By on February 25, 2016

Trunk Vent on BMW, Image: Unknown

Sebastian writes:

Every time I see a car on the street without its rear bumper, I see these vents under where the bumper is supposed to go. I have an Oldsmobile Cutlass Supreme 1993 that also has these vents inside the rear doors.

What is the function of those vents?

Sajeev answers:

I was shocked at the number of forum threads discussing trunk vents: people really wanna seal them up, too! More advanced Googling netted this great article, and here’s the good part:

The flow of air from the passenger compartment to the trunk compartment is desirable for several reasons. It is desirable that there be positive air pressure within the passenger compartment to cause air to go through the trunk compartment so that the trunk compartment, which is typically neither heated or cooled, maintains a temperature closer to the selected temperature of the passenger compartment. It is desirable that the passenger compartment have a higher air pressure rate than the trunk so that fumes, moisture or odors which may enter into the trunk from either outside the vehicle or from things stored inside the trunk do not enter into the passenger compartment.

To allow for the continuous flow of air from the passenger compartment into the trunk compartment, an exhauster must be provided. An exhauster is a vent that acts as a check valve to relieve air that is delivered into the trunk compartment from the passenger compartment to the exterior of the vehicle. Additionally, the exhauster also functions to relieve pressure when the doors, trunk or cargo hatch of a vehicle are closed. The pressure relieving function of the exhauster is vital to passenger comfort and to the prevention of glass damage to the vehicle.

So yeah, these things are important. That’s all we can say … right, Best and Brightest?

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74 Comments on “Piston Slap: Truncated Thoughts on Trunk Vents...”


  • avatar
    PRNDLOL

    I’m pretty sure they’re there to prevent the glass from blowing out during the severe pressure changes experienced during tornados.

    It’s science-based.

  • avatar
    DenverMike

    They’re necessary for a smooth/easy closing of the doors and relieve pressure from airbags exploding.

  • avatar
    ReSa

    Vents also allow the awkward exhaust fumes from passengers to pass…
    https://youtu.be/epjrWjo9ZMY

  • avatar
    CoreyDL

    I have moar question.

    1) Why do some cars have them in the rear door area, and others don’t? How do those cars which don’t get by without them?

    2) What really was the touted GM “Flo-thru” ventilation, or whatever it was called in the ’60s?

    • 0 avatar
      JimZ

      they can be anywhere on a vertical surface open to the outside. My Ranger has them in the doors (the door trim has vent openings along the bottom,) other pickups have them on the rear cab wall.

      • 0 avatar
        CoreyDL

        I’ll have to check on me M to see where they are, I’m curious now.

        • 0 avatar
          This Is Dawg

          You’re referring to vents like on the D pillar here, yes?

          http://rentacarinlosangeles.com/2001%20Chevrolet%20Blazer%20LS%20Sport%20Utility%20Vehicle%20(lt.%20rear).JPG

          • 0 avatar
            CoreyDL

            Nope. The vents I mean are in the door sills on some cars, and other places apparently for other cars. Not visible from the outside.

            http://www.texas4x4.org/gallery/albums/album462/CIMG0007.sized.jpg

            Visible there.

          • 0 avatar
            This Is Dawg

            @Corey

            Oh weird. I don’t know if I’ve ever noticed vents like those. Do you know if the D pillar vents serve the same purpose? I remember always wondering what they were on a friend’s 2001 Tahoe.

          • 0 avatar
            CoreyDL

            I think the vents on that era Tahoe and the Blazer models were fake. Just trim bits to cover up panel seams. Same reason the Express vans just had a big plastic panel there above the brake lamps.

    • 0 avatar
      55_wrench

      Corey, I’ll take a stab at it.

      Air came in thru the slits in the front cowl at the base of the windshield, blew down between the firewall and the rocker panels. Several GM cars had a cable-actuated floor vent on the driver and passenger sides that I really miss..at 60, you had loads of cool air, and if your car lacked A/C (and lots of them did back in the day) it felt better than nothing at all..
      The air continued thru the rocker panels and out a rubber flap near the arch of the back wheels, the idea was to keep the rockers from rusting out.

