Basslicks

You can get any FM muffler in virtually whatever inlet/outlet diameter you want. 2" all the way up to 3"

http://www.flowmastermufflers.com/50-series-delta/

(or 2.5" to 4" for the HD 50 series)

http://www.flowmastermufflers.com/50-series-hd/

Basslicks

I really wish we could get someone to dyno their Jeep before and after increasing diameter to 2.5"

I can't say "boo" about it because mine already had 2.5" on it when I got it. I will say though that when I was driving around the loaner XJ with the OEM exhaust on it, it had very much less ***** than mine.... mine is an auto w/ 3.55 gears and at the time had 31" tires on it, the loaner was a 5 spd with 235's and 3.73 gears... and ran better than mine!

All that being said, I really think the 1/4" difference in diameter is negligible. Now if you went from 2" to 2.5", you'd notice a difference one way or another.

5-Speed

Ill throw mine on the dyno for a before and after. Got one a couple hundred feet away from me. But its going to take some time since I wont be doing the exhaust for a while...

Basslicks

EXTREMELY interested in seeing the results

Battle

Manifold : Gibson stainless steel header
Downpipe: stock
Catalytic Converter: stock
Muffler: 2.5" Vibrant stainless steel
Tailpipe: 2.5"

Other noise modifications:
K&N cold air intake
62mm throttle body

Basslicks

Nice!

Waynerd

Larger exhaust diameters will equal less low end. It's relative to exhaust velocity. Not sure a quarter inch will change too much. But I've always believed in the fact that exhaust size is optimized from the factory. I mean if they could have gained any power by using a larger diameter exhaust then why wouldn't they? The cost increase would probably not be much if any.

Also straight piping is the worst. When I was in school at American Motorcycle Institute we dyno'd a Harley with straight pipes, with and without baffles. Without baffles it lost 5 horsepower. That was a stock motor though. Any other mods obviously may have gained that 5 horse back. Larger exhaust's are only really for high rpm applications. Making a ton of power at crazy high rpm then the stock exhaust will become more of a restriction and the larger exhaust will then accommodate for higher velocity without turbulence.

So possibly a 4.0 stroked, cammed, ported and polished with larger valves and a high flow intake and bored throttle body would benefit from the larger exhaust. Stock configuration not so much.

Waynerd

This is my favorite sounding of all I have heard so far. It's fairly quiet with a bit of a growl. Very clean sounding and not obnoxious. I have a 40 but with no cat at the moment. Mines a bit raspier for some reason. I need to record a video and post.
I had one on my last phone but the touch screen isn't working right now so I can't use that. Maybe I'll do a cold start vid in the morning. It's only going to be about 10 degrees probably so it'll be good. It's much louder until the Jeep warms up. Then it's not loud at all. Wether driving or idling. I mean you can tell it's not a stock muffler but by no means is it loud.

Basslicks

Cost increase on a single model would be negligible, yes. Cost increase on the entire production span of the vehicle is pretty significant. The main reason, however, is to improve their NVH numbers. Larger diameter exhaust = more noticeable exhaust tone = more "noise" = lower NVH rating.

I borrowed a '93 Country from someone for a while when I was saving for a car for my wife a few years back. The thing had the OE-spec exhaust and you couldn't hear ANYTHING when it was running... not even so much as a lifter tick (jeep was babied mechanically). It was also a 5-speed.... there was definitely a noticeable difference in acceleration between the two, however. Not saying it was astronomical, but you could tell there was a little more hitch in the giddyup of mine with the larger diameter, free-er flowing exhaust.... and that's even with mine having larger diameter tires.

RocketMouse

On the good ole butt-dyno. Yeah I've worked on/ built cars before that it made a difference. But I always balanced out more intake with larger exhaust. That seemed to always work well. Keeping within the right amount of back pressure is important on some models too.

Basslicks

Okay.... I'm seeing the term "backpressure" being thrown around a lot in this thread and I'm tired of cringing.

The idea that backpressure is needed in an exhaust system is a misnomer. NO car
"needs" backpressure. What is needed, in EVERY exhaust system, is exhaust velocity and exhaust gas scavenging. Example:


The thing about the XJ exhaust specifically, is that it has gradual step-increases. It goes from a 1.5" exhaust port into the 1.5" header tube, then into a 2" collector, then into a 2.25" (or in the case of a lot of modifiers such as myself - 2.5") then into a baffled and/or chambered muffler. There is plenty of negative pressure in these systems, whether you're running the OEM 2.25" or stepping it up to 2.5" exhaust, to evacuate exhaust gases. Here's a handy chart I've come across that can be used as a guide to selecting proper pipe diameter for your exhaust according to horsepower ratings:



So if you use our XJs as an example, the 242 CID 4.0 liter inline six which ranged from 177hp in 1987 to 190hp in 2006, we see that the correct range of pipe diameter is between 2.25" and 2.5" SINGLE exit exhaust. Yes... if you're running dual, you need to stick with 2" - 2.25" pipe diameter after the split.

Also, in 1991 Chrysler increased (the advertised) hp for the 4.0 to 190, so if you use this chart (see below) as a guide, the 2.25" diameter pipe is slightly too small:



Again, Chrysler was fine with restricting the performance of these vehicles in the name of making it "quieter" which, we all know that an uncorked 4.0 is NOT quiet. Example?


This video is my XJ with a 2.5" exhaust from the downpipe back. Factory cat, and Dynomax Ultraflow muffler. As you can see, it's exhaust sound is noticeable and yes, you could hear it on the inside as well. The average american family wasn't looking for that out of their family grocery-getter.

As far as straight pipe exhaust robbing power - I never noticed a decrease out of mine when it was straight piped. I'm sure it's different for every motor and vehicle though, and the example of the Harley previously used here is not the first example I've heard of in the motorcycle world. Bike motors like baffles for some rhyme or reason. Apparently they promote better velocity and scavenging.

Anyway, that's my .02

Bugout4x4

This is a very good post and right on the money. I think the bike thing may have started with two strokes. For some reason they run better with an expansion chamber and then a restricted output. Maybe so they don't push as much of the intake mixture out of the exhaust ports on the way back up? But here's one for you... Ca in their infinite wisdom decided everything needed a restriction. They force manufactures to slide a restrictor up inside the manifold to cat pipe where it can't be seen on most cars. I guess some genius decided that if you plug the exhaust up it will put out less dirty air. Par for the course... lol

Basslicks

lol.. yeah don't even get me started on the great state in which every product under the sun contains a substance known to them to cause cancer.

Bugout4x4

Yeah, but luckily the rest of us are safe, because apparently these only cause cancer in Ca. lol

Basslicks