Exhaust for the 4.0

Hey guys, I have a 99 XJ (6" lift, 33x12.50, 4.10 gears, SYE, rock rails, Warn front diff cover, custom bumer with 9000lb winch, Rock Krawler steering system, and some other stuff...) I have been thinking about upgrading my exhaust system. I want to increase performance and hopefully gas mileage because mine sucks right now. (I swear I'm getting around 8mpg) I know better than to expect good gas mileage, but that's pretty bad. Anyway, I have a K&N cold air intake right now which I plan to modify into a snorkel eventually. I am thinking about getting a TB spacer, new manifold, high flow cat, and a new muffler. It would be nice to get a little deeper tone out of the 4.0 but I don't want it to be loud. I'm sure I should toss a new O2 sensor int here too. The engine is close to 116,000 miles right now. New plugs and wires might be a good idea too. I am considering the SeaFoam treatment also. I am looking for any suggestions on the best way to go for improved performance. I have seen recomendations for the Dynomax Super Turbo Exhaust I also constantly see advertisements on Jeep Forums for this manifold Has anyone given these products a try? Any recommendations would be greatly appreciated. Thanks guys.

I'm running the dynomax with a rugged ridge header. I didn't notice much increase in gas mileage, but the throttle responce is really quick now. I'm also interested if the high flow cat is worth the price.

instead of getting that manifold, why not step up to something a little bit more sturdy such as...http://www.naxja.org/forum/showthread.php?t=93378

you'll find tons of stuff on this manifold on here.

as far as exhaust I tend to like to keep things on the quiet side so I can hear what is going on around me, especially on the trail.

the tb spacer and k and n intake will not clear the hood fyi

i keept it old scool and put on a classic glasspack cherrybomb. it was cheaper then a regular muffler and has more of a grumble.

I have the throttle body spacer and the power-aid cold air intake and have a proflow exhuast and yes the freeflow cat is worth it. Also have Msd ing. and coil and wires.

With what you have set up on your motor so far seems ok. If you are getting 8MPG something else is going on with your motor. I have an '88 4.0 Renix with a stock intake, pacesetter headers, dynomax system (HiFlo Cat and muffler), twin electric fans and I am getting 18 to 20 Mpg on streets and close to 22 on Highway. I am also running 32's on 4.56 gears.
To be getting on 8 Mpg means you have other issues with you motor. Did you ever get your speedo corrected for tire size and gears? That has a huge amount of impact on your computer system and how it reads the engine condition. If the spedo gear is wrong it may not read the driveline rpms correctly and give a false reading to the other systems affecting fuel rates.
IMO your best bet id to regear to something 4.56 or above with 33's. I have'nt found 4.10's to be good for anything but street cars if you are looking for Mpg. 4.10's have a place, mind you, just not when you are looking for gas.

Thanks for the input guys. It's good to know that the K&N won't fit with the TB spacer. Kinda sucks though, but I guess I'll get over it. Yeah my engine is probably in desperate need of a thorough tune-up. I bought it around 108,000 miles and it is close to 116,000 now. I really haven't done much to the engine aside from a new air filter, tranny fluid change, and regular oil changes. There is probably quite a bit that needs to be done to realy clean things up.

Since i recently bought new 33's (much quieter ones...the old pos tires were stupid loud) I can now hear a constant clicking sound in the engine. It increases with RPM's (duh)...maybe a stuck lifter? I don't know. I'm about as far from a mechanic as you can get.

The speedometer has been adjusted. The guage is right on so I don't think that is the problem. Speaking of electric fans, mk153smaw, is it normal for my fan to come on all the freakin time? It can be cold out with the engine at idle and the fan will come on for a couple minutes, turn off for a couple, come back on, and will repeat. Doesn't seem right to me. Plus when it kicks in, you can feel it. Like maybe it is unbalanced because I get enough of a vibe that I can feel it. Hmmm...sounds like I may have some work to do haha

Oh one thing I noticed is this stupid crimp in my exhaust right where the manifold bolts to the pipe...wtf is with this? I have heard people say that Jeep did it intentionally for clearance issues, or that maybe it is necessary for proper backpressure. I don't know, it seems to me that it would be destroying airflow thus, killing HP and gas mileage...thoughts?

Thats how the factory bends it look for mandrel bends it looks like a bend on a rollcage reduces backpressure. A local shop did mine 22 x 2.25 glasspack and new pipe from there back $125

SwampDonkeyXJ said:

It can be cold out with the engine at idle and the fan will come on for a couple minutes, turn off for a couple, come back on, and will repeat. Doesn't seem right to me. Plus when it kicks in, you can feel it. Like maybe it is unbalanced because I get enough of a vibe that I can feel it. Hmmm...sounds like I may have some work to do haha

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Mine does the same thing. Doesn't matter the temperature, or if the A/C or anything is on or off. It just comes on for 20-30 seconds, then off. Does it constantly. I have my switch hot-wired (so I can force it on or off if needed), but I don't think that is affecting it... it shouldn't at least...

It can't be normal though, I figured there was a sensor that was going on / off weirdly, but never bothered to track it down.

Tenny said:

Mine does the same thing. Doesn't matter the temperature, or if the A/C or anything is on or off. It just comes on for 20-30 seconds, then off. Does it constantly. I have my switch hot-wired (so I can force it on or off if needed), but I don't think that is affecting it... it shouldn't at least...

It can't be normal though, I figured there was a sensor that was going on / off weirdly, but never bothered to track it down.

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Mine does too, but only when the ac is on.

from what I have experienced with mine... I am using the imperial theromostat, which is adjustable, and the sensor is at the upper right portion of my radiator (intake side of it). if my coolant runs low, below 50% capacity (I have leaks), the sensor gets affected by the metal of the radiator oddly. It will set the thermostat off and on erratically. When the thing is full it works ok. Mind you, I have a flow restrictor installed just after the coolant discharge at the head. With that in there causes much more heat to be transferred at the radiator leaving it hotter for longer. I think my system keeps 185deg very constant, but the ability to spill that heat off is less I think though. My fan, when it does come on, will function like you have described how yours is. I figure, oh well, the engine is getting cooled.
Yeah the clicking could be a couple of things. If you are feeling froggy, go pull a head off an XJ at the yard and get it serviced (valve job, ect...) then change out your head. Valve jobs are relatively cheap, if you install yourself from the machine shop, and have huge immediate benefits from better mpg's.

Terrific said:

Mine does too, but only when the ac is on.

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+1

8mpg seems like time for a new o2 sensor

about the fan, the fan is supposed to come on when the ac compressor tuns on, if you have your defrost on the ac is also on to "dry" the air coming into the heater core to prevent window fog up

crazydave said:

I have the throttle body spacer and the power-aid cold air intake and have a proflow exhuast and yes the freeflow cat is worth it. Also have Msd ing. and coil and wires.

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I just put and MSD Ing, Coil and Wires on mine, and man is it sweet. It's not exactly a hot rod, but my 89 is running smooth with power to spare. A noticable improvement from tired old motor with over 220,000 miles, especially in low rpm torque situations.

As far as exhaust, you can get a diamondplated offroad muffler from Flowmaster that looks sweet. Also Headers and a high flow catalytic converter would help.