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Cold air intake must be one of the popular mods there is, and one subject to the most controversy as well. Manufacturers of such systems advertise 5-20 HP gains, yet there are claims that this mod does nothing and may even hurt performance (even tests with videos can be found online).

Do you really need this?

As I see it the stock setup of most cars already contains a "cold air intake", since the factory airbox usually sucks air from an area remote from the hot components, like the fender or bumper.

So what's the point of adding a cold air intake system to your car? Is the only point to reduce the air intake restriction by adding a gauze/foam filter, and since the airbox has to be removed, piping and heatshields are added to draw cooler air and prevent power loss? Does this mod ever give a measurable increase in power?

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Does this mod ever give a measurable increase in power?

tl;dr: yes, sometimes it works well. But...

Your picture is a good illustration of some of the problems with just saying "cold air intake" and expecting that to mean the same thing to all people. Let's break down the pieces of the puzzle and talk about how those might help or hurt:

  1. Filter: notice that the filter in the picture is not a panel filter as many of us have in the airbox. If that filter is less restrictive than stock, that will certainly impede the intake's job of getting air molecules into the system less. However, is it still filtering out hazards to the motor? Is it an oiled filter? Will that oil get onto the mass airflow sensor?
  2. Piping: the actual path that the air takes is definitely important. Is this piping better at thermal isolation? The engine bay is hot and hot intake air means less performance. Is the piping less restrictive? Does the air flow better? Does it flow too well (causing a lean condition)?
  3. Location: the intake in the picture appears to be a short ram intake. That means that it only reaches to the standard location of the airbox. Will it be drawing air directly from the hot engine bay? Or is there another path for cooler air to reach the intake? Would it get cooler air sucking from directly in the wheel well? Of course, then you run the risk of sucking up water in a heavy rain....

In theory, the engine is an air pump. If the intake can reduce restrictions and pumping losses, it's all good, right? Unfortunately, even in the theoretical world, freeing the intake isn't going to solve a restrictive exhaust (can't get the air out as fast as we can get it in).

Let's talk two practical examples using my cars:

  1. 1997 Acura Integra NA 4 cylinder. I tried two different cold air intakes on that car. Both of them seemed to make the car happier (subjective assessment based on a nicer sound). However, the power increases weren't really significant until I had replaced the headers and the exhaust system with stainless steel. The car sounded great, revved well and had a lot more pickup. There was every reason to believe that I'd removed several restrictions on a car that had originally prioritized quiet over fun. However, I'd also removed a lot of weight of metal by switching to stainless (those iron headers were heavy). In this case, the CAI was part of the picture but not the biggest part.
  2. 2004 Subaru WRX Turbo 4 cylinder. This car has a reputation for ignoring CAI changes without a retune. The ECU simply recalibrates to match the increase in airflow, resulting in no increase in power. There's also a risk of damage to the MAF using oiled filters. There's also the reality that I don't need to worry about the same intake restrictions that I had on the Acura: I'm not just hoping air wanders into the engine with a turbo in the mix! With this car, the recommended change is a less restrictive panel filter that drops in place of the stock.

So, where does this leave us?

  1. A CAI has a good chance of sounding cooler than a stock intake. It will definitely be louder.
  2. A CAI will reduce restrictions in the intake path. That might lead to more power out of the box. It might not. It might lead to engine damage. Caveat emptor.
  3. A CAI may increase risk of water ingestion. This may lead to hydrolock (a broken engine and many tears).

In short, like all things car related, it depends. On an older car with a weedy intake and hot under hood temperatures, I would certainly try a CAI. On a modern car, I don't bother.

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    Well, I consider cold air intake system to be combination of a filter, pipe and possibly a heatshield for replacement of the stock airbox. A few questions. How can air flow TOO well if the MAF/MAP is watching the air content? How does the WRX recalibrate to maintain power, if there should be more air getting in? Cutting boost pressure? Why does the under hood temperature matter, if the stock airbox usually picks up air from a cooler place than most CAI systems can offer? As I understand NA vehicles have a higher chance of benefiting from a less restrictive air intake system?
    – I have no idea what I'm doing
    Commented Jan 21, 2016 at 13:52
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    The SRT4 would react similarly to stop itself from making more than stock power. Retarding the timing usually does the job. If the boost pressure goes through an ECU controlled solenoid, then that would be one solution. I've heard on the SRT4, the ECU can defeat a manual boost controller. Turbo and super charged engines tuned properly will benefit more as they are moving much more air. CAI is a balance of temperature and restriction. Some engines are said to benefit more from a short ram intake than a cold air intake.
    – rpmerf
    Commented Jan 21, 2016 at 14:31
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    @rpmerf How does the engine know when it is making more than stock power, exactly? Does it start retarding the ignition when the MAF airflow readings exceed some value? That would seem wasteful compared to decreasing boost, as the fuel is still burned, but extra power is not made.
    – I have no idea what I'm doing
    Commented Jan 21, 2016 at 15:18
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    @IhavenoideawhatI'mdoing those are interesting questions that we would have to investigate on a car by car basis. These are measurable quantities, though. The WRX was well known for frustrating early adopters who wanted to try the intake, header, exhaust standard set of upgrades. Spend a lot of money, install and zero real impact on performance on a dyno. Proof by existence. A tune is required to make use of the extra air.
    – Bob Cross ♦
    Commented Jan 21, 2016 at 16:54
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    @IhavenoideawhatI'mdoing I assume they do use the MAF reading. I don't know exactly. I'm having a hard time thinking of another way that they could do it. If the user installs a manual boost controller, then the computer would have no way of decreasing the boost other than making less power. Most of this was heard at car shows, so it may not be 100% accurate.
    – rpmerf
    Commented Jan 21, 2016 at 21:12
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The point is to feel like you've done something cool to your car and freed it from the shackles of The Man/the OEM intake. The primary benefit of Cold Air Intakes is to the bank account of the kit manufacturer, the secondary benefit is your car making a nicer noise, if you like the sound of an aftermarket intake.

