2.5 " exhaust upgrade worth it?

[big66440]

Just for kicks, would upgrading the diameter of the exhaust to 2.5" make a difference? I know it will kill some of the velocity but will it slow it down to the point were I will feel a (negative) difference in the lower RPM range? BTW I'm planning on just dumping it before the axle,thanks.

[Jeep Driver]

Won't make a difference much one way or the other.

[Eagle]

Just for kicks, would upgrading the diameter of the exhaust to 2.5" make a difference? I know it will kill some of the velocity but will it slow it down to the point were I will feel a (negative) difference in the lower RPM range? BTW I'm planning on just dumping it before the axle,thanks.

 

Yes, it will make a difference, and not for the good.

 

Way back, when my '88 Cherokee 4.0L needed its first exhaust replacement, my brother was manager of a Speedy Muffler shop. So I took it to him, and he talked me into a 2-1/2-inch exhaust system and turbo muffler. BIG mistake. The 4.0L is a torque engine, and a larger exhaust hurts low-end torque while favoring high RPM power. The problem is, you're just not likely to be running a 4.0L consistently above 5,000 RPM.

 

In my case, I could instantly feel the loss of torque and acceleration, but I have no way to document it. What was quantifiable, though, was that my gas mileage dropped instantly by at least 2 MPG under all operating conditions.

 

Fortunately, the Speedy tailpipe and muffler didn't last more than a couple of years. Normally, I'm bummed when the exhaust system goes. This was the only time in my life I was actually hoping for the exhaust to die. When it did, I replaced it with an over-the-counter OEM spec 2-1/4" tailpipe and muffler from Auto Zone, and I regained my seat-of-pants perfomacne and my gas mileage.

[Jeep Driver]

Far more contributes to back- pressure......or the lack of it.

 

Cat, no cat, muffler, dual exhaust, crimp or no crimp in down tube, header, dump before axle...........

 

Increasing 1/4" in tubing size is not the only thing to consider.

[gogmorgo]

2.5" is a 20% increase in diameter and 23% increase in cross-section area over 2.25". That's huge. Unless you've got issues with your setup, i.e. clogged cat, there's no way you'll get as much change in flow, even going to straight pipe.

[Zebvance]

 I have a 88 4.0. when I bought it the previous owner had removed the cat and welded in a section of pipe to replace it. I redid my exhaust about two years ago. I Put a Pacesetter header on and went to 2.5 inch header back. Also put a 2.5 inch high flow cat and a magnaflow muffler. When I picked it up from the shop I hit the skinny peddle hard excited to see the extra power I had gained..... Well I immediately felt like a fat kid in a vegetarian burger joint. There was a huge loss of power. I can't really even brake the tire loose now... I thought It was cause I added a cat though. SO my whole intention up until now was to rip the cat back out and expect the power gain back. IF what yall are saying is true That isnt going to do anything for me. So looks like I need to redo the exhaust again?

[Eagle]

You do realize that the Jeep catalytic converter is not a constriction, right? The exhaust front pipe is 2-1/2 inches, and the 2-1/2 inches carries through the cat. The connector between the cat and the muffler necks down to 2-1/4. It wouldn't have cost Jeep anything to keep it at 2-1/2 all the way to the tailpipe outlet, so there's a reason why they necked it down to 2-1/4.

[HOrnbrod]

Any engine modification claim means nothing unless it's dynoed before and after. First things I did after the stroker install was upgrade the exhaust system and install a 62mm throttle body. Mike Leach headers and full 2.5" exhaust w. no cat. Net result was a ~20% overall increase in peak torque and HP numbers, although at 500 RPM higher peak points.

 

But I suspect all the above is probably true on a N/A engine; the larger exhaust diameter could hurt performance. But again, w/o dyno proof, I'm skeptical.

 

[Zebvance]

You do realize that the Jeep catalytic converter is not a constriction, right? The exhaust front pipe is 2-1/2 inches, and the 2-1/2 inches carries through the cat. The connector between the cat and the muffler necks down to 2-1/4. It wouldn't have cost Jeep anything to keep it at 2-1/2 all the way to the tailpipe outlet, so there's a reason why they necked it down to 2-1/4.

 

 

What about all the crap inside a cat? How is that not restricting air flow? You are displacing open area with material. In turn lowers the amount of area for air to travel through. Which is causing restriction. correct?

[big66440]

Guess Ill stick with 2 1/4" I might just see if I can redo the bends all the way to the muffler with some mandrel bent elbows the factory bends look like absolute $#!t :eek: , I think that alone with a Magnaflow muffler will be an improvement

[FrankTheDog]

My 99 XJ's oem muffler and tailpipe were falling off.