      Of course nobody read the manual, where it said you needed to make sure the flaps were not clogged with silt & mud..

      Then GM tried in the ’70s to punch louvers in the trunk lids of their full size cars, allowing an air exit there..looked pretty silly, and the practice was discontinued by the late ’70s.

      • 0 avatar
        CoreyDL

        Thanks, that makes sense. I do recall the ’70s Toronado had some rear vents up on the trunk as well.

        http://momentcar.com/images/oldsmobile-toronado-1975-1.jpg

        Unless those were lights, now I’m not sure.

  • avatar
    spamvw

    No Vents on the VW Van, makes it an art to determine what windows are open and the amount of door closing force that is needed.

  • avatar
    r129

    The original Volkswagen Beetle must not have had these vents. I recently read this in a book about bears:
    “Another bear took a fancy to Volkswagen Beetles. Volkswagens are relatively airtight when the doors are locked and the windows wound up tight, and this enterprising black bear found a unique way to get at the inevitable cache of food inside. All it had to do was jump up and down on the car’s roof until the air pressure inside burst open the doors. The bear was so adept at springing open VWs that it began to bounce on every one it could find – until it was shot.”

    • 0 avatar
      Fred

      I don’t know about that, but it took extra effort to close the door with the windows up. Even on my old Beetle with rotten rubber stripping.

    • 0 avatar
      319583076

      Jobst Brandt wrote some fascinating stuff about Porsches and Beetles (he was a Porsche engineer for 4 years in the 60’s). Here’s his take on the Beetle door seals:

      “Take for instance their claim that their car was so air tight that you had to open the window to close the door. In fact they placed their rubber seal on the leading edge of the door to make a piston of it instead of having a rubber seal at the closure point as all other cars do. Most other cars make that seal in the last moment as the latch engages. It was a design error that was not fixed. What advantage do you perceive in a car whose door cannot be closed without opening the
      window?”

      His comments on Porsche are extremely enlightening. Google his name and look for “yarchive.net” for more.

      • 0 avatar
        Ol Shel

        Is he as weird about cars as he is about bicycle wheels?

        • 0 avatar
          319583076

          It’s difficult to answer that because I don’t know what you mean by “weird”. However, I am familiar with his theory of the spoked bicycle wheel and my opinion is that he’s generally correct about stresses in the spokes but semantically, his argument is deceptive at best. Whether this is due to working in a second language or a deliberate choice, we’ll probably never know.

          In any case, being right, wrong, or weird about bicycle wheels has nothing to do with evaluating opinions regarding objective problems with VW and Porsche engineering.

    • 0 avatar
      cimarron typeR

      isn’t that why they were ablt to float? I always wanted to try that in my dads old 73 Beetle.

  • avatar
    FAHRVERGNUGEN

    Better ventilation for a tied-up Joey Baggadonuts than what you can do with a .38.

  • avatar
    JimZ

    they’re called “air extractors.” And the reasons for them have already been discussed:

    1) prevents complaints about having to slam the doors to get them fully latched, and
    2) improves the flow of the HVAC system when pulling outside air.

    it doesn’t matter whether the vehicle has a trunk or not, they can be elsewhere in the body. the F-150 has them in the rear cab wall, my Ranger has them in the doors.

    • 0 avatar
      Geekcarlover

      My Dodge Caravan had them in the doors too. Because it was a base model, I always thought they were part of a rear passenger AC system that came on higher spec versions.

  • avatar
    strafer

    I leave my fresh air intake always on, rarely use the recirc setting.
    Wouldn’t this mitigate aforementioned tornado, airbag deploying, etc or is this a one way valve?

  • avatar

    I think car companies had to start adding trunk vents when their rust protection improved and cars didn’t stopped ventilating themselves after the first winter.

    • 0 avatar
      JohnTaurus_3.0_AX4N

      Its as though people in the salt belt assume that *everyone* and *every car* also lives in the salt belt.

      22 year old Taurus (manufactured in Atlanta, Ga and sold in 1994 in Mobile, Al as a 1995 model): 0 rust.

