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? Posted via the PriusChat mobile app.
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: Sienna: Corolla:
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. Posted via the PriusChat mobile app.
Thinking of glossing them up, adding reflective tape, paint blue or paint red. Posted via the PriusChat mobile app.
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.
