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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Powertrain and Drivetrain Discussions > NA Engine (non-turbo) / Drivetrain / Exhaust Modifications > Basic tech on Cold Air Intakes



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      07-20-2006, 11:40 PM   #1
ksfrogman
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Thumbs up Basic tech on Cold Air Intakes

I was google-ing around and found this nice little article on CAI on answers.com. It was a contribution to wikipedia.

Quote:
cold air intake
A cold air intake is a system to bring down the temperature of the air going into a car for the purpose of increasing the power of the internal-combustion engine. A secondary goal is to increase the appeal of a car by changing the appearance of a car's engine bay and creating an attractive intake noise. These aftermarket parts come in many different colors and many different sizes, and are an inexpensive way to increase performance.
Aftermarket company, K&N Engineering first offered air intake systems in the late 1980s. Those intakes consisted of rotationally-molded plastic intake tubes and a conical, cotton gauze air filter. In the late 1990s a proliferation of intake manufacturers such as AEM, Airaid and Volant entered the fray. In addition, oversea manufacturers imported their designs lending to the popularity of Japan domestic market (JDM) air intakes in sport compact markets. K&N and many of the other intake companies now offer intake systems in metal tube designs, allowing a greater degree of customization (the tubes can be powder-coated or painted to match a vehicle).
All cold air intakes operate on the principal of increasing the amount of oxygen available for combustion with fuel. Because cooler air has more density for a given volume, cold air intakes generally work by providing cooler air from outside the hot engine bay. However, the term "cold air intake" is often used to describe other methods of increasing oxygen to an engine, which may even increase the temperature of the air coming into an engine.
Some strategies used in designing cold-air intakes are:
  • increasing the diameter of the air intake, allowing increased airflow.
  • smoothing the interior of the intake to reduce air resistance.
  • providing a more direct route to the air intake.
  • tuning the length of the intake to provide the most airflow at certain RPMs.
  • using a more efficient air filter
Intake systems come in many different styles and can be constructed from plastic, metal, rubber (silicone) or composite materials (fiberglass, carbon fiber or kevlar). Due to the limited time air actually remains inside the intake tubing, the materials often do not impact a kit's ability to deliver cool air.
The most basic cold air intake replaces the stock airbox with a short metal or plastic tube leading to a conical air filter, called a Short ram air intake. The power gained by this method can vary depending on how restrictive the factory airbox is. The placement of the filter is usually directly in the engine compartment. The overall benefits depend on the specific application. Power may be lost at certain engine speeds, only to be gained at others. Because of the increased airflow and reduced covering, intake noise is usually increased. This effect is usually amplified on applications where a resonator, a part intended to reduce intake noise on some vehicles is replaced by the intake.
Better designed intakes use heat shields to isolate the air filter from the rest of the engine compartment, providing cooler air from the front or side of the engine bay. Carbon fiber can be used for the piping instead of metal, lowering weight and insulating the air from the engine bay in some cases. Carbon fiber and other advanced composites (such as Kevlar) come at significant costs and are often more aesthetic rather than functional (unless the application is a serious race vehicle).
The most extreme designs, sometimes referred to as Complete Cold Air (CCA) intakes, route air from outside the engine bay, usually from the wheel wells, front grill, or a hood scoop. The intake can be placed such that the forward motion of the car pressurises the air coming in, creating a ram-air intake. These intakes often require additional modifications and can require body modifications or replacement panels, such as a replacement "ram air-style" hood.
The best cold-air intakes are optimized for a specific engine application, providing increased airflow at ambient temperature and raising power at all engine speeds.
When using a cold air intake, there is a potential risk when driving in the rain. This is often referred to as "hydrolock", and according to the automotive portal, MODsearch: "Say it's raining cats and dogs and you're out for a spin in your car. Normally you'd love to rip through puddles without thinking twice, but because your engine is now getting air from inside your bumper you have to be careful. If your engine manages to suck up any amount of water through the intake and into the engine you will probably have little to no horsepower left. In other words, be careful." So, it is important to take the necessary precautions when using a cold air intake so you do not end up flooding your engine. This may include installing a water shield in your intake or not driving in the rain at all. It is also notable that less damage will occur from water getting in the engine on a rotary engine car, as opposed to a piston engine car.
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      07-20-2006, 11:41 PM   #2
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      07-20-2006, 11:43 PM   #3
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I'm posting this because several aftermarket designs use different materials and methods of heat shielding. The colder the air, the denser it is, the more oxygen gets into the engine, and the better the combustion.

Metal can help shield against radiant heat, but in an enclosed engine compartment, all the hot air will be sucked into an air intake with an open design, even if it's only partially open. I can't imagine the hot air being good for performance, but perhaps the increased volume of air compensates for this.

The OEM air intake is completely enclosed. All the air comes from the kidney grill area. I believe the Simota is completely closed off.

Quote:
The short ram air intake is a form of aftermarket air intake for automobiles with internal combustion engines.

It replaces the OEM air intake with a short metal pipe and air filter inside the engine bay. This can slightly boost performance levels in a vehicle by increasing air volume intake, at the expense of increased intake noise. The high temperature of air near the engine can reduce performance in some cases, but this is often made up for by the increased volume of air. As a remedy, many short ram intakes include some form of heat shield. However, some users prefer to use different types of air intakes such as a Cold air intake.
And here's another interesting formula. We know that air density is inversely proportional to ambient temperature of the air. So this may give you an idea of the effect of hot air being sucked into the engine:
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