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Thoughts on intake/exhaust on TJs....

2K views 39 replies 15 participants last post by  EatTacoCatTea 
#1 ·
Hey guys, my '05 TJ is stock under the hood, and I'm wanting to give her a boost. Thinking of a cold air intake (K&N?) and a cat back exhaust. Any thoughts or suggestions? On Monday she gets a 4" lift and new 33" BFG KM2s. I'm replacing the oil pan gasket as well as the head gasket later next week. She's currently at 141k miles.
 
#9 ·
really, you'd be fine running the factory one. normally when we ad a CAI in, say a Honda CRV, it removes all the resonators and sharp elbows in the tubing with a more straighter, less restrictive tubing and filter element that improves air flow and cooler and denser air charge. but the factory jeep one is also a high flowing intake as an aftermarket one. so, change the filter every other oil change, and you'll be fine.

the problem with the filter elements on the aftermarket CAI is that although they improve IATs, they hinder the filtering capabilities. but if it helps you sleep better at night, i hear K&N is the worst for jeeps.

for exhaust, you're looking at an equal benefit as replacing the factory airbox with an aftermarket one. there is a nice tune that the Flowmaster 40 series and Super 44 makes, if that's what you're after. I hear thrush mufflers are also a good way to go.

FWIW, save your money for the head gasket and the regear first. just curious as to why you would need to change the head gasket after you put the lift and tires on. it's like blowing your entire savings to buy a fresh pair of Js instead of food to last through the week...but it's your money and your choice. i understand you're here for an intake and exhaust reference. ^_^

you mentioned 3.07s so it is to assume you have a 4.0L. 4.56 is where you'd wanna be, but 4.10 is acceptable for those long journeys on the Wisconsin motorways, nothing else. does help keep the rpms down, the 4.10s. i had both, and liked the 4.56, but the higher rpm annoyed me for rides longer than 100 miles on open highway. but that's just me.
 
#10 ·
really, you'd be fine running the factory one. normally when we ad a CAI in, say a Honda CRV, it removes all the resonators and sharp elbows in the tubing with a more straighter, less restrictive tubing and filter element that improves air flow and cooler and denser air charge. but the factory jeep one is also a high flowing intake as an aftermarket one. so, change the filter every other oil change, and you'll be fine. the problem with the filter elements on the aftermarket CAI is that although they improve IATs, they hinder the filtering capabilities. but if it helps you sleep better at night, i hear K&N is the worst for jeeps. for exhaust, you're looking at an equal benefit as replacing the factory airbox with an aftermarket one. there is a nice tune that the Flowmaster 40 series and Super 44 makes, if that's what you're after. I hear thrush mufflers are also a good way to go. FWIW, save your money for the head gasket and the regear first. just curious as to why you would need to change the head gasket after you put the lift and tires on. it's like blowing your entire savings to buy a fresh pair of Js instead of food to last through the week...but it's your money and your choice. i understand you're here for an intake and exhaust reference. ^_^ you mentioned 3.07s so it is to assume you have a 4.0L. 4.56 is where you'd wanna be, but 4.10 is acceptable for those long journeys on the Wisconsin motorways, nothing else. does help keep the rpms down, the 4.10s. i had both, and liked the 4.56, but the higher rpm annoyed me for rides longer than 100 miles on open highway. but that's just me.
Dude... He doesnt wanna hear it. He's gonna run 3.73's and thats it!

And no 4.10 isnt acceptable for a "long journey"
 
#11 ·
Ì really recommend a minimum of 4.10's for those 33's. I've ran that combo and as a matter of fact that's what we geared my wife's XJ.


I know it's your trash and you'll do what you want but you'll be no better off with 3.73 in lieu of 3.07. My trash has 4.88 and 35's and was a huge improvement over the 4.10's. Yeah, I re-geared twice! !!!!

Remember 4.10 is stock for a Rubicon and 31's.
 
#14 ·
I was given a K&N FIPK II as a present, and after all of the dust made its way into the intake tube, I switched to the AEM conical filter for much better results.

AEM (left) K&N (right)


AEM (for Dodge 1500) installed on K&N Intake system


With 4.10s and 35s, the intake didn't make a lot of a difference. Probably no difference in performance. I would suggest a regear in the neighborhood of 4.56 or so - depending on your transmission type.
 
#15 ·
To plainly state with has already been hinted at, the 4.0 factory intake and exhaust are non-restrictive. There will be no benefit with either "upgrade" on an otherwise stock engine.

I daily drive a Windstar cowl intake. Which, unlike K&N, is a true cold air system that routinely pulls in air at 15-20* above ambient. There is no performance benefit that I can tell.

