More exhaust backpressure

[xj92]

I have removed my cat because the stuff inside has broken loose and it rattles and I don't like having that big thing under there to hang on the rocks. My muffler is beat to death and has holes from jagged rocks and from hammering it on and off so I'm looking to replace it soon, but I want to replace it with a glasspak or something small and cheap. After taking off the cat my low rpm torque suffers a little, and it will probably suffer more if I put a glasspak in because it will flow more freely than the stock muffler. I already have slightly increased airflow going in with my drop-in K&N and 62mm TB spacer. What's a good, cheap, small way to get the backpressure where it should be without putting the cat back on and that huge muffler? A length of 2" pipe instead of the 2 1/4"?


'93 XJ/4.0L

 

[XJJPR]

Bang the pipe on a rock and smash it.

I don't know why not just put a cat on it and forget the muff. That's what I've done on my rigs.

mark

 

[xj92]

I've done the dented pipe thing but evidently not enough. I haven't put another cat on cause I'm cheap and I don't like the big clunky thing getting in the way on the rocks. I've got a 2.25" pipe in there right now replacing it, maybe I'll get a 2" pipe segment and just have the ends expanded to the proper sizes. Can you expand pipe that much?

 

[8Mud]

I welded on a two header flanges (three bolt), they come in sizes from 2-4 inches (in my mud racers, V-8, big tires). You could probably, use the extra square flange from the Cat, with the square flange on the vehicle and make a copper gasket with a slightly smaller size hole. I figured the area of the 2 1/4 pipe ID and made an assortment of gaskets in 10% (or close enough) steps, as restricters. Kind of a trial and error type thing. Restricting the flow usually gives a little more grunt off idle, to much, slows the acceleration into the sweet zone (1000-2000 RPM) and hurts top end. I used to restrict mine, until I lost 300- 500 RPM on the top end, under load.
Some muffler shops, still have a restricter wheel, looks like a big pipe cutter, only the cutting wheel is rounded. Only works on fresh pipe and is kind of irreversable.
A reduction of 2 1/4 to 2 inch pipe is about a 25% reduction, in area. Also as a general rule, every 90 deg bend in pipe, is a 15% drop in gas velocity (backs up the pulses), which translates, to about the same reduction in flow, at lower volumes/pressure (a little extra back presure at low RPM´s). Different ways to build in a little back pressure. Widening the pipe and then restricting it again, backs up the pulses, makes turbulance, also creates back pressure (resonator) and changes the tone.

 

[xj92]

Mine doesn't have any flanges on it, just matching inside and outside diameters so the pipes fit over each other and there is an exhaust clamp to hold it. If I'm remembering my math correctly, area = pi * radius^2, which means a 2 1/8" pipe would be an 11% reduction in area and a 2" pipe would be a 21% reduction in area. I'll stop by an exhaust shop when I get the chance and either get the 2 1/8" pipe and have the ends expanded to the proper size or just get 2 1/4" pipe and have them use a restrictor wheel to take the inside diameter down to 2 1/8" in a couple spots. Thanks for the info 8Mud.

 

[lunghd]

Drill hole in pipe, insert bolt. If yer really agile and want to impress the crowd at the local muffler shop... weld a flat washer to it and use it to tune the backpressure & a nut to lock it in position..

Yes. I'm serious.
Yes. It works. Ya see it from time to time on bikes w/ un-muffled drag pipes.

 

[8Mud]

I bought a selection of flanges at Whitney, years ago (in bulk). Three bolt flanges (like header flanges) about 3/16" thick, in various sizes (inside diameters). I sometimes travel to places with leaded gas, makes swapping in a straight pipe a snap. Use brass hardware and muffler puddy or thick copper foil as a gasket. If your planning on swapping pieces in and out, it makes things much easier.

 

[xj92]

Drill hole in pipe, insert bolt. If yer really agile and want to impress the crowd at the local muffler shop... weld a flat washer to it and use it to tune the backpressure & a nut to lock it in position..

Yes. I'm serious.
Yes. It works. Ya see it from time to time on bikes w/ un-muffled drag pipes.

I like the idea and will probably give it a shot. If I put just a bolt through, I would weld both sides to make it airtight. If I use the flat washer I wouldn't want to weld the bolt in obviously; would exhaust escape out the holes on the sides ? Seems like it would. I like the washer idea though because I could increase the backpressure for off-road trips and get serious low-end torque, and then open it up on the street to increase the top end, etc.

