This article is one in a series that have been released in conjunction with Wayne's new book, 101 Performance Projects for Your BMW 3 Series. The book contains 272 pages of full color projects detailing everything from performance mods to timing the camshafts. With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any 3 Series owner's collection. The book was released in August 2006, and is available for ordering now. See The Official Book Website for more details.
There's been a lot of talk on Internet chat boards lately about cold air intake (CAI) systems. Some manufacturers swear up and down that there's hidden horsepower in the intake system. Other so-called "experts" claim it's total bunk--a myth easily circulated in the age of the Internet. I believe that the truth lies somewhere in between. On some BMW models, a properly engineered cold air intake kit can effectively cool the intake charge entering into the cylinders. Why do you want this in the first place? Cooler air decreases the likelihood of engine detonation and, theoretically, since cooler air is denser air and denser air has more oxygen molecules to use in the combustion process, cooler air should correspond to an increase in horsepower.
First of all, the stock BMW E36 intake system is already a cold air intake system. It sucks air from the outside, and insulates this intake from the heat generated by the engine. But it doesn't work too well in the area of airflow restriction. The stock system is somewhat restrictive and makes the air work its way around in order to enter the engine. Aftermarket performance intakes serve two purposes: first, to better inject colder air directly into the intake system, and second, to reduce the restrictions inherent in the stock system.
Just how much horsepower can you expect to gain? It all depends on your particular BMW model and which kit you use. A poorly designed kit used with an aftermarket air filter may actually create more restriction in the intake and cause a decrease in horsepower. In addition, an intake system that leaks because it was poorly installed will also decrease performance. I have seen dyno tests where an intake system has done nothing or has even reduced horsepower. Then again, I have also seen tests where significant gains of about 10 horsepower were recorded.
The bottom line is that a performance intake system is typically an expensive aftermarket add-on that may increase your horsepower only slightly. Some kits cost over $500, which translates into a very high dollar-per-horsepower ratio. Some other kits are less expensive and incorporate reusable aftermarket filters. While I'm not a huge fan of reusable filters, as they tend to work less efficiently than the stock ones (see our Air Filter Replacement article), there can be costs savings in the long run by eliminating the need to replace them so often.
Perhaps the only thing that most people agree on is that a performance intake system will give you a much better engine growl at higher revs. I'm personally a huge fan of growling noises, so this would be a plus for me if I were making the decision to install one of these systems. For those of you who prefer a quieter engine, you might be slightly annoyed at the new music your engine is playing.
There are two distinct types of systems available on the market. The short ram system is located in the top of the engine compartment and uses a heat shield to block heat from the engine. The filters are easy to change on this type of system. The other is typically referred to as a "cold air intake" and uses long tubes that connect the intake to inlets in the front bumper. The second type of system capitalizes on the air pressure available at the front bumper, although the filter is difficult to reach for changing or cleaning. Also, if your intake is low to the ground, you must be very careful not to go through deep puddles; otherwise, water may get sucked up into your engine intake (hydrolocking your engine).
There are literally hundreds of manufacturers of these products, from the large tuning companies to people selling homemade kits on eBay. If you install one of these on your car, do your research first and stick with a well-known manufacturer. I've heard good recommendations from people who run kits available from Dinan, East Coast Induction Systems, Cosmos Racing, and Advanced Flow Engineering.
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The Dinan cold air intake assembly funnels cool air from an inlet hole in the spoiler and channels it up into the engine. It's an expensive, high-quality system manufactured out of carbon fiber, currently certified to meet California emissions requirements (CARB). In addition, Dinan is sanctioned by BMW, and the dealer can install this unit if you'd like (about 1.5 hours' labor). If you have installed the European ellipsoid headlamps (see our Ellipsoid / Angel Eyes Installation article), you might have to modify the brackets in the kit to make it fit properly.