      I get the same questions from online forums when I buy an older used car (last 4 years or so: 1991 Ford Tempo GLS coupe 5spd, 1996 Ford Aerostar XLT, 1984 Mercedes-Benz 300D Turbo, 1986 Isuzu Trooper LS 2d 4wd and of course my 95 Taurus I drive now).

      “How bad is the rust?”

      Me: “There is none.”

      “Impossible! They all rust! Every time I see one, its super rusty! Youre full of ####! Go look again, stupid!”

      Or, like when I bought my 1978 Mercury Zephyr Z-7 (miss that car!), they claim its made up/a lie because they havent seen one in 20 years, so THEY MUST ALL BE GONE EVERYWHERE. If you dont see it on your commute in Boston/Detroit/NYC, it doesnt exist anywhere, right?

      By the way, it was about 6 years ago when I had the Zephyr. We put it on a lift to service it and there was not one tiny spec of rust on it, anywhere. We examined it from bumper to bumper, top to bottom. The car had almost 180k and lived its entire life in Washington state. I paid a whole $100 for it and had it a couple years until I relocated and couldnt take it.

      • 0 avatar
        krhodes1

        I had a 1979 Peugeot 504D that had not one single-solitary spec of rust on it. Because until I bought it, it had never been farther than 100 miles from Palm Springs, California. Doesn’t mean that a Peugeot 504 is not one of the all-time world-champion rustbuckets.

    • 0 avatar
      This Is Dawg

      Lol

  • avatar
    qfrog

    I’ve had those vents get old and stale and remain open when the edges of the rubber flaps curl up a bit. The result was a smell of exhaust in the car. I found this when changing out my rear bumper cover.

  • avatar
    Geekcarlover

    Those weren’t vents on the Oldsmobile, they were gill slits. The car you saw was a Transformer. If you had continued following it you would have seen it change into a fish and swim away.

  • avatar
    Zackman

    All this sounds good to me. However, the function they serve is necessary and quite thoughtful.

    Makes one appreciate just how complex today’s autos really are.

  • avatar
    Xeranar

    This is a moment where ‘where do you think the air goes?’ is confounding people. With the doors closed a car is fairly airtight, enough for a dog or baby to suffocate in high temps. Thus you want to have vents for pass through, also for ventilation in general. Front-facing draw vents for A/C and Heat are really bad at getting air back out of the car because of the constant forward force and the lack of reverse suction. So the air has to go somewhere and ideally it goes out the trunk.

    Though when my father drove smaller box trucks and vans he was told to crack the window to keep from popping them out because of how poorly ventilated the cabins were.

    • 0 avatar
      redav

      They don’t suffocate (asphyxiation really, because suffocation is preventing them from breathing while asphyxiation is removing/depleting available oxygen)–they die of heat stroke.

  • avatar
    Vulpine

    Strangely, not all cars, nor even a majority from what I’ve seen, have vents of these sorts. Even my JKU Jeep with a canvas top is tighter than you might expect as when both I and a passenger try to close our doors simultaneously, the overpressure tends to prevent one of the doors from fully closing. My two other cars both have similar problems; one a “hot hatch” and the other a compact pickup (Ok, mid-sized by older standards.) It’s surprising when you go to an auto show (I visit two each year if possible) and try closing the doors, only to watch as their tight seals will prevent one from closing almost every time.

    Now, the idea is not a bad one but you still have to consider the possibility of an exhaust leak into the cabin. Unless the cabin fan is blowing at least at minimum speed, the odds are high that with a trunk vent some will get in; those vent seals won’t always be tight and as they dry out will begin to leak. This may be why more modern cars don’t appear to have them any more. Even so, there needs to be a way to maintain air flow even when the AC “vent” mode is turned off. Not all of them offer that flow-through ventilation and can (and do) cause issues without that positive pressure in the cabin.

    • 0 avatar
      hf_auto

      You probably still have one, but it isn’t large enough to handle that volume of air all at once. I was responsible for PRV (what we called it) placement on a vehicle and it was surprisingly challenging. The *ideal* cubic-foot/minute flow requirements are nuts and would drive something like 2 iPad-sized vents.

