Number21

I have a 1998 XJ, and recently the muffler came off after the cat due to being rusted out. I'm currently driving it that way and it sounds like a broken UPS truck.

I already have a couple new Flowmaster 40 mufflers I was going to use on another truck I no longer have, and they are just going to waste sitting in a box. Problem is they are 2.5" and I believe the stock XJ exhaust is 2.25".

If I use a reducer on both ends of the muffler, will it sound bad? Will it reduce power from having not enough back pressure?

On the other hand, should I use a reducer on the front of the muffler, and then replace the tailpipe with a 2.5" all the way back?

The Jeep is otherwise stock, and will probably stay that way.

I AM GOD

The "back pressure" thing is nonsense.

Use'm.

Number21

Only to some degree. Power and gas mileage can be effected in different ways. For example, I drove an XJ once with an open header to get some parts - almost no back pressure, it had very little power.

Also, I'm wondering if that will sound bad, and, should I increase the tailpipe size or just use a reducer?

unidentifiedbomb

I didn't notice a difference between my 2.5 flowmaster and the 2.5 tailpipe vs the 2.25 tailpipe i replaced it with when the 2.5 rusted out. I'm actually replacing my stock cat and muffler soon and going with a 2.25 cat and 2.5 muffler again. Sounds good to me.

I AM GOD

Join and watch the entire test......and much more.

Outlaw Star

A 1/4" increase in size will not effect your back pressure or performance that badly. Majority of the aftermarket cat-backs for the XJ are 2.5".

And as for back pressure being a myth, wrong. No back pressure will effect your low end power on these engines. Will your engine still make power without it, yes. But you will have to rev the engine higher to do so. The 4.0L was designed for low end, not high rpm "screaming" If you want a racecar, wrong vehicle for that. Now if you have a stroker engine, I don't think revving it up will be a problem.

Outlaw Star

Also, A V8 engine is a different animal over an inline style engine.

Jeepin'_Aint_EZ

When my factory "cat-back" exhaust broke, I simply put together my own exhaust from parts laying around the shop. I had a muffler that used 2.5" pipe so I just adapted it to the factory cat and made a homebrew catback. used 2.5" pipe after the muffler. Did I notice any difference? No. Not one bit. Except it's louder. My butt-dyno doesn't show any kind of change in performance. Do I care? Nope because my Jeep isn't a race car.

mschi772

Roughly stock 4.0? You'll live just fine with a 2.5" exhaust. Ideal? My educated leaning is toward 2.5" at the header down to 2.25" some time before or after the muffler. As the exhaust cools, it gets denser, and the shrinking pipe helps to maintain exhaust gas velocity. Reality? Mine is 2.25" the whole way because that was what was easiest for me at the time. You have a 2.5" muffler available to you, use it (I'd reduce both ends and keep the 2.25" tailpipe you have unless it is in bad shape).

Remus Redbone

"Back pressure" is not nonsense. A change in pneumatic flow will change the RPM range where your peak torque develops. Huge open exhaust may be great for the Bonneville Salt Flats & 9000 RPM, but it's not always the best for off road. It can move the torque range into an RPM range you can't achieve without selecting another gear. I stopped on this post to point this out. I suspect others will confirm.

As for whether 2.25" up to 2.5" will matter; it's possible, but that's yet to be seen. Diameter is not the only thing that affects flow, and you know what "Flow"masters are designed to do. One on an inline six might not make as much difference as two on a V8. If it's a highway rig, it might help.

I AM GOD

Using your theory, a 2" exhaust would provide even more back-pressure, therefore increasing torque.

It wouldn't.

People believe what they believe even though there is no basis in fact for what they believe.

Rogue4x4

GOD has spoken

Turbo X_J

Almost 100% probability, a loud muffler or missing exhaust on a Jeep just makes it more apparent it's not going anywhere fast.
Try the Walker quiet flow stainless, it's quiet like Jeeps were meant to be. Plus when you step on the gas it's not like stepping on a wet sponge.

peligro113

1/4" is not going to cause back pressure issues

Jeepin'_Aint_EZ

BUT #CHERORACECAR! MUST BE LOUD!

I approve of the quiet flow walker mufflers. They're not bad and work pretty damn good.