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FAQ - Cold Air Intake

254K views 340 replies 105 participants last post by  Knuckelhead 
#1 · (Edited)
Disclaimer: This is info I gathered and some of it may be my personal opinion...in no way intended to be all-knowing fact or infallible. Do your homework, read what's presented, and make your own informed decision.

The question is frequently asked: "Do cold air intakes make a difference?", whether this be improving gas mileage or improving performance.

the simple answer is NO. If you're interested why, read further.

A Problem:
ISO Coarse Dust Test of multiple brands of air filters - K&N clog faster and passes 3-4x more dust than paper filter:ISO 5011 Air Filter Test Report

They state: "Compared to the AC Delco air filter, the "K&N" plugged up nearly 3 times faster, passed 18 times more dirt and captured 37% less dirt."

Fine dust passing through a "K&N" in the stock TJ airbox:
How good are K&N filters?

Why does this matter? Dust = silica. Read about silica here:
Recognizing the Signs and Symptoms of Silica Contamination from Practicing Oil Analysis Magazine, January 2006.

The UOA (used oil analysis) with observed silica content in street driven vehicles:
Threw away K&N's due to high silicon in UOA's - Bob Is The Oil Guy

Cold Air Intake Design:
There are three main types:
1. Stock intake tube cut with a "K&N" cone style filter from local autoparts store clamped on the end.
2. An aftermarket tube, usually larger in diameter than stock, with a "K&N" cone style filter clamped onto it. Many utilize a 'heat' shield, and some utilize a "dry" filter instead of the "oiled" types.
3. Other intake options use a snorkel of some type, to draw air from outside the engine compartment (covered later).

Performance Gains:
Here is a dyno test performed by JP Magazine:
Jeep Wrangler TJ Inktake Dyno - A Day On the Dyno - Jp Magazine

Notice they showed a gain of 6 hp and 4 lb-ft of torque at around 4500rpm.

Here's a link to a 18 comprehensive dyno runs comparing the stock intake, no intake at all and an aftermarket cowl intake.
http://www.tricktuners.com/forums/showthread.php?t=226
He saw absolutely no statistically significant difference between the runs. And to actually show how easy it is to skew a dyno, he saw a near 10 hp increase just by changing tire pressure.

Have you ever wondered what the Jeep Engineer's would have to say about the stock intake vs a CAI?
well, read about that here: JeepEngineers on Cold Air Intakes.

So What Do We Know about Cold Air Intakes:
Pros:
1. Eliminates stock airbox (may be necessary for custom fenders)
2. Improved HP and TQ by about 2-3% at ~4500rpm (Engine max: 5200rpm).

Cons:
1. Expensive - brand name kits cost around $150+
2. Cleaning - open element filter is exposed to much more dirt, mud, dust and debris from engine compartment.
3. Thin film of dust typically passing through filter, thus the engine is exposed to this dust.
4. Oiled filter versions further attract dust, dirt and debris.
5. Due to inadequete filtering, a Outwears Pre-filter, or similar filter sock is required, further adding to cost.
6. Requires frequent cleaning due to dirty environment - typical cleaning kit costs $20+.
7. Increases risk of hydrolocking - exposed element can suck water into engine easily.
8. Noise - annoying "sucking" noise can be heard, sometimes associated with a loud whistle.
9. Dirty filter can cause rough and/or high idle.

I used to run an AEM intake, but removed it after finding a lot of problems associated with it. I documented my observations here: AEM Intake observations

The Snorkel:
There are also many brands of snorkels, designed to prevent water from getting into the engine, and to introduce air from outside the engine compartment. Brands include ARB, Volant, and others. Many people construct their own snorkel systems using PVC pipe and 1990's Buick intake boxes.

Pros:
1. Introduces air from outside the engine compartment
2. Elevated air intake to resist hydrolocking engine during deep water crossings.

Cons:
1. Expensive - ARB, Volant, etc brand intakes cost over $300.
2. Most require extensive modifications to the stock airbox, or a new airbox.
3. Modifications often require drilling and cutting of the body - could lead to rust, or water leaks.
4. Often times the windshield cannot be folded down.

