Rear diffuser - INSTALLED
Thread Starter
Joined: Nov 2004
Posts: 7,353
From: Upland CA
Rear diffuser - INSTALLED
I realized that no one is gonna make anything like this for our cars, so I made one myself. A diffuser had fins on it that guides the swirling air from under the car and straightens it out thereby increasing down force and making car more stable at high speeds. Since I attached the fins to a shield, less air will hit the rear bumper, reducing the "parachute effect". It should increase gas mileage too- I have heard 1-3 mpg. Actually, the real reason I made it is cuz it looks pretty cool!!!
Anyway, I needed parts. I scoured the junkyard for suitable plastic. I found the front under tray from a BMW 325, 90's vintage, and I found a front air dam, also from a BMW, but a 2002. I used the air dam for the fins. I simply made a template using cardboard until I got the right shape. Then I traced it onto the air dam making sure it was flat areas so the fins are flat. I needed 4 fins, so I did this four times. I cut out the fins with a saw and then used a grinder to get the shape I wanted. I used a hand grinder for smaller grinds and finally hand sanded.
For the shield, I cut out a large area of the BMW undershield and trimmed it to fit. I had to notch it for the bumper brackets on each side. I used existing holes to attach, but used bolts instead of the crummy plastic things.
Then I attached the fins to the shield using small corner brackets and bolts that fit flush into the brackets. I rivetted the brackets to the fins.
I will paint it later. Here are pics:
Anyway, I needed parts. I scoured the junkyard for suitable plastic. I found the front under tray from a BMW 325, 90's vintage, and I found a front air dam, also from a BMW, but a 2002. I used the air dam for the fins. I simply made a template using cardboard until I got the right shape. Then I traced it onto the air dam making sure it was flat areas so the fins are flat. I needed 4 fins, so I did this four times. I cut out the fins with a saw and then used a grinder to get the shape I wanted. I used a hand grinder for smaller grinds and finally hand sanded.
For the shield, I cut out a large area of the BMW undershield and trimmed it to fit. I had to notch it for the bumper brackets on each side. I used existing holes to attach, but used bolts instead of the crummy plastic things.
Then I attached the fins to the shield using small corner brackets and bolts that fit flush into the brackets. I rivetted the brackets to the fins.
I will paint it later. Here are pics:
Thread Starter
Joined: Nov 2004
Posts: 7,353
From: Upland CA
Originally Posted by JSutter
props for the work, but id like to see some proof that it does something.
thats all i have to say about that. 
Senior Member
Joined: Sep 2005
Posts: 213
I’d also be curious to know how well this works. If our rear spoiler reduces wind turbulence on top of the car enough to reduce the drag coefficient from .30 to .28, it makes sense that reducing wind turbulence below the car will also have some positive affect on the wind stream. I don’t know how much air moves below the car, but this is a brilliant modification.
Originally Posted by Fastmax05
either way u made it YOURSELF... thats great... it looks great...
good work
good work
I agree 100% on this. Relative to the 3rd and 4th geners, not many 6th geners do their own work. Congrats, it came out nice and clean 
Could you take some pix of this device installed from under the car if possible?
again, GREAT WORK!
Thread Starter
Joined: Nov 2004
Posts: 7,353
From: Upland CA
Originally Posted by 95BLKMAX
Correct me if Im wrong but you mounted it by screwing it to the bottom of the bumper right? one bolt on each end?
Thread Starter
Joined: Nov 2004
Posts: 7,353
From: Upland CA
Thanks guys. I took my tint off and I am waiting to return home to see if Bats overlays look good and we approve final design.
I had a chance to check gas mileage. I got 27 mpg at an average of 78 mph for 5 hours 20 minutes. Seems very good!!!
It would be hard to tell handling as it would be effective on high speed corners. I just like the look more than anything. Its just that there may be incidental benefits like better mileage and handling. I just wouldnt be sure how I could prove any benefits.
I had a chance to check gas mileage. I got 27 mpg at an average of 78 mph for 5 hours 20 minutes. Seems very good!!!
It would be hard to tell handling as it would be effective on high speed corners. I just like the look more than anything. Its just that there may be incidental benefits like better mileage and handling. I just wouldnt be sure how I could prove any benefits.
