The Principle

• Engines burn fuel to make horsepower, and they need oxygen to burn fuel. Oxygen is the limiting reactant for gasoline engines -- the engine will always run out of oxygen molecules before fuel. Diesels are just the opposite; they lack a throttle plate and are almost always fuel-limited. Higher air density means more oxygen molecules per cubic foot of air that goes into the engine, and cooler air is denser than warmer air. This principle is easy to quantify by simply running a 0-to-60 time in the heat of a summer day, then run the same test again that night. Your car should be marginally quicker at night, owing exclusively to the lower intake air temperatures. Diesels will typically not explicitly benefit from a CAI because of the increased oxygen density but because they'll allow for more fuel injection without an increase in exhaust gas temperatures.
  
  

Testing the CAI

• The rule of thumb is a 1 percent increase in horsepower for every 10 degree drop in air temperature, but that's not the whole story for CAIs. Simply sucking in cooler air isn't going to make horsepower if the intake design and filter don't provide sufficient airflow for the engine. An example from Jennetty Racing's independent evaluation of several CAIs: The stock non-CAI engine produced 376 horsepower with intake air temperatures of 55 degrees F, and picked up 10.4 horsepower with the installation of a CAI that dropped intake air temps by 4 degrees. But another intake that dropped temps by 7 degrees only picked up 8.6 horsepower. So CAI system design and air filter flow are just as important as drawing in cold air.
  
  

Ram Induction

• Ram-air intakes don't get too much press these days, but the most muscle of muscle-car accessories are probably the oldest and still most effective of all CAI systems. A ram-air system ups the cold-air ante by utilizing a forward-facing inlet to shove fast-moving air from outside the car down into the engine like a supercharger. Ram-air systems come in a few different varieties, but you're probably most familiar with the old-school, forward-facing hood scoop. The problem with a scoop is that, while it does help boost horsepower somewhat at high speeds, those gains are offset by the massive aerodynamic drag they cause. Drag racers and top-speed racers are fond of removing one of the car's headlights and ducting air in through the hole. This obviously isn't practical unless you've got four headlights -- but it'll look cool, regardless.
  
  

Considerations

• Your engine's intake tube isn't some necessary evil inherent to using fuel injection -- not these days, anyway. Air doesn't flow smoothly into your engine; it actually starts and stops in accordance to pressure reversion waves or "sound" coming out of the engine. The right intake and intake resonator -- no, it isn't just a muffler -- can control these pressure waves to keep them from propagating up in the tube and restricting airflow. In practical terms, you'll want to use the shortest and fattest intake tube possible, and the largest, best-flowing air filter and air-box that will fit; very long intake tubes are a breeding ground for reversion wave pressure buildup. And don't chuck your intake resonator on the general principle that it looks like a useless plastic tumor on your intake tube. Making that mistake could cost you some noticeable power.