Alternative DIY & Snorkel Write ups:
Buick/Hummer snorkel #1: TJ Hummer Intake Write-up (Buick Airbox)
Buick/Hummer snorkel #2: Write-up for a Hummer cold air intake
Low buck Buick airbox: Buick Airbox instalation
Homemade Cowl CAI #1: My Final Word On Cold Air Intakes
Homemade Cowl CAI #2: Cowl Induction - snorkle (WRITE UP)
Another cowl intake: Cowl Induction - snorkle (WRITE UP) - Page 4
Autozone Cowl intake: http://www.jeepforum.com/forum/f9/cold-air-intake-860538/
Buick airbox mounted directly to the throttle body: http://www.jeepforum.com/forum/f9/i-am-liberated-863818/
Homemade snorkel with K&N: http://www.jeepforum.com/forum/f9/homemade-snorkel-intake-w-pics-910113/

My personal favorites:
Windstar Airbox Cowl Intake: http://www.jeepforum.com/forum/f9/cowl-intake-windstar-air-box-566973/
Windstar Airbox Cowl Intake #2: http://www.jeepforum.com/forum/f9/windstar-cowl-intake-1064377/
Mustang Air box CAI: Mustang GT Air Cannister
Homemade Aluminum Snorkel: homebrew aluminum snorkel

Fun video:

 
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#207 ·
those that always reference the JP Magazine for Intake Shootouts should read this:

Sometime between the last test and this one, we had formed a working theory that it wasn't so much the air filter as the air tube that makes the power in these kits. With most, if not all, aftermarket air intakes, the air filter itself can flow more air than the Jeep can use. Sure, there are varying degrees of restriction from one filter to the next, but on a stock 4.0l engine we just aren't convinced that a significant result will be seen at the wheels from the air filter alone.
from their recent "Intake Shootout"
Jeep TJ Wrangler - Intake Shootout - Jp Magazine
 
#210 ·
Good idea. If you want a real cold air intake call the guy at Skinnypetal.com and ask him if he has anymore MCAI's available. I have the older version of this on mine and it works great. I can actually feel the power increase and it has the added advantage of filtering the cab air when you have the top on and it dual filters the intake air. It also gets the air inlet up a little higher.

This is the intake:
http://skinnypedal.net/store/index.php?main_page=product_info&cPath=7&products_id=108



He doesn't show the cowl scoop and filter, but call and ask.

The cowl scoop and filter was originally developed to just filter the interior/cab air so dust can be eliminated entering the A/C system and plugging the evaporator and heater core and had the added benefit of letting you breath better on dusty trails. Skinnypetal bought the idea from that guy but it could be it is no longer available. It used the stock Jeep air filter in the cowl vent.

Here is the cowl scoop and filter on mine.



I have the older intake design which uses a GM paper filter. The skinnypetal uses a conical filter.

I suggest you call or email them and ask.
 
#213 ·
What works for me, and is by far the most affordable is:

- replacement K&N
- cut the stock air box on the side facing away from the engine bay. I basically cut that entire side of the box out.

Sounds better under more throttle, air is cool (feel filter after driving a while), and i think that i feel a little more pep on the butt-dyno. :thumbsup:
 
#217 ·
The big thing with the factory air intake system on the TJ is the restrictive filter box and the "shock boot" that attaches it to the intake tube. The K&N fipk gives you more dyno proven horsepower because it eliminates the bottleneck of the air box and eliminates the turbulence created by the "shock boot". Air is lazy and likes to take the path of least resistance.......so the easier it is for air to flow, the more power potential there will be. Simple engineering, simple to understand. As for the actual filtering ability of a K&N, well that is up to you to decide if you want to run one or not. Personally, unless I lived at the end of a mile long dusty, dirt road that I drove up and down multiple times per day, I'd have one on my Jeep. Wait, I do have one on my Jeep. lol
 
#220 ·
The big thing with the factory air intake system on the TJ is the restrictive filter box and the "shock boot" that attaches it to the intake tube. The K&N fipk gives you more dyno proven horsepower because it eliminates the bottleneck of the air box and eliminates the turbulence created by the "shock boot".
Now this is going to lead to many days of reading enjoyment...:D
 
#226 ·
Is there any other Mopar motor that was a straight 6 that might have a better flowing head? I cant think of anything...but maybe someone here would know....

I am asking because I slapped on stock Explorer heads on my Mustang and picked up 30+ horse. (dyno tested) 100% bolt-on, they are just much better flowing heads due to Explorers additional weight and need for torque

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#227 ·
there is no mopar/chrysler head that flows better than the 0331....but there are aftermarket heads. A ported & big valve 0331/0630/7120 will flow better than the stockers tho. Also, Hesco & Patriot make an aluminum head, and Golen & Alabama make a high flow cast iron head.
here's all the flow specs on the various cylinder heads:
http://jeep4.0performance.4mg.com/tech_specs.html

Also note the stock airbox, intake tube, throttle body and intake manifold all flow more air than the stock 0331 cylinder head can use...and still more than the big valve & ported 0331 head can.
 
#228 ·
My turn - "~$FREE - DIY Cold air intake!"

So here I am much later on.... at last returning to this thread. This thread taught me that the only CAI worth doing was COWL and I promised myself I'd either get a used kit or fab one up. Well guess what, I'm going to show you that Jeep already provided you with most everything you need for a DIY CAI (minus a hole saw and a clamp or two and some RTV....along with a cone filter of course).

