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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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#109 ·
That cai issue is behind me now. Sat. I put the old stock filter unit back on did a lot of around town and
local freeway driving. Did not notice any diference what so ever. And that little horn on the stock unit
should draw in less dust out on the trails. Moving on.....
 
#112 ·
Interesting thread. It seems that Bigbob is the only one here who might have a setup worthwhile (assuming he didn't spend much money since it's largely homemade). I'm certainly glad I didn't just "assume" the K&N FIPK claim of 12 HP was true. I just bought my 05 Unlimited and the first thing I noticed was "that's a small throttle body ....I doubt an FIPK setup would help HP much". Now from what I'm reading it looks like it's all big enough and that the engine just can't make much use of anymore air. I never realized the K&N's are so poor at filtering ...makes me a little nervous about the fenderwell setup I have on my 450+ RWHP LT1 Z28 ....but at least it's in the fenderwell and covered up fairly well.

Bigbob ...you need to post some pics of your setup please. I might be willing to try yours if it could be fabbed up for a reasonable amount. I figured a cam swap would be the biggest power gain on these (of course that means giving up some of that wonderful off idle torque). I'd like a little more ....like 210 HP. The torque is fine I'd just like a little more at the top (without giving up the torque. Since FIPK / CAI seem to be controversial at best ....ummm, what about a Hypertech or something? Let me guess ....for it to be worthwhile you have to switch to an aggressive enough program that premium fuel is needed, otherwise you incur detonation....correct? Sheesh, I'd love to get some easy/cheap/free HP ..but it doesn't look likely with the 4.0 from what I've read so far (downside of a relatively low power engine from the factory).
 
#113 ·
Interesting thread. It seems that Bigbob is the only one here who might have a setup worthwhile (assuming he didn't spend much money since it's largely homemade).

Bigbob ...you need to post some pics of your setup please. I might be willing to try yours if it could be fabbed up for a reasonable amount.
Not homemade, but also not on the market yet. There are many of these around. Being patents are pending on it I can't post pictures. I did modify mine quite a bit from the stock design. My modifications were to make the filters easier to service and had nothing to do with performance.

As far as easy to fab one of these up yourself goes, duck soup.
 
#114 ·
Well after I was at a huge outdoor local car show near here last weekend, and viewed every air cleaner I saw. So many with a large conical shaped filter as the Banks, and most not in a housing as mine, but just there on the end of a fat air duct, I went home and enlarged the opening on the Banks from it's 4"x 6" to twice that.
I'm very low tech, But there sure is a faster response at the tap on the gas. This Jeep is new to me, but I have
not had as much fun with a vehical since my old VW bug's. coldbeer52.
 
#117 ·
I dont have a CAI on my jeep, but I do on my gas chevy truck and my diesel duramax and they make a world of difference. I wouldnt ever put one on my jeep because it sees alot of water and mud, it is stupid for a jeep unless you stay on the roads. Solid product just not made for my jeep!
 
#120 ·
If you had a CAI you most likely wouldnt be hitting up mud like that if you were smart?


mud? never been a problem for me. i'v been in some deep sloppy stuff . so much so that mud clogged the throttle body spring and it was stuck open. that just happens to be the only picture i have from under the hood



the filter stays clean. flows much better then stock and i don't go past mid grill at the most in water. never going back to the stock junk
 
#125 ·
you guys are all forgetting something here, besides mud what about the fine dust? no mud in the desert, great, but what about all that fine sand and dust? I could be wrong here but I haven't seen any brand of oil gauze filter that actually filters better than a paper element. with that said what advantage(s) are you getting with one vs a stock paper filter.
 
#126 ·
"my butt dyno says i have 100 more HP, and i know it's working b/c it whistles"

a real dyno says you have <10+HP, and that's at above 3000 RPM.

i had K&N cone filter on the stock tube (cut the stock box off), in reality it did nothing except make the insides get dirtier faster. Now i have a foam filter that's WAY to big, it filters better than the K&N but still not as good as stock. I put something else in place of the stock box so i'm stuck w/ the shorty 'C'AI style, for now. Come thursday i may be buying a true CAI that goes through the cowl.
 
#128 ·
Most off roaders back in the 70's-80's ran K&N.

Anyone on this forum who says K&N is good will get promptly spanked with a huge wet noodle. There may be a possibility K&N builds crappier filters now than back then. I used to run them as filter #3 in a 4 filter set-up on a fleet of trucks and equipment I managed in extreme dirt and dust conditions. Filter #1 was one of those simple spinner type separators that gets out the big trash. These would be emptied every couple hours. Filter #2 was a cheap paper filter (Wix 2260) that would get blown out every couple hours and sent it to be washed once a week. Filter #3 was a K&N stock filter replacement. Back in the 70's K&N would build filters for your application. Filter #4 was the stock inner "last stand" filter. Normally we'd only clean the K&N once a month or so. In almost a 10 year period I never changed a filter #4 as no dirt ever got to it. You want the K&N to fill with dirt as it works better filtration wise and looses very little flow.

