,500 RPM).

 

IMHO, the stock 2-1/4" is the preferred setup. That had to be the only time in my long life of playing with with cars that I was actually hoping for an exhaust system to rust out so I could justify replacing it.

[CWLONGSHOT]

Personally, I would stay with 2.25" all the way back.

 

Like already mentioned, larger diameter exhausts can KILL lo end torque and that is counter productive to what Jeeps are used for.

 

CW

[jeepthing07]

Personally, I would stay with 2.25" all the way back.

 

Like already mentioned, larger diameter exhausts can KILL lo end torque and that is counter productive to what Jeeps are used for.

 

CW

 

X2

[TNT]

I'm wondering as well... do I spend the money and get a mandrel-bent 2.25" or a do a cheap non-mandrel 2.5" or it does it matter at all? I know there will be a slight torque drop off with less backpressure, but I'm not really worried about it. I will say that I'm all but certian that I'll go with putting the exit between the spring and frame rail on the pass. side... nice and out of the way :D

 

You have the right plan, mandrel-bent 2.25" or a do a cheap non-mandrel 2.5".

 

I ran 2.5 mandrel-bent with a highflow cat with my Borla on my XJ and was happy with it. I didn't have a cat and it was even better. Muffler is a Flowmaster.

[A-man930]

Well I'm hearing two different stories now... TNT, did you notice a drop off in torque and/or gas mileage with the 2.5" mandrel-bent system? What series flowmaster did you use?

[Car RamRod]

Not to jump off topic here, but I am planning to do exhaust on my truck, and I'm wondering if the stock manifold is sufficient for a stock engined truck. In other words, is it choking me up?

 

And what are you guys using for mufflers?

[mvusse]

F100 had a Cherry Bomb glass pack 'cause it was cheap.

Suburban currently has Thrush glass pack . Love the sound on both of those.

Geo had stock muffler. Was afraid a glass pack would make it sound like an angry mosquito.

MJ will get glass pack as soon as the current muffler rusts out. Probably less than a year.

[TNT]

Well I'm hearing two different stories now... TNT, did you notice a drop off in torque and/or gas mileage with the 2.5" mandrel-bent system? What series flowmaster did you use?

 

Keep in mind that mine is a HO engine.

 

Fuel economy was the same to about one mpg better. As for torque I did loose a slight about under light throttle applications below 2500rpm. Overall power was definitly better. I removed the tholle body and sanded th step out of it. This eliminated any torque lost and I could also drive from 55-70mph without the AW-4 having hunting and downshifting on the cruise contrtol with 4 people and about 400# of weight.

 

Using the Borla header , the Walker DynoMax high flow converter and the Rustys cat back with the step removed out of the throttle body really improved the power and economy.

 

The best benefit is the improved drivability and incredible throttle response.

[HOrnbrod]

Keep in mind that mine is a HO engine.

 

Fuel economy was the same to about one mpg better. As for torque I did loose a slight about under light throttle applications below 2500rpm. Overall power was definitly better. I removed the tholle body and sanded th step out of it. This eliminated any torque lost and I could also drive from 55-70mph without the AW-4 having hunting and downshifting on the cruise contrtol with 4 people and about 400# of weight.

 

Using the Borla header , the Walker DynoMax high flow converter and the Rustys cat back with the step removed out of the throttle body really improved the power and economy.

 

The best benefit is the improved drivability and incredible throttle response.

 

After reading this thread, I thought I'd dig out my old Hesco dyno readings. The BEFORE was done on my fresh stroker engine in Mar 2005; the AFTER was done after exhaust mods in May 2007. The BEFORE exhaust was with a 2.5" Mike Leach header, hi-flow cat, Dynamax muffler, and 2-1/4" exhaust exiting behind the right rear wheel. The AFTER exhaust was with the same header, a Cherry Bomb 2-1/2" muffler replaced the cat, a 2-1/2" Walker HP muffler with dual outlets, and 2-1/2" dual pipes exiting under the rear bumper. And these were not mandrel bent exhaust pipes. These were the peak torque and HO vs. RPM at the rear wheels:

 

BEFORE: Torque 228.7 @ 4200RPM; HP 182.4 @ 4200RPM

AFTER: Torque 276.8 @ 3950RPM; HP 207.2 @ 3950PM

 

Peak Torque and HP averaged about a 13%-15% increase across the board starting at 2000RPM, with no changes but the exhaust mod between the two runs, and of course the engine was fully broken in. Mileage decreased approx 1 MPG. So it appears going from 2-1/4" to 2-1/2" is a good move for the HO engines. I don't understand why the Renix engines would not see similar results. Why?? :dunno:

[mvusse]

Weird that peak torque and peak HP are at the same RPM. High RPM even.