Don't bother with 4 port or higher flow injectors either.

Depending on your engine/transmission, you want a minimum of 4.56 gears to match 33" tires.

Nobody asked Billy what his engine/transmission combo is before giving their recommendations. Though everyone is correct in saying that 3.73 is a mistake. Especially when you are paying for a regear, rather than swapping in another pair of factory axles with lower gearing.

www.grimmjeeper.com will tell you everything you need to know. Plan around your OD gear. The 4.0 likes to cruise above 2500rpm.
 
#16 ·
Billy, it's your choice and your jeep. we're here to give you advice and recommendations from our experience. as i said, 4.56 is where you wanna be for them 33s, and per the aftermarket and all the jeep networks.

but hell, if you have money to throw around, experiment with the 4.10s (idk if 4.11s were an option for our jeep axles) and 4.56s to see what you consider to be comfortable for driving daily in, if you decide to go against your wishes that is. the way i see it, your input just might help someone else new to the jeep world. but we can sit here all day and say 4.56 is the correct gearing to go with for 33s, but it's what you want or like is important to me too. :wavey:

i can understand if you're already sitting on a set of 3.73s or 4.10s on the side, that you picked up for free or a few bucks, and ready to put in just to hold you over. well, that's just the benefit of the doubt. but ultimately, the only better way is up. =D

back on topic, you have our input on the CAI. IMO a snorkel would be a better trade off in terms of functionality compared to a CAI. the factory airbox will filter better than an aftermarket CAI. and if you haven't figured it out yet, there wasn't much performance gains to notice with a CAI that we've experienced. mufflers, pretty much the same. they do sound nice though, at least to me anyway.
 
#23 ·
Billy, it's your choice and your jeep. we're here to give you advice and recommendations from our experience. as i said, 4.56 is where you wanna be for them 33s, and per the aftermarket and all the jeep networks.

but hell, if you have money to throw around, experiment with the 4.10s (idk if 4.11s were an option for our jeep axles) and 4.56s to see what you consider to be comfortable for driving daily in, if you decide to go against your wishes that is. the way i see it, your input just might help someone else new to the jeep world. but we can sit here all day and say 4.56 is the correct gearing to go with for 33s, but it's what you want or like is important to me too.


i can understand if you're already sitting on a set of 3.73s or 4.10s on the side, that you picked up for free or a few bucks, and ready to put in just to hold you over. well, that's just the benefit of the doubt. but ultimately, the only better way is up. =D

back on topic, you have our input on the CAI. IMO a snorkel would be a better trade off in terms of functionality compared to a CAI. the factory airbox will filter better than an aftermarket CAI. and if you haven't figured it out yet, there wasn't much performance gains to notice with a CAI that we've experienced. mufflers, pretty much the same. they do sound nice though, at least to me anyway.
Thanks bro for all your input. Much appreciated. Clearly I need to rethink the gearing and go with 4.56s. Shy do so many people install K&N intakes when they aren't doing squat? Are folks sold on the name and the concept?
 
#17 ·
It is your jeep and you posted nothing to deserve insults but as for the info you posted

I removed a K&N and threw it away as for me it passes too much fine grit to the engine

I am stuck with not using factory air box as I have an onboard air York compressor blocking stock air box location

I ended up with factory air tube shortened and connected to throttle body adapted to a early90s Buick/Olds air box with a paper filter

My rock it CAI tube never fit the throttle body as well as the stock air tube and it has been removed in favor of a shortened stock air tube

Do any cat back you want but buy for sound not power as you will likely get very little gains

As for gearing first somewhat dependent On auto vs manual tranny

I have two manual 4.0 with 33 tires

First is an 04 Rubi with spring spacer plus small body lift and motor mount lift stock 4.10/4.11 gears and even where I like at 7-8000 feet above sea level it is a very pleasant daily highway driver with metric 33" on the stock 16" wheels
If I keep highway rpm in mid 2000 range and speed 65 or below it gets an honest 16-17 mpg and does not feel under powered
Stock air box and exhaust

Next is an 98 sport 5 SPd wit 33 inch summer and 31 inch winter tires on 15 inch wheels and 4 inch spring lift
Running D44 in back, and ARB front and rear with 4.55/4.56 gears
Honestly a little less pleasant on highway then the 410/411 and a little better off road ( like the Rubi it also has a 4wd low of 4:1 but via a terra low rather than a Rubi transfer case)
Best mpg that rig gets on highway with speedo healer corrected odometer spot on is low teens
So living with both if mostly on road with 33 and 4.0 5spd I prefer the 4.10/4.11
The 98 is the one with onboard air and relocated air box