There's been a few debunkings of CAIs over the years, this is one of the more entertaining ones All the investigations I've seen of CAIs have shown that they usually reduce power output. OEM air intakes are generally a lot better optimised than they look and there's a lot more room for making them worse than there is for making them better.

The myth behind the CAI is that it lets you suck in denser (because it's colder) air than the stock intake, which, as you correctly pointed out, will already be sucking air from a fairly cold location. Cone filters are supposedly less restrictive than OEM paper panel filters but there's probably not much difference as the OEM filter will already provide adequate airflow for the engine in its stock state of tune. Few manufacturers are going to throw away 2bhp just to save 50p on the air filter. Changing the adequate OEM filter for a filter that flows more air will not make an appreciable difference because it's not a limiting factor on the engine's performance.

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It depends.

Just because an intake can flow more air mass doesn't guarantee that the engine will utilize it.

The intake is part of a system of components. The engine produces power by managing air flow into and out of the combustion chamber. There are usually other actors involved:

  • Intake side. Carburetors, throttle bodies, intake manifolds, intake valves, crankcase vacuum...
  • Exhaust side. Exhaust valves, headers, resonators, catalytic converters, mufflers...

The potential of any engine will be held back by the component that is acting as the greatest bottleneck to air flow, much like how the reliability of a system of components will be limited by the one that is least reliable.

With this in mind, what @BobCross says should not surprise anyone:

If the intake is bottlenecking a vehicle's performance, a CAI will make it look like a hero.

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    This is a great answer Zaid! Spot on :D
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Jan 22, 2016 at 17:04
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Almost never? Mainly because they're really a 'hot air intake system'. This is especially true for cars using forced injection due to the high under-hood temperatures.

If you want to reduce intake restriction look into a less restrictive 'panel' filter like a K&N. Even then, it really only matters if your car is intake (vs exhaust) limited and your ECU will increase fuel delivery in response to the additional air vs retarding timing or (for old timey cars) doing nothing, resulting in a leaner air-fuel mixture.

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    It really depends on the "type" of CAI. I agree with you if the filter element isn't isolated from the engine bay, but there are many brands/models which are isolated from the engine bay and could be considered true cold air intakes :D
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Jan 22, 2016 at 0:40
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The working cycle of an 4 stroke internal combustion engine is like:
(1) Inlet of the fresh air
(2) Compression
(3) Working Cycle - expansion
(4) Exhaust

Cooler intake air is especially beneficial for part (1). Since cooler air has a higher density, it means that the gas velocities at the inlet are lower. Therefore lower pressure losses at the cylinder. More fresh air mass in the cylinder. -> Potential for higher power

This is also the reason why turbo-charged engines often have a charge-air-cooler (CAC) is applied.

That's the theory. I don't have experience on how much benefit a retro-fit device delivers in reality. Additional factors like the pressure loss of the system itself and the parameterisation of the engine control unit (ECU) might influence this effect.

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I didn't see anyone mention it but I believe any increase in power would only be at wide-open throttle (WOT).

So if you are racing or really aggressively pulling away from stop lights or accelerating, maybe it helps. Day-to-day driving at less than WOT the engine management system will keep the air-fuel ratio at an acceptable value. If it doesn't get enough air, it will either reduce the fuel or open the throttle a bit more.

So if you need more power you just press the gas pedal a little more.

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After many years of tuning and operating competition type vehicles of all description (carts, off-road and rally trucks) I can say confidently that a stock vehicle meant for daily driving does most likely not benefit from a so called cold air intake. The difference you experience is a perceived increase in power because of the sound you hear from a basically wide open intake pipe. That's why we used to turn our air cleaner covers upside down on carburated engines so it sounded beefier and did absolutely nothing for perceived performance.

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