Things to note in the pics:

One 3.5" length of the factory intake pipe should be removed from the intake tube (I zip tied it for the picture) to correct the geometry for it to reach to the firewall in the proper location. Use a dremel to smooth the edges and angle them so that the tube points up just a tad. I used gorilla tape (probably all I needed to be honest) and a flex clamp from Home Depot as seen in picture.

Note that the ground wire has been relocated to the upper left of the tube on the firewall. The firewall-nut and ground wire were in the perfect location for my 3 5/8" hole so I decided they had to be moved, piece of cake with a drill bit, one nut and two washers.

I decided the hole in the firewall should be the exact diameter of the filter ...so it had to be dremeled out to more like 3 6/8" and the filter pushes in to make a perfect seal. You put the factory accordian tube into the filter and cinch it down with a clamp (temporarily I am using velcro to fatten up the outside of the accordian tube to match the inner diameter of the filter ....I will later use a 3.5" O.D. / 3.0" I.D. x 2.5" long rubber tube / radiator hose to better adapt the tube to the filter. It made sense to me to do it this way because the accordian tube is forgiving and heck, it's already provided!

The IAT sensor had no issues, stock wiring still reaches just fine. I disconnected the PCV hose from the front and used a breather I had laying around. I look at it like this, I am doing this as a cold air mod ..why pollute the intake tube with warm/dirty crankcase air? ...it already has some PVC action from the rear anyway. This way no cutting/driling/epoxy necessary on the intake as well.

Note the SUPER EASY and FREE rain-shield on the cowl, I simply cut a chunk from an old clear floor mat and RTV'd it around the two rows that come near the filter (the way it's positioned it is NOT squarely underneath the cowl vent ....only barely under the passenger edge). I creased some of it down that was not RTV'd for additional rain shielding. A simple alternative to this would be to glue/RTV a ziplock bag ...talk about easy yet effective. I will do something more permanent later ..like hard ABS plastic epoxied onto the small portion of the vent that's just over the filter. It has already poured the rain on my Jeep several times with the temporary rain shield, no issues whatsoever.

For those of you who think it's a big deal to remove the cowl, it took me four minutes to remove it with a hand held screwdriver ...simply to take these pictures for you all. It's harder to get back on but not so bad if you think to clip the center of the hood weatherstrip down before you begin so it doesn't get snagged on the cowl as you try to drop it back in. Folks ....it is NO BIG DEAL to R&R the cowl! I like keeping mine intact and I don't want a filter heat soaking in the engine bay.

In the close up shots, note the new placement of the ground wire and the careful positioning of the hole for the filter. I took care to not overlap the hole saw onto seams or tack welds, making for a clean cut and a cleaner installed look.

Hahaha ok why am I using a K&N you ask? ...because I already had it laying around. It came with a supercharger kit for my '94 Z28 but it was so tiny it was ridiculous, it was never used but replaced with a much larger filter. This little filter will flow PLENTY for the little 4.0 however. I read the awful test data in here and decided I would pre-filter. So temporarily I am using the stock paper filter ...I just cut it out and pulled it flat, wrapped it around the cone filter and zip-tied it on there. This works fine for now until I decide what kind of prefilter to use.

Hahahaha FREE FREE FREE was how this whole project went! Now I have additional storage in my old intake box too! ...might put a sweatshirt or two in there for those cold nights with no doors, pop the hood and grab a nice warm sweatshirt lol.

I chose this setup because I believe the only improvement you get in HP and MPG is due to the cold air, not improved flow. As already proven ...the stock intake outflows the engine by a comfortable margin, probably even more so when it is shortened and run straight to the cowl with a high flow cone filter. So I decided to re-use it.

I am baffled at how different this SOUNDS!!! Holy crap! The first time I floored it I looked beside me to see if someone else was passing me but the sound was all ME lol! At low / medium loads it makes some hissing sounds that are sometimes pleasant, sometimes weird ...a bit like a itty bitty turbo that you only hear at certain loads. It sounds a lot like a blow off valve in-between shifts. The worst sound it makes is at cold start, the choke it gets at startup makes it sound really weird ...like heavy metal rolling on heavy metal but muffled ...kinda like a train wheel just rolling on a track by itself. Fortunately this goes away quickly as the engine warms up and it begins to sound nice, a pleasant hiss. It's not audible at low load cruise or warm idle. At WOT it ....ROARS!!!! Reminds me of a 4V carb ...seriously....lol I love it, it's fun. It sounds faster than it is but what the heck.

