alpine.adrenaline

I'm hoping to improve the flow through my engine and I know the header is a good place to start. The majority of the search results turned up recommendations but I need more of a walkthrough... I've pulled an engine in these things before but that just involved disconnecting it at the downpipe. Beyond that, I'm not sure what I'm getting into.

If I use APN's (ineedparts.com) header# 15343 with a 2 1/4" outlet, how do I find a downpipe to match, or is my best bet to have one made? I'm looking forward to getting rid of that restrictive downpipe that looks to have a dent in it.

Anyone have issues bolting that header to a '99?

Next, a high-flow cat, right? Then a MagnaFlow muffler or something like it, though I'm not interested in sound as much as performance. Do these all come ready to fit to each other and tightened with a clamp, or is welding preferred? When I order them, I just need to make sure everything has a 2 1/4" inlet/outlet, correct?

I appreciate the advice!

413maxwedge

Go with the 2.25 all the way or into the muffler with the 2.25 and out of it with 2.5 or 3 ''. Go with your choice, as far as the downpipe I beleive that the dent is factory, you can buy a new one (parts store) or have one made when you get the exhaust installed or when ever you want it.

5-90

2.25" from the collector to the muffler - 2.5" for a large stroker. Going larger can (and usually does) cause internal flow turbulence due to uneven cooling of the exhaust gas pulse.

Going larger at the muffler exit isn't as big a problem - but I wouldn't go any larger than 3".

Going with a larger-than-optimal pipe size causes the exhaust pulse to slow down, the outer annulus of the pulse to cool, and that causes slower flow, rather than faster.

The idea of going with an exhaust as large as possible comes from the turbo world - but the turbine wheel breaks up the pulses and causes the exhaust to go as a constant flow, so pulse cooling becomes a non-issue. Since you don't have coherent pulses, you don't have to make use of the low-pressure area between them, and you don't have to worry about internal turbulence (since the high-pressure zones are gone.)

The riceboi kids don't understand this - and they actually lose performance when they throw three-inch tubes on their 1300c/c engines. There's really a very good reason that most of these compact cars with small engines run an exhaust pipe under 2" ID...

JeepingDan

X2 ^^^

alpine.adrenaline

5-90, thanks for your input. I'm curious, do you teach? You've a knack for explaining the science behind things, like the necessity of controlled flow in an engine.

How would a 4.5L stroker do with a 2.25" outlet on a header? I'd like to build one down the road (pending completion of the garage) so that's been a consideration. Wish I could upgrade the exhaust during the rebuild but the header repair is somewhat pertinent.

If the increasing the diameter after the muffler isn't influential, I'll probably just go with 2.25" throughout. Will a high-flow cat create unwanted or too much turbulence?

The ricerboi effect- something I certainly want to avoid. No chrome tips either, thanks. Wish it could still sound stock despite the larger diameters, but a deeper tone shouldn't hurt.

5-90

I only teach informally - I'm "NDGA" at the moment (No Degree, Good Anyway. I worked for a guy who signed himself "Joseph C. Chen, MBA" - and I've always found that pretentious. I started signing myself "Jon D. Kelley, NDGA" shortly after. He got seriously butt-hurt when I explained what it meant, and even worse when I explained why, but he dropped the "MBA" on his signature.)

I've just made a huge hobby out of knowing how and why things work, and I've been in positions where I had to explain it after I understood it. Therefore, I look for ways to explain difficult concepts in simple language, and I usually seem to manage.

Engine capacity of 4.5 litres is "borderline" for 2-1/4" - that's the point where you want to either step up to 2-1/2", or keep the tach below 3500rpm. At that point you'd also benefit from a slightly larger collector at the header - but the construction of the XJ exhaust is such that it's less of a problem than you'd think it would be (and if you were going to have a new header bent up, you'd have to make the primaries about twice as long as the longest on the OEM just to get the torque band of the thing where you really want it to be - down around 2800-3200rpm. The lower the crankshaft speed for peak torque, the longer you want the primaries to be between the port and collector flange - this is, again, to take advantage of the low-pressure volume between exhaust pulses. This is what makes "equal-length" headers so useful, but they have to be length-tuned to time the exhaust pulses to be of maximum utility. You end up with the effect of each exhaust pulse "drawing" or "pulling" the next one out, which is why primary tube length becomes important.)

A high-flow cat should not cause any trouble - and can actually be of benefit! Bear in mind that a catalytic converter needs to achieve a high operating temperature in order for the catalytic reaction (reduction/oxidation) to take place, so you don't really lose heat in the thing. And, most of it is ceramic, so the only real loss of heat is through the cat case (and not in the catalytic element proper.) I understand that Random Technologies does an excellent job with their catalytic converters - but the last time I had to replace one (on my RENIX,) I just got a cat spec'd for a Big Block Chevvy (single cat application) and bolted that in - which got me better exhaust flow, and improved throttle response. I don't have "before/after" dyno pulls, so I can't give you more definitive results than that (I didn't check 440 times before/after either, and I'd have to weigh the rig to make such information useful anyhow.)

1991Jeep_Man

Well, not to jack the thread but I think my question might help the OP's original goal, but the dent (for driveshaft clearance issues) in the down pipe wouldn't cause a huge issue would it? I would think that since it is only about a 2" span, the exhaust gases would only momentarily pressurize at that point and then just exit with a higher velocity, then equalize further down the downpipe. Plus, with the header not far away, those gases have plenty of force to deal with that dent dont they?