Others claim to know better and maybe they do but I drive both and offer my real world experience and opinion not just something I read somewhere

I have No experience or recommendation for auto tranny gearing in a TJ

The fact that you asked for some info before making YOUR choice indicated you are at least interesting in considering info from others and any who feel you must take their advice are being somewhat rude or at least that is how it looks to me

Enjoy your jeep however you want to
 
#20 ·
Wow krisbman, why must you be such a dickhead? I asked a simple question, and regardless of whether you disagree with my gear choice, you have no reason to be such an asshole. I joined this forum for advice, not criticism. Yeah, I'm a Newb, but only to this forum, and to owning a Jeep. But make no mistake, I'm not a newbie to offroading, as I grew up ripping mudholes in my '79 K5 Blazer back in the '80's, not to mention touring the country with my stepdad and his world record holding mud bogger. (Jim Weber-"American Made") Thank you for keeping count of my posts, that's very flattering. Loser. But a sincere thank you to all that gave me sound and honest advice. I truly appreciate it. Clearly some people need to feel like a tough guy behind a keyboard.
 
#33 ·
you planning anything else besides changing the head gasket? like, what seems to be the problem? it would be a good idea to update the header while you have the head out, if there's anything you could do to the exhaust system. makes putting the header bolts on and torquing them properly a whole lot easier.

IDK, i found it quite difficult to do the head gasket on mine with the 4 inch lift and 33s, and i'm 6'0". the head alone felt like it had to have been damn near 80lbs LOL. and unless you're standing directly above the engine block to drop the head on it, could result in a hernia and blown o-ring.

the head cracked between the 3rd and 4th combustion chamber, but the block was fine. i removed the head bolts in the reverse order from the installation order per the FSM. i guess this is to avoid warping the head upon removal. DO cover the lifter valleys while you're working so junk doesn't fall in so it can circulate through your pushrods and stuff...
 
#28 · (Edited)
Billy, I personally would fix the head gasket first. I would dump the idea of any sort of gains from intake or exhaust mods to a 4.0L because of trash being sucked in, first off. Secondly, the 4.0L is MAXED OUT, which is one of the reasons it was dumped with the JK.

It comes down to advertising hype and whether you believe that some maker/seller of bent tubes and filters (that allow so much particulate past them and into the engine) know some secret that the Jeep engineers could not figure out?

Also, a CAI can help a car that is running flat out, doing very high RPMs all the time. We don't race our jeeps, though. Any minute advantage we *might* see are in a performance category we do not generally do much with.

The proper gear ratio for a 4.0L with 33" tires is 4.56 if you have a manual. For an auto I *think* it is 4.88, but it might be 4.11 -- I do not have an auto, so I have never memorized that bit of info. Sorry.

Just remember to spend your shekels on FIXING WHAT IS BROKEN before you do any modding.

By the way, I speak from experience. I have a 2003 Sport with a D44 and factory 3.73 gears on 33s. It lugs on hills like my old 4 cylinder TJ did. It is better than 3.07 but only a little bit. (I have some nice, new 4.56 gears from Yukon and Revolution that will be going in soon and I am stoked about this!)

The expense of re-gearing is SO high you need to get it right the first time. I suggest you do a lot more reading before buying anything or making any decisions.

Good luck and keep us posted on what you do. (And remember to post a lot of pics. All of us love TJ mechanical pics and write-ups.)
 
#29 ·
.... For an auto I *think* it is 4.88, but it might be 4.11 -- I do not have an auto, so I have never memorized that bit of info. Sorry.

...
The 42rle needs a minimum of 4.88 to run 33's.

I am running 33's on 4.10/5 speed. It is workable, but I would never pay money to arrive at this combination. OD is barely usable unless I am on flat ground and pushing 75mph. 4.56 or even 4.88 would be very nice in the mountains.
 
#36 ·
Forget the ratios that are appropriate for a V8 Chevy truck, the gearing requirements for V8 trucks are entirely and totally different than for a six cylinder TJ. Even the transmission type affects what the right ratio is.

4.56 is ideal for 33's when you have 33" tires and 5-speed transmission, it's the long-favorite ratio for that size tire for the highway... and of course that ratio works superbly offroad as well.

As an example of how the correct ratio changes according to the engine and transmission type:

33" tires:
2.5L with 5-speed... 4.88
2.5L with 3-speed automatic... 4.56
4.0L with the 5-speed.... 4.56
4.0L with the 3-speed automatic... 4.10.
4.0L with the 4-speed automatic 4.88.
 
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