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Old 01-01-2004, 10:12 PM   #1
Unabomber
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Exclamation Intake FAQ: Read if you are thinking of buying one!

Intake FAQ

The primary purpose of an aftermarket intake is to increase the amount of air flow the engine receives.

HP gain is 0-15HP. These HP figures are a range as different intakes have different dyno results.

What is an aftermarket intake? Aftermarket intakes can be broken down into two types:
1. Short ram: AKA shorty or ram pod.
2. Cold air intake: AKA CAI.

What does the stock intake look like? To gain a perspective on how aftermarket intakes function, the stock intake features will be shown and discussed for further clarity.

This is the stock intake as viewed from the driver's side.
The "U" shaped assembly and the tank that connects to it on the left is commonly referred to as the snorkle, snorkus, silencer, or resonator. It is actually hidden in the fender.


This is the stock intake as viewed from the passenger's side.


This is the stock intake as viewed from above.
1. Stock "ram intake" funnel.
2. Stock air filter box.
3. MAF sensor.
4. MAF sensor piping.



What is a short ram? This is an aftermarket intake that removes the stock fenderwell snorkel components and draws air from the engine bay. To generalize, it is essentially a filter unit that replaces the stock air filter box. Some aftermarket short rams also replace some or all of the stock MAF piping with aftermarket piping.

What is a CAI? This is an aftermarket intake that removes the stock fenderwell snorkel components and replaces them with aftermarket piping. These systems, depending on the manufacturer, may or may not replace the MAF piping as well. These systems draw air from the fenderwell.

Which type of Subaru does this information apply to? This post is generally catered towards turbocharged models. Naturally aspired Subarus actually see good, solid, and safe gains from an aftermarket intake. The STi appears to accept some intakes without as many ill effects as the WRX and little data is known about other turbocharged Subaru models. The information about aftermarket filters' filtration capabilities is applicable to all vehicles though and is a point of consideration for some owners.

Is the stock WRX/STi system adequate? Yes. The majority of major Subaru tuners agree that the stock intake is adequate to power levels in the 400HP range. It also has the following advantages:
1. The standard air scoop is mounted high in the engine bay. This makes it resistant to ingress of water. The design of the fender snorkel helps to remove any water that is drawn in by this scoop.
2. Air drawn in is cool, since the air comes from the edge of the engine bay, and at speed, all air will be drawn from outside.
3. The standard system is quiet since the resonator in the fender helps filter out noise from the air flow.
4. The stock ECU is calibrated to work perfectly with the stock intake system.

Are there any downsides to aftermarket intakes? There are potential problems with some intake systems. Most manufacturers utilize less dense filtration media in their filters. This equates to increased pariculate matter injestion via the intake. This depends on the manufacturer's choice of filter media and is something to be taken into consideration. CAI can be suseptible to water injestion. This is because through their design the filter media is quite low in the fenderwell. Extreme caution should be used when driving through standing water or other areas where water is present. The major downside of intakes are problems with the air/fuel ratio.

Which intakes do not cause problems with the air/fuel ratio? The only manufacturers of intakes that claim in writing not to cause any ill effects are:
a. TurboXS
b. APS
c. Perrin (based on my personal email correspondance with Perrin)
d. K&N Typhoon (based on testing by nhluhr and testing by AaronWRX, both NASIOC members) These tests were performed on an STi and their effect on WRX A/F ratios is untested.
e. Cobb Tuning
f. SPT

Do intakes cause problems with the air/fuel ratio? Many seem to believe that since their XXX intake does not cause a CEL or exhibit any driveability problems, then their XXX intake must be good. Though the apparent increase in power gives them peace of mind, what they don't take into account is their intake is actually causing unseen harm to their engine. This is generally due to two reasons:
1. The intake piping diameter is incorrect. This can cause a lean condition in the case of larger than stock diameter piping or a rich condition with decreased diameter piping.
2. The air flow is not laminar. The stock piping, through it's apparent unusual design, is constructed to smooth out turbulent air flow. Aftermarket systems do not take this into consideration. When turbulence occurs, the MAF sensor will suddenly see more or less air then it had previously. This will cause the ECU to respond by making the engine run leaner or richer for as long as the error from turbulence occurs.

Can the intake problems be corrected through aftermarket engine management? Yes. There are a wide variety of engine management systems that have the capability of modifying the mass air flow values to correct intake piping that is non-OEM size. Some intakes have especially troublesome turbulence problems. Since this turbulence can occur at different points in the power band, based on both engine RPM and engine load, all that can be done to make the engine run safely is to recalibrate the engine with a relatively rich overall tune to compensate.

Is there something I can do myself to free up my intake? Many owners remove their snorkel and utilize an aftermarket filter in the stock air box to free up the intake system somewhat. Instructions for this procedure can be found via this link, this link, and this link.

Which manufacturer is best? This topic is highly debated. There have been no reported consistent "bad" intakes on the market. Obviously, there may have been bad intakes sold, but not enough to report as "bad" overall.

Which intakes have the best gains? There is no irrefutable evidence that any intake has better gains than another.