Went to my local garage and asked him if he had anything that would last longer than the 2 yrs I've been getting out of the parts store walker stuff.

He said a cherry bomb and his heavier exhaust tubing would do the trick.

"NO !! It has to be quiet. So he put a muffler he carries on it and bent a new 21/4" tailpipe and me and the dogs can't even hear my wife pull into the yard now. Perfect, didn't seem to lose any power either.

What ever you would gain with an otherwise stock engine just isn't worth the noise. In Don's case his engine has a bunch more power than stock so it could use a larger exhaust. Otherwise stick with stock size.

[JENSSEN]

On the 1998 Jeep Cherokee 4.0L Auto 4x4 (stocker) I just did the fully welded dynomax Ultra Flow muffler (PN 17512) and the ugraded tail pipe that comes in their cat back system (PN 55014). I wanted a bit more growl which I got with this combo, what I didn't expect is a gain in MPG. In the city I have been seeing as much as 3MPG gain since the system went on according to the data log from dash comander. No real hiway data to share and no dyno results, it does feel a bit more peppy but I also did a trans filter and fluid when I id the exhaust. The MPG gain could be from the trans service or a combo of the two repairs.

 

Seat of the pants dyno says it helped it breath a bit ;)

[FrankTheDog]

On the 1998 Jeep Cherokee 4.0L Auto 4x4 (stocker) I just did the fully welded dynomax Ultra Flow muffler (PN 17512) and the ugraded tail pipe that comes in their cat back system (PN 55014). I wanted a bit more growl which I got with this combo, what I didn't expect is a gain in MPG. In the city I have been seeing as much as 3MPG gain since the system went on according to the data log from dash comander. No real hiway data to share and no dyno results, it does feel a bit more peppy but I also did a trans filter and fluid when I id the exhaust. The MPG gain could be from the trans service or a combo of the two repairs.

 

Seat of the pants dyno says it helped it breath a bit ;)

I think the tail pipe on that exhaust is still 2.25" it's just mandrel bent so it has a bit less restriction. I had one on my ZJ and I liked it.

Just watch that polished tail pipe tip.

They like to rust off at the weld if you live where there is salt.

[Jeep Driver]

I give short answers because no one really cares for the long answer.

 

Read- http://www.autozine.org/technical_school/engine/Intake_exhaust.html

 

You alter the exhaust side without altering the intake side.............and who knows what the outcome will be.

 

And further-

 

I rarely if at all have read here where anyone has really given consideration to cam, cam timing, intake, ignition timing (yeah I know), fuel delivery.........etc......... when asking about exhaust.

[FrankTheDog]

That's why it's not worth going to a larger exhaust unless you are going to play with all that. Like in Don's case, stroker with other mods.

[HOrnbrod]

I rarely if at all have read here where anyone has really given consideration to cam, cam timing, intake, ignition timing (yeah I know), fuel delivery.........etc......... when asking about exhaust.

That's why it's not worth going to a larger exhaust unless you are going to play with all that. Like in Don's case, stroker with other mods.

Exacerly. Bore it out a little, mild cam, roller rockers, maximize the suck and blow sides with modded intake and exhaust systems, then dyno balance it all out and fine tune to correct AFR and fuel pressure. Hard to improve the ignition system - it's already pretty good. Gotta do it all to make a significant difference.

[gogmorgo]

You do realize that the Jeep catalytic converter is not a constriction, right? The exhaust front pipe is 2-1/2 inches, and the 2-1/2 inches carries through the cat. The c

What about all the crap inside a cat? How is that not restricting air flow? You are displacing open area with material. In turn lowers the amount of area for air to travel through. Which is causing restriction. correct?

 

 

A little bit of catalytic converter 101:

Going down to basics, a cat is a series of tubes lined with a catalyzing agent that triggers a chemical reaction, transforming bad exhaust gasses to less bad exhaust gasses.

These tubes are completely free-flowing with no obstructions. In most cats, they're perfectly straight, and you can kinda see through it, although the tubes are really small so you have to be looking at exactly the right angle. The tubes are small so there's lots of surface area for better exposure of the exhaust gasses to the catalyst.

If you've ever looked at a cat, you'll notice it's a lot fatter in the middle than your exhaust pipe is. Some of this is heat shielding, cause they like to run HOT!, but mostly this is so that the cross-section area of your exhaust doesn't go down. I won't say there's no constriction at all, because the flow does get a little jumbled going into the tubes and there's some turbulence on the way out of them, but it's not going to make enough difference to even be noticeable on the butt dyno. It is enough though to be noticeable on a real dyno, but you're looking at very low single-digit percentage gains by deleting the cat. 2% is a number I often see cited for a brand new cat being deleted. There are more power gains to be had by deleting an older cat, but that's mostly dependent on how clogged it was.