Cowl induction has been used since the early days of Grand Prix racing in 1910. This method is a proven, if somewhat antiquated, way to make horsepower. Cowl induction hoods use a backward-facing scoop in the hood to draw in some of the cool, high-pressure air that collects on the windshield at speed. Though a properly engineered cowl-induction system looks good and works well, there are a number of options available on the market that don't carry many of this system's drawbacks.
As a car punches through the stagnant air at high speed, there are a number of areas that tend to accumulate high-pressure pockets. The front bumper, grille, headlights, windshield and wheel-well are the most common areas.
Since engines make more power with cool outside air than the hot ambient air found in the engine bay, it seems logical to tap these high-pressure pockets to feed air to the engine. In addition to lowering air temperature, increasing the air pressure around an intake artificially crams more air into the engine than the static air could, which is similar to how a supercharger functions.
Cowl induction hoods are used for a number of other reasons as well, prime amongst which are intake clearance and cooling. Since many performance intakes -- particularly those found on carbeurated vehicles -- are taller than stock, there may not be enough room under the hood to fit an air cleaner.
The second ancillary function of cowl hoods has to do with cooling. Though these hoods draw in cool air while at speed, when idling or cruising at low speed they go from intake to exhaust. Since performance cars tend to produce excess heat when idling, the ability to vent hot air from the engine bay may be the difference between a fun night out and a melted motor.
Cowl induction hoods can help to smooth the airstream passing over a car at speed, but can be more of a detriment to high-speed handling than anything else. If the cowl inductor is not sealed to the intake, the cowl can actually pressurize the engine bay at high speeds. This can lift the front of the car slightly, causing a dangerous understeer condition at speeds over 150 mph.
The main drawback to a cowl induction hood is weather. Though some hoods are available with manual bypass valves and catch pans for water, many choose the cheaper alternatives that do not. Even in hoods that have these features, some water may work its way into the intake, particularly if there is a sudden and severe rainstorm and you don't remember to open the bypass.
Cold air intakes that draw air from the wheel-wells carry many of the same benefits of a cowl-induction set-up, without the weather worries or aerodynamic concerns. These intakes also tend to be a great deal cheaper than cowl hoods, but won't win you any cool points at the local cruise-in.
So-called "ram-air" set-ups are ducted to a forward-facing inlet in the front bumper or grill, since these areas experience much air greater pressure than the sloped windshield. With the right ducting, a proper ram-air tube can be up to 25 percent more efficient than a cowl induction hood and may actually generate as much as 2 to 3 psi of boost at very high speed.
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