Where do I buy an intake? Every Subaru/Import performance store sells intakes. For purchasing, support your local economy or the NASIOC Vendors.

How hard is it to install an intake? Allow around one hour for install time. Professional installation, depending on your area, is around $75. This is one vehicle modification that is very simple and can be successfully accomplished by even the greenest shade tree mechanic.

Do I need engine management/tuning with an intake? For most people, yes. Though there are a few that are or claim to be plug and play, tuning will make any intake shine or turn a "cheap-o unsafe" intake into a safe and power making unit.

How do I install an intake? Refer to the intake manufacturer's instructions. Below are links to some of the better known intake installation instructions:
TurboXS's Instructions (with photos)
scoobymods.com instructions (with photos)
Cobb Tuning's instructions (with photos)

Should I reset my ECU after this mod? It is never a bad idea to perform an ECU reset after a mod. The traditional route is to disconnect the negative battery terminal, press the brake pedal for a few seconds to bleed the system of charge, and reattach. Some use the more advanced Vishnu Reset.

What about just using a drop in air filter like K&N etc.? Many people will install an aftermarket air filter with two lines of reasoning:

a. longer lifespan due to end user cleaning = cost savings
b. freer flowing = more HP

While both are probably true, there are costs associated with using an aftermarket drop in air filter.

a. Comparing costs of the OEM paper filter at $10 vs an aftermaket unit at $40 mean you will have to use the aftermarket unit for 150,000 miles before you see any return on your investment based on change frequency schedules.
b. freer flowing means more contaminants work their way into your engine. Filtration study and Filtration study
c. HP gains are not well documented if at all.

Editors Note

My thanks to Trey Cobb of Cobb Tuning for writing an excellent intake article and to EcuTek for their excellent intake article which both provided a lot of the information contained within this FAQ.

This post was created because I wasn't able to find a good intake FAQ. I came up with the text based on LOTS of searching here. It was also created to be intentionally brand neutral so that it serves as a stepping stone for further research. Upon reading this you should have an idea of what type of intake best suits your needs. The manufacturer is up to you.

If you find an error in this FAQ, please PM me with factual details and I will update this post. Responses such as, "I have XXX's intake and it's great!" or "XXX's intake is boggy" are not appreciated here, that is what the Car Parts Review Forum is for.
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Last edited by Unabomber; 04-08-2009 at 11:41 AM.
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Old 01-02-2004, 04:17 PM   #2
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more excellent info by the unabomber
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Old 03-05-2004, 06:42 AM   #3
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yes thank you this is exactly the type of run down i was looking for, im picking up a TurboXS shorty intake from a friend on Saturday for cheap so this helped...Thanks!

Kevin..
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Old 03-05-2004, 09:07 AM   #4
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Thanks for the info. The pics you posted are a lot better than the ones in the service manual!
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Old 03-05-2004, 09:08 AM   #5
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Quote:
Originally posted by KevinWRX
yes thank you this is exactly the type of run down i was looking for, im picking up a TurboXS shorty intake from a friend on Saturday for cheap so this helped...Thanks!

Kevin..
Just one question... I'm thinking of getting the TXS, but it looks like it pulls air from the engine bay...any chance pulling this "hot" air will actually decrease power?

My car is stock now...but will be at Cobb Stage II as soon as my AccessPort and Exhaust ship... So I'm looking for an intake to compliment the exhaust...even if its just for the sound...(and no LOSS of power)....

Thanks
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Old 03-05-2004, 11:31 AM   #6
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Your best bet is to call Cobb on the phone and ask for their recommendation. I researched the devil out of intakes when writing this FAQ. I was amazed to see that some manufacturers make "worry free" intakes. My experience:

I called Cobb to order my AccessPort and asked them specifically about these "worry free" intakes to see if they had maps for them or since they were "worry free" if I even needed a special map. I was greeted with a fierce chorus of, "don't waste your money" to all of my very pointed questions. My intake Q&A lasted about 10 minutes and the advice I was given never wavered. To sum it up, I was told when they tune for an intake it is generally so conservative that it removes most of the gains that these intakes claim to make. Most manufacturers post the dyno results with gains, but don't post the helter skelter A/F ratio charts. Once tuning occurs to fix the A/F ratios, much of the gains go away as well. These unusual A/F ratios are caused by incorrect piping diameter, incorrect MAF sensor placement, and lack of turbulence smoothing in the piping itself.

Obviously, this is the opinion of one tuner. Cobb themselves are in development of an intake as well, so maybe they know something the other tuners don't. Also to their credit, they are the only tuner on the block that tunes Subarus as a soul source of income. This FAQ was written as a good baseline for further research, I suggest you reasearch some more and come to your own conclusion as to wether an intake fits your overall plan.
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Old 03-05-2004, 12:22 PM   #7
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This post isn't a sticky yet?
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Old 08-29-2004, 02:37 PM   #8
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What about Injen CAI?
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Old 08-29-2004, 03:03 PM   #9
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Quote:
Originally Posted by randyspeed
What about Injen CAI?
Until proven otherwise, it's crap.
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Old 08-29-2004, 04:56 PM   #10
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Quote:
Originally Posted by randyspeed
What about Injen CAI?
I have a Injen CAI sitting in a box on my bedroom floor. I too called COBB to get their advise about the Injen CAI because they have a map for it. You know what their response was?

