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Cold Air Induction
As any chemist knows, the colder that air is, the more dense it is, and denser air will provide more oxygen in any given volume (i.e. inside the engine cylinder), allowing your engine to burn more fuel and generate more power. A common rule of thumb is that decreasing the air intake temperature by 10 degrees F will increase horsepower and torque by 1%. The converse is also true. The problem with most stock engine intake systems is that they consist of just a box with a filter located somewhere in the engine bay where it fit. This box justs draws in the hot air from under the car's hood and is probably not in the best place to draw cooler air. Furthermore, they tend to be quite restrictive, in an attempt to reduce underhood noise. Aftermarket "short ram" air intakes (that look like a cone on a pipe) are less restrictive and might provide a few more extra horsepower. However, they suffer from the same problem as stock systems in that they draw in the underhood air. Therefore, they work nicely after the car has been sitting overnight, but actually decrease in performance once the engine (and thus under-hood temperature) heats up. The better aftermarket systems try to locate their intake tip in an area under the engine bay that might have cooler air, such as near the bottom of the car or by the grille. This arrangement helps out by drawing slightly cooler air than that generally found under the hood and thus results in more of a power gain.
The only way to solve the problem of hot under-hood air is to get air from somewhere else. The solution is what is called a true cold air induction system, but is commonly called "Ram Air" (Pontiac invented that name). There are two common techniques used by these systems. The first is to draw air from the outside either through holes in the hood, or through openings in the grille into air ducts that channel the air through the air filter and into the carburetor. In almost all cases, the carb is sealed off from getting any air from under the hood. In fact, you can easily stall the engine by covering the holes on the hood or grille that feed the carb because the carb does not receive air from any other means. Oftentimes, the hood vents will actually be scoops or are raised from the rest of the hood, to capture air as it flows over the hood. Performance can be increased if these scoops are at least 1" above the surface of the hood, because the air flows faster than right at the hood's surface. Thus, the air flowing over the hood is "ramed" into the scoops, through the air ducts, and into the engine. While most scoops either point upwards or forward, Chevrolet pioneer their "Cowl Induction" system which featured a backward facing scoop right at the base of the windshield. This scoop actually received air that was forced backwards after hitting the windshield.
The second technique is to move the entire air filter & carb assembly out from under the hood. The most common examples are Shaker hoods where the air filter (usually in a protective and decorative cover) physically juts out of the hood through a hole in the hood. Thus, the air is drawn directly from the outside, without having to go through any piping. It is called a Shaker hood because the air filter cover actually can be seen shaking as the engine vibrates. This technique also has a greater benefit the higher off of the hood it is. Most high performance race cars use this technique and, with carb risers, often have intake systems 6" - 10" higher than the rest of the hood.
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