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Rear bumper diffusers

Discussion in 'Prime Accessories and Modifications' started by Sezy, Feb 11, 2020.

  1. Sezy

    Sezy Active Member

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    Been toying with different universal rear fin diffusers and none fit perfect. First were too skinny and hard to notice. Second a 7 fin 34 in piece fits in the middle but leaves the sides looking like something is missing. Next is a large fin 4 pack but has a curve & will take cutting to fit flush. Tried two 5 find which looked good but would need trimming & too close in the middle. Anything else out there to try besides fiberglass a custom thing? 20200204_163205.jpeg 20200130_171314.jpeg 20200130_171629.jpeg 20200208_144255.jpeg 20200204_163328.jpeg 20200208_144342.jpeg 20200210_220956.jpeg

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  2. Jon Bloom

    Jon Bloom Member

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    Educate me: What is the purpose of a rear bumper diffuser?
     
  3. Vman455

    Vman455 Senior Member

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    First we need to clear up the terminology: what the OP is trying to fit to his car are vanes, not a diffuser. A diffuser is simply a flat panel angled so that it effectively forms a divergent duct with the ground at the rear of the car. Air velocity through a divergent duct slows as the flow regains pressure; higher pressure under the very rear of the car reduces lift acting on the forward part of the car and reduces drag as the now higher-pressure air exiting from under the car increases pressure in the wake. The Prime, as in preceding generations of Prius, already has a rear diffuser; for Gen 4, Toyota got smart and repositioned the muffler to function as a diffuser where earlier generations used plastic panels.

    Where vanes are helpful is in changing the direction of the airflow. Typically, under a car the airflow spreads out and exits at an angle of up to 15 degrees or so to the direction of travel. If you look under any 3rd or 4th gen Prius (or the current Camry, Corolla, Sienna, etc.) you can see two (or more) vanes already under the car, in the center part of the diffuser or further back, like in the current Prius and Prime; these straighten out the airflow a little so stability is improved. Extra vanes at the back of the diffuser could help to increase that effect, but they would need to be sized, shaped, and positioned correctly. I suspect the OP is going for a visual modification, in which case that doesn't really matter.

    Here are the vanes on the Camry:
    [​IMG]

    Sienna:
    [​IMG]

    Corolla:
    [​IMG]
     
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  4. Sezy

    Sezy Active Member

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    Took my largest ebay fins to a bench sander to make them flat. Filled with foam and shaved. With a helper I used L brackets to screw into the fins then bumper. Wanted them spaced out more but the exhaust cutout did not allow for more space without having the L brackets more visible. Will need to add more pictures in the sun. Snapchat-397103679.jpeg 20200225_182706.jpeg 20200301_180147.jpeg 20200301_180133.jpeg

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  5. Sezy

    Sezy Active Member

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    Thinking of glossing them up, adding reflective tape, paint blue or paint red. 20200302_111932.jpeg 20200303_133840.jpeg

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  6. Kenny94945

    Kenny94945 Active Member

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    FWIW My vote: I would not paint or add tape.

    I for one, would like to know if you see a fuel mileage increase (might be minuscule, so may not be noticeable).
    or any performance increase if tested - g meter, straight line downforce, all the good stuff a diffuser is to offer.

    As an appearance modification, appears as a clean install with your bench modifications ! Awesome.
     
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