Originally Posted by auditech79

Ok kids, I really shouldn’t have to make a sticky about this topic but there is some confusion on the boards about what a cold air intake really does equipped on a turbo/NA engine. We can start with basic physics.

1. Colder air is heavier than warmer air because colder air is more dense than warmer air. More dense means there are more air molecules in colder air than there are in the same volume of warmer air. Meaning more molecules results in greater mass and weight.
2. Warmer air will move to colder air.

Let's use a hot air balloon as an example. Hot air is in a state of excitement, meaning the molecules are moving around faster than cold air and they are spaced farther apart. The result is it will be lighter than the dense air surrounding the balloon so the balloon will rise. If you stop adding heat to the balloon, the heat will move to the cold air outside and will begin to descend back to earth. Now lets analyze this pertaining to an engine. I’ll start with an N/A engine first as its simpler.

N/A Engine equipped with a CAI.

Now, and engine that doesn’t have a turbo charger must rely on atmospheric pressure to supply air to the engine. That ratio is 14.7:1. Now if we use our example above colder air has more molecules, more molecules means more oxygen, engines burn oxygen, so the more you can cram in there the more power you can achieve. Since there is no turbo or intercooler on an N/A engine the only way to get colder air into the intake is to extend the pipe to the fenderwell or down below the engine where the air is cooler. On this example more oxygen is getting to the engine thus creating more power. On the flip side, you have extended the intake pipe about 3X the normal length meaning it makes it harder for the engine to pull the air into the engine. So a shorter length pipe would be ideal, but that would put the filter into the engine bay sucking in hot air. You would actually LOSE horsepower here because you are pulling in a lot less oxygen. In short, people using short ram intakes on an N/A motor are actually shooting themselves in the foot. The stock airbox is more efficient.

Turbo engines equipped with a CAI.

I know you kids love your APR Carbonio intakes but you should understand exactly why your car performs the way it does with it installed. So now you understand that colder air is more dense, so pulling in colder air should create more power on my 2.0T right?……..Not exactly. Turbo cars have these things called intercoolers. When air is sucked into the turbo and compressed the air is extremely freaking hot, like burn your damn face off hot. This gets pushed through the intercooler where the temperature is massively reduced then sucked into the intake. I want you to think about this for a second, will there be a difference sucking 10-15 degree cooler air than normal into the turbo that’s usually around 800-1000 (more if the freaking thing is glowing) degrees? Well you guessed it, its not. I can hear you thinking, well then why does it feel like my car gets up and goes better with it installed!? Well I can see why you would say that, the intake is shorter and has a higher flowing filter, thus decreasing spool time. That’s the only gain you will get from an intake, slightly increased spool time. If you put your car on a dyno, you would be lucky to get a 1 horsepower gain. So once again the best intake you could have is a short ram intake with a high flowing filter. Just look under the hood of a 600 horsepower EVO, you won’t find a cold air intake on that. Ever notice that turbo cars at the drag strip have one of the headlights removed? That’s because they are trying to ram as much air into the intake as possible. Case and point, if you want power, buy a good FMIC kit, a larger turbo, tuning, and a higher flowing exhaust. Beyond that, an engine build.

Originally Posted by Yearight

On the turbo cars (my specialty) it's true, they have intercoolers, but routing the filter outside of the engine bay, for an average of about 15 degree cooler air, still helps the intercooler with it's efficiency. It won't have to work as hard, and contrary to popular believe, the post IC air charge will be cooler. It's a fact. NOW, whether or not you'll get any appreciable gains from that, is another story. The cost difference between a short ram, and a true CAI, is usually around 150 dollars. Is it worth it for an additional 5-7 hp? Nope. Not for most people.