In the interest of reducing weight and adding lightness I was thinking about trimming the rear bumper on my integra similar to the miata I found a picture of. I'm not looking to gain any style points here, and I was wondering how functional this modification might be. Would I be better off trying out some homebrew undertray panels instead to smooth out the air flow under the car?
If you're going to cut up a part, make sure it's not original. Once you cut, you can never go back.
You are pronbably going to make it lighter by less than a lb by cutting that much of a plastic bumper.
There are guys who have done that to various levels in an attempt to reduce the "parachute effect" of the back bumper. Do some looking around on ecomodder forums and you'll find a lot of knowledge on the subject. It will do virtually nothing for weight savings but may help you pick up a MPG or so if done right.
Trimming the bumper on a Miata seems like it should make a difference. The rear bumper appears like a huge scoop when viewed from under the car. I did the bumper chop on both the G10 and the B3 Miata and did not find any improvement in MPG....at street legal speeds. I'm sure MPG gains are not what your looking for, however it is worthy to note. Anyway, there may be some gains at higher speeds where aerodynamics make a bigger difference. If nothing else, it looks cool.
Doing so on any '78 and later Datsun z or Zx knocks 43lbs off the ass and 35lbs off the nose. But they had massive bumpers made from steel and foam, not this plastic stuff they use today...
It's not tucking the cover that saves weight, it's deleting all the stuff under it.
I don't think the airflow works for the rear bumper to act as a "sail". But it is the first step in a diffuser, and that is useful.
The rear bumper on my Miata is cut because the part I used was off a car that originally had a dual exhaust, and I didn't at the time. So it looked dumb with the extra hole.
Rear bumper removal was good for like 5-6mph on the front straight of the Roval at autoclub speedway so it does reduce high speed drag
You should drill some mad JDM tyte speedholes in your rear cover.
Trim and add a real, aerodynamically functional diffusor... that will reduce drag and get rid of lift at speed, probably netting actual downforce at the rear. Should add a front splitter to re-establish F/R aero balance. Never tried this with the Integra's I've had though...so can't say what the actual effect will be for cutting it - should not be anything detrimental though.
I'm currently in the process of converting to custom center exit exhaust with diffusor for my FR-S. Getting rid of that rear lower bumper section with aftermarket exhaust has really helped with a little more stability at the rear at speed. Should be even better once I have the custom exhaust and diffusor made. A custom diffusor for the FR-S has been made and tested which reduces rear lift by ~108lbf @ 100mph, netting ~10lbs actual downforce.
Jaynen wrote: Rear bumper removal was good for like 5-6mph on the front straight of the Roval at autoclub speedway so it does reduce high speed drag
What car?
Sean Thompsons NA 1.8 swapped Miata Challenge/now global time attack car. At the time he was testing with removing the bumper, hes cut it now as you can see.
Well there Ya go, hack your bumper off and gain some top end. 
Well since OP wasnt talking about a Miata either no guarantee it would help. Also the speeds I am talking about are well over 100mph and not really useful anywhere but a track. Which by its name (Roval) should give you some idea its a straight and half oval with a road course section in the middle.
Interesting, first I'd heard of actual tested results. Was that a pure A-B test, full bumper vs no bumper? Or where there other mods done at the same time.
That's about the same difference I'd expect to see by adding a hardtop.
Jaynen wrote: Rear bumper removal was good for like 5-6mph on the front straight of the Roval at autoclub speedway so it does reduce high speed drag
the NASCAR guys never seem to distraught over the wrecked aerodynamics of their cars if the rear bumper should happen to accidentally fall off after a minor incident with another car.. they'll spend 4 pitstops during a yellow to seal up the nose of the car- even going so far as to tape a new nose and fenders on top of the smashed up nose and fenders- before the race goes green, but they'll only do what they absolutely have to do at the rear of the car to keep the car legal, which generally only involves taping the trunklid back into place...
Anything on a vehicle that the air can't pass through on a vehicle looks like a solid to the air. Blocking off rear bumper has no effect on lower air flow unless it is part of a full belly pan. Cutting the bumper can raise lower airflow making it closer to the perfect aerodynamic model. If there is something lower than the cut out area, like an axle or a fuel tank.
The only way to find out is A to B to A testing. Probably a coast down test would be best. Making a low pressure area at the rear can create more down force, less efficient, but faster on the track.
I know some A-B testing with "speed holes" in the rear bumper has found that the holes actually increased drag on certain cars...in fact I think one of them was a Civic like the one shown above.
Keith I can ask Sean. I will get some clarification. He and Ryan are doing aero parts as Singular Motorsports so they have tested a lot of this. Ryans car runs a full DTM style Aero package (red turbo 1.6 with sharksteeth)
Oh, I know who Ryan is. I have some Singular parts on my car and we've talked about a few things. Ryan usually does some pretty good testing (other than on the modified front fenders, nobody's really checked them out too closely).
5-6 mph is a massive difference.
I just asked Sean again and he said it was a "few" at least. This was well over a year ago so I could be off but it was noticeable improvement just from removing the bumper
GameboyRMH wrote: I know some A-B testing with "speed holes" in the rear bumper has found that the holes actually increased drag on certain cars...in fact I think one of them was a Civic like the one shown above.
Usually you want air going around the car not through it. That is why upper grill blocks helps on some cars. On the other hand adding vortexes like those holes could make can fill in the back with air and actually help. It all depends on the shape of the car and where the holes are.
Someone else just did a coast-down test on cut vs uncut Miata bumper and found that the cut actually made it worse.
Looks cool, though. 
More info on this test? I dunno, pretty sure Moti from Blackbird Fabworx has also done some cuts and I would trust him regarding miata aero as well. https://fbcdn-sphotos-h-a.akamaihd.net/hphotos-ak-xfp1/t31.0-8/1655663_755999977786350_3552175467741351685_o.jpg
I dont know how to thumbnail on this board and dont want to link the big picture in line.
You'll need to log in to post.