 

[What Rd]

I've got a completely different take on this. IMHO intentionally restricting the exhaust flow as you describe is counterproductive. You went to the trouble and expense of installing a freer-flowing filter and straight-bored throttle body. You must have intended to benefit from these mods. By bottling up the exhaust you're preventing the engine from making use of it's capacity to draw more air in.
Assuming for the sake of argument (which to me seems a Big fat assumption)that you could actually feel a seat-of-the-pants loss of bottom-end torque when going from a bashed up exhaust with a broken catalyst brick to a straight pipe in place of the cat, I say it's because you're running too lean now that your engine flows more air in and out. You don't need to add backpressure. You need to add fuel.
After my intake improvements and Borla header and exhaust, my plugs showed I was running lean. I added an adjustable fuel pressure regulator and the engine really perked up. It started more easily, responded better to throttle input (especially off-idle), and revved more readily.
Just my personal experience.

 

[8Mud]

A lot of inter related things going on in a motor. I´ve studied the theory some, but tend to deal with the physics. On a heavy vehicle with tall tires, it becomes, seat of the pants obvious, with say open headers, it doesn´t pull from idle as well, as a header with a longer collector and/or a restrictor cap. More obvious with a header (pushing a ton of mud), noticeable with a larger exhaust. Most guys agree (and it seem logical to me, from my experiences and screw up´s) that a stock crossover pipe, works out better (as a restriction) between headers and a larger or more open exhaust, for low RPM applications. A little more restriction (in addition to the diameter of the crossover pipe), could help a little at lower RPM´s. Have to try it and see or put it on a dyno.
Exhaust is an iffy thing, an open (low restriction) exhaust, will often help the motor to rev faster into the peak torque ranges. Sometimes moves peak torque, up the RPM band, which lowers torque at lower the RPM bands (off idle to say 1300-1600 RPM´s). Torque (pull) is a peak, in a fairly narrow RPM band, that can be moved higher and lower in the overall RPM band, but is hard to widen out (lowering the lift and duration of the cam will do it, restriction?). Inter related with the gearing of the vehicle, vehicle wieght and other factors.
Got to remember, exhaust is a pulse and the motor was designed, cam, compression, timing, to work in a sympathetic fashion with the other functions. Adding fuel, often tends to slow the burn a little, as will retarding the timing. Both I´d guess, could be seen as a restriction. Theroetically the closer the mixture gets to critical mass (pretty lean 15-16 to 1) the more horses/torque it produces. Don´t know if more fuel will help with low end torque, know too much will bog the motor at low vacume, low advance.
Theoretically the larger the throttle body, the more air/fuel, the more horses. Practically, the larger the opening the lower the vacume, big enough opening and the vacume gets close to zero. Floor the motor and it goes, flump assgas: and dies. Won´t idle and doesn´t start making power until high RPM´s. Open (low resriction) exhausts tend to work better (build horses and torque) at higher RPM´s in my experience.

 

[What Rd]

We're talking XJs here - not V8s with crossover pipes and not race motors running zoomie headers with practically zero backpressure. My comments were directed at real-world practical XJ performance where it belongs - a mix of on and offroad. The stock XJ 4.0 does not suffer from having an exhaust that is too free-flowing. Granted if you slapped a 4 inch exhaust off a diesel on a 4.0, it'd lose practically all its scavenging and a bunch of low end with it, and it would never use all the capacity such an exhaust offers even running past redline. But we're not talking about absurdities here (or at least I wasn't).

 

[8Mud]

Go look in the book for the early 4.2, the 4.0, the H.O. and where the peak torque is found (goes up the RPM band, with most every production series as does the the diameter of the intake or the exhaust). I´ve seen 2 1/4" pipe on motors a whole lot bigger than 4.0. I´m talking about accelerating from idle (or trying to) stuck in the mud or up against a rock. Maybe I´m talking through my exhaust opening. I´ve spent a lot of time trying to get tall tires through thick mud (V-8, I 6, whats the difference). My 4.0 will run well over triple digits, don´t need it. More often I´ve needed a little extra grunt, to get me through the gumbo.
I guess the old time engineers were absurd, when they designed the exhaust. Compromise between low end and high end.
Restricting the exhaust is no more absurd than increasing the diameter. :dunce:

 

[8Mud]

I´ve got a stock Jeep straight pipe, in my garage. actually a resonator with a restrictor welded in it, designed by Jeep to replace the CAT in countries, that still run leaded gas. Guess the engineers at Jeep are wrong. Seems silly, to build something that complex, that doesn´t work, is unnessary or doesn´t have any function. Noticed when I put the Cat in, that my peak torque moved lower. Always thought the straight pipe needed a little more restriction, to match stock, torque objectives.

 

[Dr. Dyno]

A Jeep 4.0 engine that has only bolt-on performance mods doesn't need an exhaust pipe bigger than 2.25". In fact, that is the optimal size for this engine so the Jeep engineers got it right from the factory. They got the header primary pipe size right too at 1.50".
Engines don't "need" backpressure. Backpressure prevents exhaust gases from leaving the cylinders causing contamination of the next fresh intake charge and a loss of performance. The key is to use the optimum diameter piping with mandrel bends that cause little restriction, a low restriction muffler, and a high-flow cat, so that exhaust gases are scavenged from the cylinders at the correct rpm range and to maximize exhaust gas velocity.
I have several performance mods on my 4.0 and I'm still using 2.25" piping on my exhaust. No need for bigger piping just yet.

