So yeah, horsepower is a bit of an issue with my rig. ('97 TJ, 31x10.5s, auto) I'm not really into hauling ass, I've never not been able to go anywhere I wanted to go offroad for lack of horses-I think of my jeep as an oversized Willys. And for most of my running around town, it's more than adequate. I was a pretty fast driver when I was younger, but honestly, now I kinda like to travel in what I call "Jeep time." A little slower and cooler than the millions of identical BMWs around Los Angeles. The only time it becomes an issue is up really big hills, and there are a lot of them up near Acton where I go to ride horses. It's either keep it to the floor and cycle the daylights out of the engine, or just downshift and hit the right lane, keeping her at around 4000 rpms.
Recently, my cat failed. I started hearing this godawful rattling that sounded like my crank was flopping around, turns out the aft matrix (there are two sections inside) in my catalytic converter had jarred loose. It probably still functioned ok, but I couldn't live with the sound. In the process of diagnosing, I finally nailed down where the sound was coming from (whew) and cut the cat off to have a look inside. It was definitely loose, so I grabbed a section of 2 1/2" tubing, u-bolted it in and headed to the zone. I was kinda surprised- there was a little increase in pep, but nothing major. So it got me thinking, and reading all over the Net, and from all I could find, replacing factory exhaust with larger diameter tubing reduces resistance to exhaust gas flow, so I decided to upgrade. I went 2 1/2" all the way from the down-tube, back, welded every joint meticulously so as not to inhibit flow, really put a lot of work into it. Magnaflow cat, Raptor muffler (cheap, but from every review I could find, people seem to be really pleased with them.), stainless, mandrel-bent tubing.
First, I have to say, it sounded awesome. Really low sort of rumble. Badass. Loved it. The only problem was, it ran like POO. It was like I was towing someone. At highway speed it was a little better, but when I decelerated, I got backfiring. I'd like to say right here that I do understand the need for back-pressure. I'm certainly no elite level pro mechanic, but I've been lying under and standing over engines since '78, made my living at it a few times early on when I was starting out in my real job. But I have to admit that virtually every time I take on a new project, I find out how little I know. I expected that the smaller diameter (2") of the forward part of the down-tube would accomplish that. After the section of 2" that lowers the system down from the manifold, it widens to 2 1/2". I took that width all the way back. It seemed to make sense.
I won't go into all the profanity and process-of-elimination tail-chasing that ensued, but I have to say, it was crazy-frustrating for a while. Then, when I was looking for a cat based on a part-number a muffler guy gave me, I ran into this page at Magnaflow. (
http://www.magnaflow.com/wideopen/performdata.asp FYI, that's not an endorsement for Magnaflow, but it is pretty useful info.) I was WAY over the maximum.
Now, I'm normally suspicious of anything that's written on a commercial site. I have the dubious distinction of being in the entertainment business AND was in sales when I was younger, so I know virtually every syllable related to a commercial product is selling me. But this was sort of principle-based, or seemed to be, not product-specific. So I decided to try implementing it as best I could, yet with some of the parts I already had. It was a holiday weekend, so no parts were available.
So here's how it ended up: after the down-tube which is entirely stock, which starts at 2", then widens to 2 1/2", I welded in the Magnaflow cat p# 46736 (priced it out with Kragen/O'Reilly, not Magnaflow directly. Difference between $800+ and $230. Crazy.) which is also 2 1/2". Then, I stepped-down with a short section which reduced from 2 1/2" to 2 1/4" for the entry into the Raptor muffler, then, I stepped down even further with another reducer to the original 2" tail-pipe which I cut off the old, OE muffler. Honestly, it was kind of a lark. I was really kind of just trying it because I didn't have anything else to do, and I like to weld.
It's never run so well. I have better low-end-torque than I ever remember having. Ever. And I had to run into Hollywood sort of last-minute the other night and it handled merging and higher speeds better than ever too. Not that it's all of a sudden a six; it still downshifts on hills, but it's definitely better.
Of course, after I started all this, I realized I probably could have simply welded a spot through the converter wall that would have basically created a shim or pucker inside, holding the element in place, but I didn't think about that possibility until it was too late. Plus, I think in CA it's illegal to do any type of mods to a cat, even benign ones, so it may not have flown anyway. But either way, I'm really glad it happened. Dudes, I pulled out of my driveway the other day and gave it the same gas I used to, and it actually chirped the tire a little. Which has
never happened. Again, I'm not saying it squalled the tire, but it was definitely trying to break loose.
There was a statement on that site that I think is the key to all of this really, where it says (product-specific, sales-oriented wording removed): "
Keep in mind that the goal is to improve exhaust flow. In most cases, just changing the restrictive OEM muffler and replacing it with the same size straight-through, performance muffler will do this job."
I'm not sure if this would work exactly the same way for anyone else, but there may be something here that some of you guys can use. When I have time, I'm leaning seriously toward the throttle-body (and associated mods) swap. I just really want to get comfortable that it's not going to screw with the reliability first.
