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.

 

hahaha i have seen the shooting in the foot before, and i laughed, my friend had his eclipse equipped with a CAI that sat directly over his manifold haha he said "man it looks awesome, all chrome, i bet it get 15hp from it!" haha i have seen a few good applications, but mostly it all depends on the engineers who designed the car and the space they allow you to run the shortest intake to the coldest location. My biggest question is about intercoolers, i come from ford world where intercoolers are CAC and freakin bigger than my audis entire radiator! so for my audi is a bigger intercooler worth it or do i loose precious boost with more sq in to fill? i know the pressures there but the longer it takes for it to reach the engine the more lag right?
~sorry for a long post

 

Right, you don't want to run an intercooler or piping thats too large for the turbo, you will lose PSI. The turbo on a powerstroke diesel is freaking massive so the intercooler has to be........large.
They sell kits like this one http://www.atpturbo.com/mm5/merchant...egory_Code=AB7
which will work up to 400 horsepower. You still might lose a PSI or two using that core on a stock turbo.

 

Not quite.. Slower air has more pressure so extending it down to the fenderwell is actually pushing more air in that overcomes the adding length (less Delta). Which, when you look at the systems that actually work, you'll see - They all go to the fenderwell or other 'low velocity-high pressure' area.

You mean decreasing spool time?

Again, there is no 'ram effect' since faster air has less pressure.. in this case, they are looking for the shortest least restrictive distance for the fastest spool up time.



edit: wtf? had to edit to post? forums are slow...

 

There are two aspects to air pressure: static pressure and dynamic pressure. Static pressure decreases with speed, dynamic pressure increases. Total pressure is the sum of these two, and it is constant. For a forward facing intake (ram), you get the full total pressure (static + dynamic). If the intake is at right angles to the air flow, you only get the static pressure.

Technically you need to take Mach number into account, but at the speeds our cars go, it's not an issue.

 

Bernoulli's principle

Basically, if air is moving at higher speeds, it has a lower pressure. Sure, dynamic pressure plays a part, but the 'ram air' has already been shown to be less desireable giving a choice.

 

OOps, totally right about that. Blooper.

 

I'm an aerospace engineer, so I do know a little about air flow properties. Now, perhaps turbulence in the ram air entering the intake decreases performance more than ingesting static air, but if so it's not due to a pressure difference. The total pressure (which is what matters) is the same. In fact, if you study the equations carefully in the link you posted, you will see this.

 

So total pressure is always ambient? I'm lost. if that was the case, how does a wing create lift?

 

At low Mach numbers (such as those experienced by most cars), total pressure is ambient. Total pressure is the pressure that is recovered when air has been slowed to a stop.

Wings create lift slightly due to Bernoulli's effect, because what matters in that case is static pressure, which decreases as dynamic pressure increases with speed. Mostly, though, it's due to the air flow being deflected downward (see http://en.wikipedia.org/wiki/Kutta_condition).