_______________________________________________

Modified 1992 4.0 XJ Laredo UpCountry, 169k miles, AX15, NP231, D35c rear, D30 front
Estimated 240hp@5250rpm/280lbft@3500-4000rpm
1/[email protected], race weight 3490lb
Future mods: 4.6L stroker
Website: Jeep 4.0 performance
Webpages: Electric fan, IAT sensor relocation, Intake manifold heatshield, Oil temp. gauge install, Jeep 4.0 cam specs, Jeep engine dyno graphs, Automotive formulae, CTS resistor trick, MAP adjuster

 

[DDCxj]

I dont know what kind of emisions laws you have where you live, but here in Masachusetts you gotta have a cat. Mine started rattling just like yours, so I took the exhaust off right behind the cat and stuck a pry bar inside to break everything up then started up the jeep, reved it up, and everything shot out, I had to do this a few times to get everything out. So now my cat is still there but nothing is in it, and it wont matter if you bang it on rocks, its not doing anything anyway. If the inspection dude looked for my cat and saw nothing there, I would have a huge fine coming in my mailbox. Oh yeah, it did pass inspection.

 

[What Rd]

Back up. We're not talking about using any pipe any larger than the stock 2.25 (at least I wasn't). This thread started with the notion that it was necessary (OK, let's say at least beneficial) to place some sort of restriction in the exhaust when removing a cat and/or going with a less restrictive muffler.

Side note - don't remove your cat. Federal law requires that it not be removed or disabled and it serves a valuable purpose. It's no problem to get a cat these days that flows plenty for what we're talking about.

My statement about a 4 inch exhaust was simply intended to make the point that we weren't talking about extreme examples or absurdities, but rather practical XJ performance. I suppose I could have suggested that by xj92 or 8Mud's logic, he'd get HUGE torque gains at idle by stuffing a cork in the tailpipe. Perhaps then no one would have thought that that was something I was seriously advocating.

The point here is that you modified the intake tract presumably so it could flow more air into the engine. I'm sorry, but IMHO this makes no sense unless you also increase the engine's ability to breath out. And this does not mean anything extreme. It means using 2.25 inch tubing with mandrel bends. It means using a muffler and cat that are less restrictive than the stock ones. For some it means using a header with whatever combination of primary diameter, primary length, and confluence design will produce the desired results (whether that be to increase low end grunt or top end horsepower or some compromise in between). Otherwise all you are doing is wasting money on intake mods that will never show any advantage because the engine is struggling to push its spent exhaust out. If you're only going to do one or the other, you're much better off to improve your exhaust and leave the intake alone.

One last thing - 8Mud's cat replacement straight pipe with the restrictor welded in is not something I'd draw any conclusions from when discussing modifications to a 4.0, and for one simple reason. (Assuming it exists as described - I know nothing about it whatsoever) That was a stock piece for a stock motor that was in every respect designed and built to run with a cat and a stock muffler and exhaust. xj92's rig is not a stock rig and he's talking about modifying it further.

Hey - I sound like I'm preaching here. At least I do to myself. So ignore me. It's just my opinion, but it's your rig.
Damn - I need to go wheel!

 

[xj92]

I put my cat back on for inspection every year, and sometimes it takes me a couple weeks to get around to taking it back off, so I've been able to see the differences and compare them quite a few times. I can definitely tell that I lose low-end power (500-1700), but the engine handles higher rpm's a lot better with the cat off. Since the straight pipe I currently have cost me all of $6, I'll proably try drilling the hole and using the bolt and seeing what happens. (I can always remove the bolt and weld a patch over the holes or just throw a different pipe on. It's a lot more fun to tinker and fine tune when it's really cheap or free). I don't know all the theory behind it, I just figure there is a balance between intake and exhaust. I have increased the intake and have increased the exhaust flow, but evidently by different amounts. Now I'm just trying to get a good balance back, which should still perform better than the stock combination did.

 

[Fryphax]

What I did was replace my cat/muffler with 2" glasspack with the ends expanded to accept a 2 1/4" pipe. You could probably do the same with a 1 3/4" pipe for more backpressure.

 

[8Mud]

I´m putting in a high flow Cat (honey comb)(mines toast), bought a more restrictive muffler (more free than stock, more constipated than what I have now). My peak torque is a little higher (above 2500) than what I would like. I´ll be happy if the torque band doesn´t change. Seems to have a flat spot now, right where I don´t need it. But things could be worse. Hope they don´t change too much.

 

[Lucas]

I like the idea of no muffler and a cat. ALso, JC whitney sells a turbo muffler that is the shape of a glasspack. More backpressure, all the size benefits of a glasspack.