 

Normally peak torque is at a lot lower rpm than peak HP. Usually makes sense mathematically because HP is torque multiplied by RPM (and then multiplied by a constant depending on what units are being used).

[Eagle]

BEFORE: Torque 228.7 @ 4200RPM; HP 182.4 @ 4200RPM

AFTER: Torque 276.8 @ 3950RPM; HP 207.2 @ 3950PM

 

Peak Torque and HP averaged about a 13%-15% increase across the board starting at 2000RPM, with no changes but the exhaust mod between the two runs, and of course the engine was fully broken in. Mileage decreased approx 1 MPG. So it appears going from 2-1/4" to 2-1/2" is a good move for the HO engines. I don't understand why the Renix engines would not see similar results. Why?? :dunno:

Because the Renix engines weren't designed to run in that RPM range.

 

There are three (or four) “generations” of the 4.0L engine. The “first generation” was introduced in the 1987 model year. This version used a Renault/Bendix (“Renix”) designed, multi-port fuel injection system. The original 1987 model was rated at 173 horsepower at 4,750 RPM and 220 foot-pounds of torque at 2,000 RPM. The horsepower rating for this version was raised to 177 horsepower at 4,750 RPM and 224 foot-pounds of torque at 2,400 RPM for model years 1988 through 1990.

 

As a result of the various changes Chrysler made in the 1991 model year, the second version of the engine (the original HO) was rated at 190 horsepower at 4,750 RPM and 225 foot-pounds of torque at 3,950 RPM. The increased horsepower was perceptible in street driving, resulting in an engine that “felt” considerably faster than its predecessor even though it was not, in fact, significantly faster. However, the horsepower increase came at a price: although the rated torque was unchanged, the torque peak was now at nearly 4,000 RPM, which is higher than the engine would likely see in daily street driving, and considerably higher than what is desirable for off-road use (other than mud bogging).

 

The “third generation” 4.0-liter engine: In 1996 DaimlerChrysler again revised the electronic engine controls, to comply with a Federal mandate to make all vehicles OBD-II compliant. The horsepower rating for the 4.0-liter engine remained unchanged at 190 horsepower at a slightly lower 4,600 RPM, but the torque peak RPM was lowered, to 225 foot-pounds at 3,000 RPM. An additional running change was introduced in the Grand Cherokee in model year 1999 and in the Cherokee for model year 2000: the single ignition coil was eliminated and ignition was handled by a coil rail system, in which each spark plug has a dedicated coil mounted to a rail directly at the spark plugs. This arrangement eliminates the high voltage spark plug wires of older (“conventional”) ignition systems. The author considers this to be a variant of the “third generation.” However, it can be argued that the change is sufficiently significant that the 2000 and 2001 (and 1999 Grand Cherokee) 4.0-liter engine should be considered to be the “fourth generation” of the design. This version produced incrementally more power than the preceding version: 193 horsepower at 4,600 RPM and 231 foot-pounds of torque at 3,000 RPM.

 

The 1991 HO engine had completely different heads with a higher flow and higher ports, different intake and exhaust manifolds (which you negated by going to aftermarket headers), a tweaked cam profile, and a different ignition and injection system using larger injectors and a completely new fuel map. The HO engine is operating near its "sweet spot" in the 3,000 RPM range, which is while it feels so much stronger than a Renix from 3,000 to 4,000 RPM even though the horsepower ratings are virtually identical. The Renix engines are in their sweet spot between 2,000 and 2,500 RPM. You can feel it when you drive it. Mine will rev past 2,500 ... but I'm not doing anything at higher RPMs, I'm just making noise. What I consider to be the "natural" shift point is around 2,250 RPM. By contrast, my 2000 XJ 5-speed really likes to be shifted at 3,500 RPM, and hills I can drive up in 4th gear with the old Renix at 30 MPH require downshifting to hold 30 MPH with the new 2000.

 

You really cannot extrapolate from one engine to say anything meaningful about the other (except that they're both Jeep engines and will run forever if you change the oil).

[HOrnbrod]

Weird that peak torque and peak HP are at the same RPM. High RPM even.

 

Normally peak torque is at a lot lower rpm than peak HP. Usually makes sense mathematically because HP is torque multiplied by RPM (and then multiplied by a constant depending on what units are being used).