"Dont bother, it's not going to give you more power. Just stick to the stock airbox. If you want to put it back in the car feel free we do have a map for it".
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Old 08-30-2004, 01:26 PM   #11
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Oh cool, looks like everyone is running 350hp+ then. Because if you are not, then your stock intake is more then enough. This should be a mod that you perform at the end. Save your money! And please please please stay away from Ingen
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Old 09-02-2004, 03:36 AM   #12
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Default cold air intakes: read this first

Before all you overly eager, mod happy freaks go out and blow a bunch of coin on something your car doesnt really need, but absolutely must have take a gander at this ----> http://www.mkiv.com/techarticles/filters_test/2/
Injen and AEM both make their own filter elements and are comparable to K&N's. APS uses a K&N filter element with their kits as do a lot of other manufacturers, if not you can usually buy a K&N element to fit most of the aftermarket ones. Wet foam type filters also have 1 major drawback, over time your MAF sensor may get fouled by the oil and throw a CEL, or worse, eventually damage it and cost you even more $ for repair/replacement (trust me on this) lol, plus, they're no fun to clean and re-oil. If you don't care about looks under the hood or a cool BOV sound keep your stock air intake, save your money and just upgrade your panel filter. Hope this helps, cheers!
P.S. If you live anyplace overly wet you'll regret blowing all that $ on a CAI, fair warning.
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Old 09-02-2004, 09:41 AM   #13
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so is all of this info on intakes just as equally relative to a non-turbo suby?
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Old 11-06-2004, 09:49 PM   #14
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Old 11-07-2004, 03:49 AM   #15
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So from everything I've read in this thread, am I to believe that I would just be wasting my money buying an aftermarket Cold Air Intake for my stock 2002 Impreza 2.5RS since I don't plan on putting anything on it that will make it have more then 250 HP? Would I just be better off buying an airfilter?

Last edited by dabigmooish; 11-07-2004 at 03:59 AM.
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Old 11-07-2004, 08:24 AM   #16
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yep!
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Old 11-07-2004, 09:44 AM   #17
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The Arc box is a very good intake if you just have to have one. Never had a problem with it compared to Apexi and HKS brands that always seemed to give me problems!!!
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Old 11-07-2004, 02:36 PM   #18
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Quote:
Originally Posted by DuoMaxwell
The Arc box is a very good intake if you just have to have one. Never had a problem with it compared to Apexi and HKS brands that always seemed to give me problems!!!

but that one is an expensive piece...

why dun go with K&N Typhoon ? cheaper in price, more in gain...

heard many good things about K&N Typhoon, now going to sell my zero sport intake pipe and filter...
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Old 11-07-2004, 02:47 PM   #19
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Quote:
Originally Posted by dabigmooish
So from everything I've read in this thread, am I to believe that I would just be wasting my money buying an aftermarket Cold Air Intake for my stock 2002 Impreza 2.5RS since I don't plan on putting anything on it that will make it have more then 250 HP? Would I just be better off buying an airfilter?
Pretty much...

but hey, the CAI will sound really cool!

~ Miranda
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Old 11-07-2004, 02:52 PM   #20
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This post is catered towards turbo'd Subarus. The rules for intakes are different for N/A vehicles. I suggest you search in the N/A forum for the best advice.
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Old 11-08-2004, 12:50 AM   #21
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They really need a "FAQ" section for these posts.. aw hell.. just have a "Unabomber" section..

Sticky at least??

-Shane
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Old 11-08-2004, 02:53 AM   #22
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Quote:
Originally Posted by AzSandSlinger
They really need a "FAQ" section for these posts.. aw hell.. just have a "Unabomber" section..

Sticky at least??

-Shane
yes i agree totally!!!!
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Old 11-08-2004, 07:47 AM   #23
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Quote:
Originally Posted by kool168
but that one is an expensive piece...

why dun go with K&N Typhoon ? cheaper in price, more in gain...

heard many good things about K&N Typhoon, now going to sell my zero sport intake pipe and filter...
I dunno but I am not a big K&N fan and the Arc untilizes the stock set-up. Don't get me wrong I am not saying K&N is bad at all just not what I like! Besides I bought my arc used for $100 through yahoo auctions Japan. I know not everyone can be so lucky but for that price you just cannot beat it!!
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Old 11-08-2004, 07:48 AM   #24
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There already is one, you just don't know about it!

Unabomber's Manifesto

Next FAQ will be my Transmission FAQ which I hope to complete in 2 weeks. Look in the transmission forum or visit my manifesto to see when it will arrive.
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Old 11-08-2004, 12:06 PM   #25
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<<<<<<< kicks himself becuase he knew unabomber already had a manifesto but forgot about it.

Last edited by Bishop; 11-09-2004 at 01:36 AM.
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