I don't use them anymore as I think they use thinner gauze and less of it now.
 
#130 · (Edited)
I'll provide some interesting reading on this subject-

"Recognizing the Signs and Symptoms of Silica Contamination" from Practicing Oil Analysis Magazine, January 2006:
Recognizing the Signs and Symptoms of Silica Contamination

A complete air filter analysis using the PTI Coarse Dust Test, which includes several brands, in addition to K&N:
ISO 5011 Air Filter Test
Spicer Filter Testing

Now K&N's & 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

Read all links thoroughly and decide for yourself.
 
#131 ·
FYI The CAI I use is now available to the public.

YouTube - MCAI First Look

I have the first design which uses a metal tube.

My secondary filter also looks a little different.



This is who sells them.

Skinny Pedal Products

Double filtered engine air, filtered cab air, better mileage and performance.
 
#132 · (Edited)
FYI The CAI I use is now available to the public.

YouTube - MCAI First Look

I have the first design which uses a metal tube.

My secondary filter also looks a little different.



This is who sells them.

Skinny Pedal Products

Double filtered engine air, filtered cab air, better mileage and performance.
You finally got them on the market heh? Has been a long time in R&D.

Nice!

BTW, my cowl CAI with OEM air box and filter is still doing what I want it to.
 
#135 ·
Pretty sweet. I like the idea of the prefilter, though I wonder how it'll work with dust and mud getting on it and such. Maybe wrapping the prefilter in pantyhose or something might be a good idea?

$350, though? Really? Are there at least any real-world miles on that setup with oil analysis showing silica levels and such? Any dyno runs or meticulously logged mileage numbers (without a change in driving habits/styles, of course)? I mean, I like the idea, and I don't want to be sold a pack of marketing lies, especially for that price, but I think I could maybe be talked into it...
 
#143 ·
Can someone expound upon this a bit? ...like which Buick intake (I own a Buick Riviera haha)? I have another question about the whole cowl thing in general .....does it interfere with clearance for the windshield being folded down? Not that I'll use that feature often if ever ...just thinking ahead .....

I so desperately want more power for this 4.0 but nothing seems practical or worthwhile. Like I said it seems like an LS3 swap or a used Vortech kit might be somewhere in my future ...but I'd like some minor but significant power upgrade right now, esp if it would help mileage. We just don't have any real dyno numbers yet for Hypercrap or CAI or other ....would be nice at some point. Anyone experiment with hotter or colder plugs or anything like that? .....I think an LS-3 would be soooo awesome. Wonder how much torque the 6-speed and stock clutch can handle? Not that I'd be racing and banging gears ...I'd respect it but at least hope it could hold WOT in the power gears. Gotta do something ....this 4.0 isn't bad but I just want more. At least to be on the plus side of 200 HP would be nice .....
 
#142 ·
Here's another pic for your thread. It seems weird that after driving for a little while, my 'cold air intake' is actually very warm. The heat from the engine compartment warms the aluminum tube that moves the air from the 'cool' spot to the TB. So yes, it pulls 'cooler' air but as it goes to the TB it is warmed up. I do not believe this 'cold air intake' is worth anyone's time. If I didn't buy the Jeep like this then I wouldn't have a 'cold air intake' on it.

Use the pic freely for anything. Also- it's a KoolVue CAI
 
#149 ·
My own personal opinion is that 31's on 3.73's with the 6-speed will be fine. Not quite as quick as stock, but if someone had never driven one like that, they'd never know the difference. 33's, though, are...tolerable. 35's would just be a pain in the ***. Ask me how I know :)
 
#150 ·
So, I have read through all 10 pages of this post....I see alot of opinions and statements of testing regarding the CAI. Both for and against. It's tough to ingore the voice of experience also. I have thought about this mod. It's something I would do myself, so it would be a low budget improvement. So my question is, why is it when it is 90+ degrees here in FL my jeep feels sluggish and now that it is cooler (50'-70 degrees) I defineitely feel a better throttle response? I like it for sure, but is that the only real gain for the 4.0 by installing a true CAI? I'd like to have the throttle response during the summer months. Will pulling outside 90 degree air result in better throttle response vs pulling from the hot engine compartment?
 
#151 ·
Will pulling outside 90 degree air result in better throttle response vs pulling from the hot engine compartment?
if you remember back to page 2 and 3 those of us with ScanGuageII's reported some IAT sensor results under varying conditions. Regardless of what kind of intake box you have, if its under the hood its pulling in hotter air.

