95Cherokee

Actually Quadratec has the tail pipe for half the price of Performance curve, but they don't list the diameter.

http://www.quadratec.com/products/51409_11.htm

5-90

2.25" ID is about the stable optimax for the 6-242 at usual crankshaft speeds - any less, you're restricted by pipe volume. Any more, and you create a restriction due to uneven exhaust gas pulse cooling.

Fortunately, the OEM plumbing is right about 2.25" ID, as I recall.

The two big restrictions in your system are the catalytic converter and the muffler - I mention the restrictions because those are the power robbers in exhaust (pumping losses.) On my RENIX, I went "oversized" on the cat (the unit I got was for a G-van with a Big Block Chevvy, single unit. So, not quite twice the displacement was intended, which improved exhaust flow) and get a muffler with a gas path as close to straight as possible. (If you don't mind noise, go with a glasspack. If you want something reasonable, I have had excellent luck over the years with the Walker Turbo II series. Don't go by the catalogue, just get a muffler that fits, that's rated for more engine than you have.)

The only real problem is if you have OBD-II and you try to use a "universal" cat - but that's an easy fix. On the pipe stub you know you'll need, have the exhaust shop weld an M18-1.5 bung in to accept the HEGO.

And, do yourself a favour and do NOT eliminate that four-bolt flange in front of the cat! You have NO idea how much work you're letting yourself in for if you eliminate that handy, easy-to-separate junction in the system...

(If you're going whole hawg through the exhaust, I would also suggest having your downpipe "broken" and put a 4-6" flex section in. Collector weldment failures on the header are typically caused by failed/failing engine mounts, coupled against a rather rigid exhaust system. Allowing that bit of flex will allow you to merely replace engine mounts, not engine mount AND exhaust manifolds!)

The working idea here, as far as "exhaust for economy," is to knock pumping losses down as much as possible. You may only gain 1-2 measurable mpg (probably closer to 1,) but this can compliment other efforts - kinda like "indexing" spark plugs in a high-output engine. By itself, it's very little - but it can make other mods work rather better than originally anticipated...

terzdesign

It's interesting that the OEM setup does not feature a flex pipe. My toyota celica has one.

it sounds like 2.5" is the way to go. Also, my 02 sensors are going bad as the CEL was on and the code was P0138 so those will have to be changed. Do I need to make adjustments for those myself? How will opening the exhaust up like 2.5 affect them? I haven't been under the car to see what they even look like yet.

terzdesign

I think I like the magnaflow best for muffler since it doesn't seem to be too loud. Which one is the best for a stock xj?

BlueRidgeMark

Ah, that would be why weekend racers often disconnect their exhausts completely to gain a bit of power. Sometimes they even take off the manifolds. They risk warping some valves that way, but they do get some power.

And that would be why real racing machines, I mean high dollar, big buck sponsored teams with real engineers, often have little more than short, straight pipes on their engines.

Yeah, that would explain it.



This is why a properly tuned system will work better than a cobbled-together system that is larger.

Science is what wins this battle, not just size. Bigger is not always better.

You cannot beat an engineer with a computer modeling system. He'll win every time, as long as he doesn't have his hands tied by corporate bean counters.

What does that mean to us?

It means buy an aftermarket SYSTEM that is designed for your vehicle. Don't just cobble parts together because somebody said, "This is the really cool thing that works great!".



Same place they get the "Dielectric grease is BAD! It's a INsulator, Maybel! Run fer yer lahf!!"

It's from people who didn't pay attention in science class, so they have no unerstanding of basic principles, repeating what they heard from some other guy who also didn't didn't pay attention in science class. Urban legends, passed around and around and around until they are Unquestioned Wisdom.



Um, maybe not. Please re-read salad's post and 5-90's post.

Bigger is not always better.

salad

Unscrew old sensors, screw in new sensors. Done. Apply heat and curses as necessary.



Dielectric grease is an insulator, and I've seen it break things. The right stuff to use on electrical connections is an electric grease that promotes conductivity instead of preventing it.

These products are designed for that: http://www.k1ttt.net/technote/antiox.html

Speaking of people who didn't pay attention in science class... lol

BlueRidgeMark

Yes, except at the contact surfaces. It's too thin there to insulate. Research "contact pressure". Fun stuff.

I can't imagine how.



That's the LAST thing you want in a multi-pin connector. (Unless you like stray currents going place they weren't intended to go.

That's what dielectric grease is designed for.

On single-circuit connectors (battery connections, grounds, etc.) conductive grease is great.



Or speaking of electrical engineers who know quite a bit about things like this because they have taken a lot of science classes and passed them easily and work with it every day?

5-90

NNnnngggg - I don't know who started this myth, but I'd like to look him up and punch him in the throat!

Backpressure is never needed for engine performance. Period. Full stop. All of the definitive literature agrees, and this is stuff that was determined through testing, experimentation, and measurement - not merely theory.

Say it with me, class: "Backpressure is never necessary for operation of an engine."

Doesn't matter - two-stroke or four-stroke. Diesel or gasoline. Even if you're not set up to scavenge every little bit of exhaust gas from the engine (usually through valve overlap and resonant tuning,) backpressure will increase pumping losses for your engine combination, which results in parasitic losses.

ALWAYS

Now, that's not to say that backpressure hasn't found a use in emissions control - that's how ChryCo eliminated the EGR valve for 1991-up 6-242, by using trace backpressure to encourage exhaust port reversion, which accomplishes the same "useful function" (those who have followed me know what I think about EGR...) without use of a failure-prone part. But, it is NEVER for performance.

So, one more once - backpressure is never actually useful to engine performance, often dampens economy slightly (anything that hinders combustion efficiency OR increases pumping losses USUALLY is harmful to economy,) and anyone who tells you otherwise is pushing bogons.

("bogon" - Quantum particle of BS.)

Sorry about this - but science and observation both bear this out. Think of "backpressure" as having a kink into your exhaust pipe. However small it may be, how's it going to help performance?

As far as exhaust sizing goes, our 6-242 falls right in between wanting 2.25" or 2.5" ID plumbing, but falls closer to 2.25". A stroker will usually push that to be closer to the 2.5" end of things - but no so far that the backpressure created will be significant at low RPM - which is where our 6-242 (OEM or stroked) really shines. Therefore, you're actually a bit better off going with a size smaller than you could ever be with a size larger (considering exhaust optimising calculations are often done using redline RPM - and we're usually down around half that.)

salad

Okay, so you aren't some flunky singing the tune of "it says so on the Permatex package".

I agree with the above. Though it should be very obvious how dielectric grease breaks things outside of the lab - connectors with very low contact pressure and too much corrosion will just eat the stuff up and pits will fill in with it. Presto, no worky. Think 10-year old headlight receptacles. A little smear on the pins works wonders.

Carry on...

BlueRidgeMark

Yep. Most things work better when you follow the directions.

Carry on...

djb383

There are myths on the Internet and in forums?????

salad

No. Only on History Channel.

Radi

There is only truth on the internets. Something I read on the internets says so!

Radi

Yes? You rang?

5-90

Yeah, but the backpressure myth is rather older than the Internet...