You've heard the rumors/horror stories about people ruining their engines with a cold air intake and driving in heavy rain. AEMs patented Air Bypass Valve virtually eliminates the chance of hydro-lock in the event the filter of your Cold Air induction system becomes submerged in water. On most systems, the valve installs along the upper portion of the inlet pipe and shuts down induction at the filter when it becomes submerged, rerouting air through its external diaphragm to keep water out and air flowing in. Protected by Patent #6,394,128, the AEM Air Bypass Valve is CARB exempt (50 state street legal) . Keep in mind that, this item should NOT be used on forced induction (turbo or supercharged) vehicles. Virtually eliminates chance of intake system ingesting water if filter becomes submerged Installs along upper portion of AEM intake systems (or most other brands of intakes)with minor modifications (you will be required to cut your Air Intake System tube with a hacksaw or bandsaw. Instructions are included). Complete hardware and installation instructions included If you have an AEM V2 Intake System, or your vehicle is turbocharged or supercharged you should NOT use this product. If you are not sure what the diameter of your air intake system is, you can easily measure it with a ruler. The part number of this AEM Cold Air Intake Bypass Valve is 20-401S and it fits on intakes with a 2.5" diameter.