413maxwedge

same thing I said without the drawn out explaination for you, execpt I gave you the choice of tailpipe size since it wont hurt.Talk about someone trying to look smarter than everyone else,such bs.

Jamie57

I much prefer the EXPLANATION.
I get so pi%^$#ed at the smart azzes that just tell you something and it's supposed to be like the word of god or something.
i think it's because although they know the answer you embarass them by asking how they know it. Because they don't.

Keep up the good work Mr. Kelly.
I always learn something when you post.

neeland

It was said that the extremely large dent in my down pipe right after the exhaust manifold is supposed to be there? I noticed it the other day an thought "my god how did I manage that" what could possibly be the purpose for such a restriction right off the motor? Not trying to thread jack I'm just curious an it seems u have the right people here to answer it.


Lol I feel like a douche now. Didn't know it was already asked 3 post up. That's what u get for hitting reply before the very bottom of the thread.

5-90

No, that's OEM - to clear the front driveshaft at full stuff on stockers. As soon as you throw on a lift, it becomes unnecessary (but it is not an abrupt change in section, and it's relatively smooth, so the restriction is minimal.)

If you didn't have that formed dent, you'd form one as soon as you put the driver's front wheel to full stuff - and it wouldn't be as clean, as neat, and it would probably hole the pipe.

DFWAI.

5-90

Am I smarter? Don't know. I've merely spent a good deal of time reading the theory and evaluating the practise - and I've got plenty of experience.

And, I've been teaching various subjects for years. I've found it does no good to simply posit information as "Word of God" beyond the basics - if a topic can be reduced to more basic topics and assembled, the tutees will learn far more than if you just say "Q.E.D." Ergo, the explanation.

If the length of the explanation bothers you, do not read it. Simple, no? If you want to know, read it - and feel free to ask me questions. Chances are, I can stick my paw out and grab a reasonably authoritative reference that you can probably find in your library or at your bookstore, give you the basic information out of that, and give you the information on the reference itself if you want to dig some more.

I don't "try to look smarter than everyone else" - I simply try to teach people who ask questions. Why? Because, by posing the question in the first place, they've demonstrated a willingness to learn. That should be rewarded by teaching. How do you think I learned as much as I have? Most of it didn't come from school...

white91

I know that exhaust has a science to it, so why did jeep up the size of the head pipe in 00. I measured mine and it is a oem 2.5" pipe through the cat. it necks down to 2.25 at the muffler. I know they had to do this for the stricter emissions but wouldn't that have changed the pulses and earlier explained.

5-90

Probably in an attempt to balance reducing the primaries (which was done to keep exhaust pulse temperatures up, and improve catalyst light-off.)

Once the catalysts are lit - the cats are in the manifold 2000-up XJ, 1999-up other models - and the exhaust gas has flowed through the catalyst bed, it does tend to diffuse the pulses slightly. And, some of the heat got dissipated in the catalytic reaction (reduction of NOx to N2 and O2, oxidation of HC and CO to H2O and CO2,) so going to a slightly larger collector can make sense.

Necking down to 2.25" from 2.5" at the muffler entry doesn't make a whole lot of sense - but they may have done that to continue using the parts from 1999 and earlier. Are the part numbers for the mufflers the same? I don't know offhand.

No-one ever said that everything that comes out of the factory makes sense - you can get more sense out of custom aftermarket parts than you can from most OEM parts...

honesT

Welding the pipes together with a small bead will keep them from seperating on the trail if they ever get snagged.

I had the dent removed from my down pipe by a welding shop for a steep $90 after searching the wreckers with no luck of a ding free pipe.

Dent in down pipe? Quote from Jeep forum from cruiser 54 :
Absolutely factory. Was there when we did the Pre-delivery Inspections at the dealership. It's not there for clearance either. It's disguised to be there for clearance but was added to the pipes in order to heat up the catalytic converter sooner to please the EPA.

If you are standard height, it will hit in some off-road conditions. With a 2 to 3" lift, you'll be fine. The factory could have put a sweep in that pipe, had no crush, and had tons of clearance. I have a local muffler shop make me a downpipe with two 45s in it instead of the 90 like stock. End quote.

I up graded to a high flow exhaust system after reading how it improves gas mileage due to the trans shifting into OD sooner. MY jeep is still STOCK with stock sized tires & I’m at 1250/1300 RPMS at 70 Km per hr. I will post on another thread what hwy RPMS are later.

I kept the stock down pipe (from manifold to cat) due to many conflicts stating that I would lose low end torque, I’m still on the table with this but why take any chances... AS explained earlier swapping the header sounds like the culprit for losing low end torque, not to rule out the stock pipe. I`m happy enough with the results of the high flow cat & muffler so I`ll save my $$$


I don`t know what the gas mileage was before but with city stop & go traffic & putting the pedal down a few times I`m at 315km with 1/4 tank left, so it looks like 400-450 km per 75 litres (FULL TANK) of gas in city driving. With stock sized tires


The high flow is a little loud but with all the windows down still comfortable enough to have a conversation with your passenger, no where close to those stupid cherry bombs the kids that rush into everything toss on there cars with absolutely no power gains