 

Yes, fat-fingered it. The charts start at 3000 RPM and continue to 5000RPM. 3000RPM was peak torque on both runs. Should be:

 

BEFORE: Torque 274.7 @ 3000RPM; HP 182.4 @ 4200RPM

AFTER: Torque 296.0 @ 3000RPM; HP 207.2 @ 3950PM

[HOrnbrod]

Peak Torque and HP averaged about a 13%-15% increase across the board starting at 2000RPM, with no changes but the exhaust mod between the two runs, and of course the engine was fully broken in. Mileage decreased approx 1 MPG. So it appears going from 2-1/4" to 2-1/2" is a good move for the HO engines. I don't understand why the Renix engines would not see similar results. Why?? :dunno:

Because the Renix engines weren't designed to run in that RPM range.

 

There are three (or four) “generations” of the 4.0L engine. The “first generation” was introduced in the 1987 model year. This version used a Renault/Bendix (“Renix”) designed, multi-port fuel injection system. The original 1987 model was rated at 173 horsepower at 4,750 RPM and 220 foot-pounds of torque at 2,000 RPM. The horsepower rating for this version was raised to 177 horsepower at 4,750 RPM and 224 foot-pounds of torque at 2,400 RPM for model years 1988 through 1990.

 

As a result of the various changes Chrysler made in the 1991 model year, the second version of the engine (the original HO) was rated at 190 horsepower at 4,750 RPM and 225 foot-pounds of torque at 3,950 RPM. The increased horsepower was perceptible in street driving, resulting in an engine that “felt” considerably faster than its predecessor even though it was not, in fact, significantly faster. However, the horsepower increase came at a price: although the rated torque was unchanged, the torque peak was now at nearly 4,000 RPM, which is higher than the engine would likely see in daily street driving, and considerably higher than what is desirable for off-road use (other than mud bogging).

 

The “third generation” 4.0-liter engine: In 1996 DaimlerChrysler again revised the electronic engine controls, to comply with a Federal mandate to make all vehicles OBD-II compliant. The horsepower rating for the 4.0-liter engine remained unchanged at 190 horsepower at a slightly lower 4,600 RPM, but the torque peak RPM was lowered, to 225 foot-pounds at 3,000 RPM. An additional running change was introduced in the Grand Cherokee in model year 1999 and in the Cherokee for model year 2000: the single ignition coil was eliminated and ignition was handled by a coil rail system, in which each spark plug has a dedicated coil mounted to a rail directly at the spark plugs. This arrangement eliminates the high voltage spark plug wires of older (“conventional”) ignition systems. The author considers this to be a variant of the “third generation.” However, it can be argued that the change is sufficiently significant that the 2000 and 2001 (and 1999 Grand Cherokee) 4.0-liter engine should be considered to be the “fourth generation” of the design. This version produced incrementally more power than the preceding version: 193 horsepower at 4,600 RPM and 231 foot-pounds of torque at 3,000 RPM.

 

The 1991 HO engine had completely different heads with a higher flow and higher ports, different intake and exhaust manifolds (which you negated by going to aftermarket headers), a tweaked cam profile, and a different ignition and injection system using larger injectors and a completely new fuel map. The HO engine is operating near its "sweet spot" in the 3,000 RPM range, which is while it feels so much stronger than a Renix from 3,000 to 4,000 RPM even though the horsepower ratings are virtually identical. The Renix engines are in their sweet spot between 2,000 and 2,500 RPM. You can feel it when you drive it. Mine will rev past 2,500 ... but I'm not doing anything at higher RPMs, I'm just making noise. What I consider to be the "natural" shift point is around 2,250 RPM. By contrast, my 2000 XJ 5-speed really likes to be shifted at 3,500 RPM, and hills I can drive up in 4th gear with the old Renix at 30 MPH require downshifting to hold 30 MPH with the new 2000.

 

You really cannot extrapolate from one engine to say anything meaningful about the other (except that they're both Jeep engines and will run forever if you change the oil).

 

Eagle, thanks for the excellently detailed explanation. My block is a 96 (my original 91 block could not be bored safely over .030), but the head and intake are from the original 91. And since I changed the gearing from 3.55 to 4.10, highway cruising @ 70 MPH raised the RPM to just below 3000RPM, closer to the "sweet spot", and I'm getting a little bit better highway mileage too with the AW4. But this was offset by the decreased mileage in town. Many thanks, I understand now. :cheers:

[Eagle]

Eagle, thanks for the excellently detailed explanation. My block is a 96 (my original 91 block could not be bored safely over .030), but the head and intake are from the original 91. And since I changed the gearing from 3.55 to 4.10, highway cruising @ 70 MPH raised the RPM to just below 3000RPM, closer to the "sweet spot", and I'm getting a little bit better highway mileage too with the AW4. But this was offset by the decreased mileage in town. Many thanks, I understand now. :cheers:

Another thing you younger folks don't take into account, because you weren't driving and paying for gas when it was a problem, is that AMC was oriented toward gas mileage. Especially in the days of gasoline shortages and being able to buy gas only on odd-numbered or even-numbered days, to match your license plate number.

