From what I've read, rear bumpers can act like a sort of parachute, scooping a bunch of air up at speed. I've seen people cut holes in the rear bumper, slice the bottom off, even a bunch of Miatas that just removed the rear bumper completely. What do the aerodynamic experts say on this? I have a 1995 Civic challenge car and cutting things is free. Is there much to be gained by trimming my rear bumper and if so, what guidelines/tips should I follow?
The biggest advantage will be at the fast end of the drag strip. At autocross speeds it MAY be a benefit, but it will be hard to quantify. As far as how much it would help on the drag pass, probably less than getting a good launch. One aspect of the competition is the magazine wants cars that look good, so make sure you do a good job trimming if that is the way you decide to go.
Trimming/slotting/holes: lay down and look up/back from the rockers. If you can see large amounts of material blocking your view out the back AND it is rolled under the most rearward part of the bumper, cut it off. This will likely mean trimming the rear bumper near the bottom edge. Try to keep the sides of the bumper as long as possible, That will help drag reduction. Holes in the top/vertical faces of the rear bumper could help, but again they will be small gains.
Awesome, thanks! I'm removing my bumper today so I can plan for this. Maybe if I can find some free material I can smooth out the underside a bit leading into the rear bumper.
So looking from underneath my car, the bottom of the rear bumper curves under and pretty much attaches to the spare tire well. Part of me wants to cut the spare tire well out and make that area flat so I can chop the bumper a little and maybe even make a basic rear diffuser. I'll add that to my ever-growing list of crap I want to do but need to learn more about first.
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