I recently strolled through a junkyard with my friend Taylor when he asked: “What are these flaps on the rear corners of all these cars?” Realizing that he may not be the only one wondering, I decided to write this post about these devices, which act as vital pressure relief valves.

Normally, you don’t see these air extractors (also called exhausters or pressure relief valves) because they’re hidden under a plastic bumper cover on the outside, and under trunk trim on the inside. But valves like this exist on pretty much every car, and consist of simple plastic housings with elastomer flaps that blow open when the pressure in the cabin is sufficiently higher than outside, but that otherwise remain closed under their own weight.

One of the reasons that these exhausters exist is to relieve pressure that builds when someone closes a door, as high pressure can cause discomfort to occupants, increase door closing efforts, and even put undue strain on windows and closures, possibly causing them to unseat from their seals and make noises as air leaks out.

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On top of that, to increase passenger comfort and more optimal window defrosting, exhausters facilitate continuous, fresh circulation from the air intake of a vehicle (usually from vents at the cowl—the base of the windshield) through to the very rear of the car, and out of the flaps. This way, occupants get steady ventilation without having to deal with the noise and potential debris associated with opening a window.

The one-way valves also makes life easier for the HVAC blower motor, which has to push against the cabin’s pressure to feed air through the ducts and out of the vents in the dash. Chrysler corporation describes the importance of exhausters in a 1965 patent, writing:

Perhaps most importantly, it reduces the back pressure against which the heater blower has to work, thereby appreciably improving heater performance and efficiency. Also, in vehicles of the hardtop type, it precludes the understrained upper edges of the windows from being pushed outwardly away from their seating surfaces by excess interior pressure, thereby precluding the noise and promiscuous air flow that would attend such outward movement of the window upper edges. The selective outflow of exhauster air also has the effect of continuously purging the passenger compartment of smoky or otherwise contaminated air.

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So yeah, rubber exhausters—things you probably never see—are actually fascinating, and also a hell of a lot of fun to play with at the junkyard.