 

Both my 1966 Rambler American (I-6, 28 MPG highway) and my 1968 Javelin (343 V8, 24 MPG highway with an honest top speed of 147 MPH) were geared at 24 MPH/1000 RPM. That meant 60 MPH was eactly 2500 RPM, and 3,000 RPM took you 72 MPH.

 

We didn't have overdrive top gears then. The Rambler was a 3-speed manual, the Javelin was a 4-speed. Top gear was a 1:1 ratio. The way the government economy tests were set up in the early days, just adding on a 5th gear as an overdrive and dropping the RPMs, along with an engine designed to produce maximum torque at low RPMs, was the way they went about generating good numbers on the tests. 60 MPH cruise in my '88 XJ is only around 1600 RPM. It's really operating BELOW the torque peak. I think I would get better fuel mileage if I were to swap in 3.54 or even 3.73 gears ... but with 273,000 miles on the thing now, it doesn't make a lot of sense to be dumping big money into it.

 

This is also why people who liked to drive at RPMs much higher than the engine's torque peak couldn't match the official fuel consumption numbers. I never had a problem. I always did better than the official numbers. (The '88 XJ 4.0L was NOT rated at 28 MPG, which I got once, or at 24 MPG, which I could repeat regularly. I still get 22 MPG highway.) Once Chrysler took over, they were more interested in selling to people who wanted speed and performance, so they designed the vehicles around different parameters. We all realize that a Jeep with a torque peak at 4,000 RPM is rather silly, but if it spends most of its time on pavement and it's fun to drive ... we sort of ignore that elephant in the living room.

[87mjdriver]

holy crap this thread got long winded. but real good info. Eagle's post really explains why I enjoy driving my renix MJ, It rarely sees 3,00 RPM's. The post opened my eyes to my engine behavior.

 

I can remeber when I had 3.07 gears and 30 inch tires and got an honest 21 MPG. my step dad could not belive that my old truck got better milage than his new izuzu.

 

Anyway,

 

Renix motor. mandrel bent exhaust. 2.25 or 2.5. I am not going for noise. just mileage and not to lose power.

[Eagle]

holy crap this thread got long winded.

Sorry ... I tend to get carried away on occasion. As a friend says (about himself) on another forum, "Why write a quick note when you can write a novel?"

[dakal]

great article. i have a renix with 4:10 gearing. mileage 14 to 19 miles per gallon. will check what i have and run some tests.

[Eagle]

Gear ratio alone isn't the entire picture. Tire size enters in. As I've posted before, my '88 MJ got the 3.73 gears I originally bought for the XJ, and for the XJ I was going to run those gears with stock 225/75R15 tires. The MJ ended up running those gears with 31x10.50s. In terms of speed to RPM, the actual final drive ratio was exactly the same as my wife's XJ with automatic and 3.54 gears on stock tires.

 

And set up like that, the MJ got 19 to 21 MPG on the highway, with the tailgate up. I'm now driving the MJ on stock 215/75R15 tires. It's a great ratio for street driving, but the speedo and odometer aren't accurate now so I haven't calculated the gas mileage with this setup.

[Car RamRod]

With my 3.55s I usually cruise at 22-2300 rpm. Lately I've been going in the slow lane at around 2050-2100rpm and the mileage difference seems noticable. I might throw in my 3.07s and downsize tires a little bit for the summer.

[Eagle]

With my 3.55s I usually cruise at 22-2300 rpm. Lately I've been going in the slow lane at around 2050-2100rpm and the mileage difference seems noticable. I might throw in my 3.07s and downsize tires a little bit for the summer.

If you're on stock tires that means you are cruising at 75 MPH. If you're on larger tires, it's even faster. 3.07 gear will NOT help your gas mileage, and may make it worse. The problem is you're driving a brick on wheels, and aerodynamic drag increases exponentially with speed. Back when I briefly owned a 99 WJ 4.7L, which had a more slippery shape than an MJ, I found that the gas mileage increased about 3+ MPG if I dropped the cruising speed from 70 to 60. I suspect the difference will be greater with an MJ.

 

At 2300 RPM you're in the "fat" part of the torque curve. You won't gain much gas mileage by gearing to run well below the torque peak. That worked for AMC to beat the government economy tests, but those were run on a dyno, with no aerodynamic drag to factor in. That's why those mileage figures were pretty much invalid except as a comparison between different vehicles. They were useless as far as predicting what mileage you'd get in real world driving.

[Car RamRod]

Thats a good point.

 

If I drop it, and put a big spoiler on the tailgate, will that help my aerodynamics?

[Eagle]

Running with a net instead of a solid tailgate helps, or a cap or a full bed cover