      This is a big problem in something like an SUV or pickup cab because of noise intrusion targets. In a car, you stick the PRV in the trunk and the rear seats block road noise. In a truck, we now have 2 iPad-sized holes in the vehicle. That’s when we start getting into weird air channels filled with foam that is transparent to air yet blocks sound.

      In a car without a PRV, you can barely close a single door at all. Jeep probably reduced the size of your PRV since it’s relatively uncommon two have two doors close at exactly the same time. Some vehicles (I believe Audi) will actually open the HVAC fresh air intake when you open a door to act as a makeshift/supplementary PRV as well. Really smart design that mitigates the packaging problems of these parts.

  • avatar
    Drzhivago138

    John Deere’s ComfortGard cab on the 6000, 7000, 6000 TEN and 7000 TEN series is so well-sealed that if the hinge on the side vent windows is loose at all, slamming the door will make the side windows pop open.

    • 0 avatar
      ihbase

      Yeah… or it just blows back through the fresh air filter. So not really that big of a deal and certainly not unique to ag cabs.

      The 7000 Series and 7X10 Series cabs and HVAC systems leave a lot to be desired when compared to the 8000 Series tractors. At least the 7x2x cabs were an improvement.

      -Mike

      • 0 avatar
        Drzhivago138

        Definitely not unique to ag cabs, but definitely notable, because the SoundGard cabs before that were the first insulated cab on a wide-use tractor, IIRC. Before that, tractor cabs were uninsulated metal boxes sitting on the fenders. We came a long way in less than 25 years.

        • 0 avatar
          ihbase

          That is factually untrue. Sound Guard was not the first insulated cab in the ag market. It wasn’t even the first insulated cab used by Deere & Co.

          -Mike

          • 0 avatar
            Drzhivago138

            As I said, “IIRC.” And I didn’t. In the interest of learning new information, what was?

            Perhaps instead of “insulated,” I should have said “integrated.” The first integrated cab from Deere (it was often called the Sound-Gard body, since it wasn’t just a cab mounted on the platform).

        • 0 avatar
          Lou_BC

          Drzhivago138 – My dad used to say that heating and cooling the cab involved reversing the radiator fan. In winter you would want to have air drawn through the rad and blown towards the machine operator. In summer the fan was reversed and blew air forward.

  • avatar
    kurtamaxxguy

    vents like ones shown help the HVAC send air into the vehicle. Without the vents, air has to leak out from whatever chassis or body openings are available.

    Classic examples of bad ventilation are early VW Beetles. Yes, they could float for awhile, but were prone to fog-ups and slow heating in winter. The Super Beetle had tiny body vents behind the rear windows as well as forced ventilation: both helped better ventilate the Beetle and reduced winter fogging

  • avatar
    Irvingklaws

    I suspect my ’10 Golf must have them. Somehow mice have been getting into the cabin with all the doors/windows closed.

    • 0 avatar
      This Is Dawg

      A quick Googling tells me your vents are on either side of the hatch. Check out the seventh post on this forum:

      http://www.golfmkv.com/forums/showthread.php?t=148492

  • avatar
    anomaly149

    Almost every car has an air extractor, either in the trunk, in the side door, or somewhere else. Otherwise you can’t close the doors. (The Corvette even has these cool electrically controlled ones IIRC) if you have a remotely modern car, there’s at least one of these buried somewhere.

  • avatar
    shaker

    Hm. More junk in the trunk than I woulda thunk. Thanks for the info.

  • avatar
    Sigivald

    “Glad your trunk isn’t damp from condensation? Thank the vents.”

  • avatar
    Numbers_Matching

    Hey – nice e39 M5 – I’d take it even without the rear bumper.

  • avatar
    hawox

    some years ago fiat was forced to pay damages to some lancia dedra owners.
    trunk vents were located above the tail pipe, the evidence was that exaust gasses were sucked in the compartment.
    i don’t know it properly but some cancer cases were proved caused by exposition to the dedra’s exaust

  • avatar
    415s30

    I know on the early 240Z they changed the vents around because people’s ears would pop, and the rear ones also sucked in exhaust. I would think pressure and the flow of air in all driving conditions with the windows up or down etc.. would make pretty significant changes.


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