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BMW Land Barge
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It provides an alternate path for air into the engine in case your intake is submerged in water. Basically, it keeps you from accidentally killing your engine.
 

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it's not really a 2nd air filter, it's an emergency air inlet that opens when there a change in the pressure caused by the filter being submerged into water. just a little insurance to prevent water being sucked into ur engine. While this might never happen, I think better safe than sorry. It allows u to bring ur intake closer to the front where it might be more susceptible to water. I think a magazine did a review where a guy had his car with AEM bypass valve put onto a clear plexi tubing intake. Revved the car up and then put the end of the clear tubing into a bucket of water. The water sucked half way up to the bypass valve after which the valve opened and air entered from that point.
 

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Like han-han said.... when water starts coming in the filter, air is drawn instead through the AEM bypass valve...
 

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INDEED
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1,135 Posts
unless you're driving in 6 inches of water, you probably don't need it. I live in Seattle, and yes, there's lotsa rain here, and been fine without one.
 
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