SOTP difference? ...very slight I would say. It's hard to discern with all the extra noise. I think some who believe it's making a big SOTP difference might be fooled by the vibration you feel in the pedal due to the resonating roar at WOT, this may make it "feel" more responsive. Honestly, I think it is a little more responsive ...but only slightly. I'm just going to guess ....like ~7 HP or so. The biggie for me was better MPG, I will post results when I have them ...I am hopeful that it will make at least a modest improvement, that alone will make it all worthwhile! I also did this ...just to have something to do, you know how that is ...gotta tinker. I love it, no regrets, especially for $6 in clamps and a bit of tape/RTV and a $18 hole saw ....hahahaha to imagine ******* out $300+ for a system LOL!!!

I don't mean to brag but from the looks of things I bet my setup performs as well or better than any other in here, especially in the bang for buck category. I like the filter in the cowl because it doesn't get heat soaked in there. The stock intake is shortened to get to the firewall so any incoming air doesn't get heated up much before it hits the TB ..not enough time, plus the stock intake is THICK and probably doesn't heat soak as bad as a thinner tube would. This was well thought out but rather easy really. If I can do it anyone can! Hope this inspires someone to do it my way.

lol I'd love to hear this setup with the 260 HP 4.7 I6 ...talk about an intake ROAR!

To all who took the time to contribute and post in this thread, thank you ...without the info I would have never bothered.
 

Attachments

#229 ·
Two more pics, my temporary prefilter (stock paper filter element cut out and one layer is wrapped around the K&N, remember, it is also shielded by the clear floormat that is cut to fit and RTV'd in place on the cowl vent where the filter just peeks under the vent a tad ...not as much as the two rows covered though, so it is very adequate):
 

Attachments

#230 ·
btw I've learned a little tip I want to pass along. Ya know how our 4.0's are twitchy from idle and it's hard not to rev it too fast from a stop or buck the throttle? Well I already tried the obvious-add a throttle return spring ...and that helped a lot once I found the right spring (you can see it in the first pic of the previous post). What helped even more was to speed up the idle adjustment screw. Either tap into it to turn it or remove the throttle body (what I did) and remove the screw ...then turn the screw around and re-install so you can access it from the top. I cleaned the TB while off (really easy to remove) and flipped the screw over as I explained after taking a measurement as to how high it should hold the throttle. I raised it a bit more than stock so the blades would be open a little more. My theory in doing this was that if the throttle blades are more open to begin with at idle, the transition from idle to light throttle will be smoother ....indeed I was correct. It changes the nature of it in spite of the IAC motor compensating. By virtue of the fact that the throttle blades are physically open more, it makes more vacuum at idle and closed throttle cruising. Engine braking is reduced and the transition from off the gas to on the gas and visca versa is smoother. I think this is nice on the street. When I hit the trails I want the blades closed as much as possible for better engine braking. Now that the idle screw is flipped over as it should be ...I can adjust it easily anytime.

Cheers for another easy and free "mod"
 
#233 ·
LOL I AM SO SORRY (ignore the thread Unlimited04 just posted lol) that I posted pics with a stupid K&N and crappy hacked up prefilter. IT IS ONLY TEMPORARY!

DO NOT USE A K&N!

USE A NICE CONICAL PAPER ELEMENT FILTER or use an intake from another vehicle so you can change filter elements underhood if you don't want to R&R the cowl (I find it easy) with filter changes.

....or ...simply enlarge the firewall hole a little and use a grommet that will allow your conical paper air filter to be changed without removing the cowl. The grommet will keep it isolated from hot underhood heat. I should have had everything "complete" before I posted my pics and data....I got reamed lol.

IGNORE THE K&N and if you have one DO NOT use it.
 
#237 ·
I found a pic of the OP's dog when they went to the beach.Clearly,he wants to be prepared for any situation.
 
#241 ·
I like the cowl idea, but I wonder if the top section of the stock alrbox would fit under there. The air is pulled in from the top half of the airbox, so the bottom half wouldn,t be needed. I like the idea of cold air, but don,t want to give up my AC Delco element. It would need a 90 degree elbow to mount it sideways, and you would have find another way to fasten it to the top half of the airbox, since the clips are on the bottom half. Anybody ever try?
 
#246 ·
Andy5150 said:
I like the cowl idea, but I wonder if the top section of the stock alrbox would fit under there. The air is pulled in from the top half of the airbox, so the bottom half wouldn,t be needed. I like the idea of cold air, but don,t want to give up my AC Delco element. It would need a 90 degree elbow to mount it sideways, and you would have find another way to fasten it to the top half of the airbox, since the clips are on the bottom half. Anybody ever try?
EDIT: it took me too long to type up and i was beat to it vvvv

I have thought about it and came up with the idea of cutting the top of the lower half of the box and fastening that permanently and still using the original clips and air filter, however i dont see any possible way to fit it on firewall....

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