Either way, if you drive long enough distance, the intake tube will eventually reach an average temp based heat transfer from the engine and outside air.

You're best bet is a cowl or snorkel CAI, and wrap the plastic tube with some heat-resistant insulation, like header wrap.
 
#152 ·
If I do this it will definitley be something similar to the hummer set up or MCAI setup. Here's another question, Most of us know what throttle response is and can describe it,but what is the "mechanism" behind it? Is it due to more efficient combustion due to denser air (more air) being introduced into the cyclinder?
 
#153 · (Edited)
On our Jeep's, throttle response has more to do with the efficiency and flow of the manifold & throttle body.

Introduction:
Throttle response - Wikipedia, the free encyclopedia

the technical stuff is covered on page 8:
http://www.jeepforum.com/forum/f9/faq-cold-air-intake-558437/index7.html#post7833663

Basically imagine if you have no manifold at all - the engine wants to turn at max rpm constantly because it is sucking in as much air as it can use. A prime example is when people don't get their intake manifold seated on the gasket properly when removing the intake and exhaust manifolds. The manifold is basically wide open - there is no restriction, the engine instantly revs full throttle.

To modulate this, we have an intake manifold and a valve that limits the amount of air that enters the engine. This valve can be a throttle body or a carb, they function in the same manner we're talking about here - strictly air modulation on an air pump. When you open the valve wide open, the engine goes to max RPM. The time it takes it to do that is throttle response. Whenever you put your foot on the pedal, you're modulating the amount of air entering your engine. The restriction of the air pump creates vacuum - a suction. The vacuum is what powers your brakes for example.

A bigger valve and bigger manifold allows more air to enter the engine - in other words - you are more closely approximating no manifold. This means the vacuum decreases. Decreased vacuum = less pressure difference across the manifold = increased throttle response. Well, sorta...a bad explaination for the race techies in the crowd, but its an OK way to think of it.

This doesn't automatically mean you make more horsepower or torque - it just means you're more closely approximating no intake manifold in less time.

On engines that don't have a particularly restrictive intake manifold, like the 4.0L, there will be minimal power gains. Any power gains, if any, will be at RPMs where air flow is the limiting factor to approximating that no manifold thing - high RPMs. However, throttle response will be improved because you are now allowing more air past the valve for every increment you open it. This is shown in the math by the very slight difference in pressure between a 58mm and 62mm throttle body.

And that throttle response/pressure difference is slight on the stock 4.0L. Not worth $300 to me, but it was worth ~$80-90 for an Ebay throttle body. It depends on your preference. But there are always better things to spend your money on.
 
#154 · (Edited)
Thanks for the explanation Unlimited04. So, bringing temperature into the discussion again. The reason I feel the increased throttle response is due to the cooler air being denser, thus bringing in more air, simulating the "no manifiold condition". Is this correct?
 
#156 ·
sort of...theres two things at play there. while there is less pressure drop with colder dense air, you have to account for the A/F ratio and adding more fuel to compensate for the denser air.

Using the calculator described on page 7 -
Throttle Body Sizing Calculator | REVTRONIX

Test the extremes:
20* F = 0.24 psi, 320 CFM
175*F = 0.18 psi, 320 CFM

So on a 155* F temperature swing there is a 0.06psi difference due to air density. Theres also a difference in fuel delivered, which I have no idea how to measure or speculate on, especially considering the PCM's propensity for rich-lean A/F behavior depending what RPM & loop we're talking about. Obviously more fuel + more dense air = more power. If you're drag racing it makes a world of difference, but in the context of the 4.0L, who cares. :rolleyes:

Thats pretty much the idea - who cares - the 4.0L isn't a wicked performance engine - its a tractor motor. Put paper filters in it for $5-10 each and be happy knowing you're keeping junk out of the engine. End of story!

sometimes when I put my jeep into park and let off the gas it decides to idle around 2500 rpm which is high for it being in park. it doesnt happen often, but could this be a problem with the intake manifold that you were talking about and the rev problem?
no thats and electrical/computer problem. Most likely your IAC or TPS.
 
#155 ·
sometimes when I put my jeep into park and let off the gas it decides to idle around 2500 rpm which is high for it being in park. it doesnt happen often, but could this be a problem with the intake manifold that you were talking about and the rev problem?
 
#159 ·
Cold air warm air whatever. Bottom line where the air intake is sucks water. A paper filter is junk after it gets wet. I plan on moving my air filter under cowl as suggested by unscathed DIY project. http://articles.jeepforum.com/Tj_Cowl_Induction_Snorkle. A regular filter won't fit. I am going to leave the old filter box where it is plugging the holes and using it as a tool box. Have a nice day.
 
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