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The rear bumper question could be tested using a magnehelic differential gauge. They basically measure the difference in air pressure between two points by using a couple tubes. They're $100 on Amazon. Maybe I'll pick one up before I start on my undertray/diffuser. You'd put one tube directly inside the bumper and one directly outside it. If the outside spot has lower pressure, you know the rear bumper is creating drag and/or lift. You could also use the gauge to establish baseline pressures between the top of the trunk and the bottom of the rear end. Ideally you'd have reduced pressure underneath the diffuser compared to the same location when not running it.
It's not fully A/B scientific, but I commuted my daily driver Accord without its rear bumper cover for a while after a mild rear-end accident, and my fuel economy went up 3-7%, sustained across several tanks of fuel. It dropped again when I replaced the cover. Those were measured using a Scangauge (I like to hypermile the Accord for fun). The bumper cover on the Accord has a similar parachute profile, but not as extreme as it is in the S2000. The speeds were also a lot lower than track speeds, so any observed effect was less than it would be at track speeds.
It might be the lemming effect, but in California all the Miata guys are running without rear bumpers now.
The phenomenon with the rear bumper may be similar to pickup trucks. Driving with the tailgate down may intuitively seem more aerodynamic, but it's been proven in testing to be less so than with the tailgate closed, which maintains a high pressure zone in the bed that forms an aerodynamic "roof" over the bed.
Agreed.
I was going to mention the same thing.
I think that once the "pocket" of air forms in the space where the rear bumper etc. is located, it prevents "new" air from flowing into that zone and catching. Instead, that pocket of air becomes part of the car, and the rest of the air flowing under the car flows under that pocket as if it were part of the car.
The pickup truck tailgate finding was a surprise to me but proves that aerodynamics can be counter intuitive. The tail gate is acting like a spoiler by trapping air in front of it and reducing the low pressure behind the cab.
These pictures show the velocities and pressure distribution for different diffuser angles. Key is that the low pressure is created before the diffuser, not under the diffuser.
Unfortunately, in front of the diffuser is a mess with rear subframe, exhaust, differential, etc. Things that radiate heat, things that need cooling, and things that need maintenance.
Password:JDM does have a complete underbody. $3000 plus shipping. It looks like it might reduce drag and create downforce, but I couldn't find anyone who had tested it. I did find an installation article: Honda S2000 Diffuser - Honda Tuning Magazine
It is clear from a quick glance, this is a complete underbody that is trying to provide a smooth airflow through the entire underbody. Not sure the effect the blister for the exhaust has on the airflow. It seems to have a NACA duct to cool the differential.
Of the diffusers, if one works, this or something like it would be it. It would create downforce, reduce drag, and improve the effectiveness of the front splitter, in turn allowing more rear wing and more overall downforce. However, $3000 plus shipping and a non-trivial installation that screams for a lift.
Yes, you would think "having no bumper at all may be better then having the stock one in place" and "Examining the stock bumper cover, it has a large area behind the rear of the car with large openings underneath and behind the rear wheels to trap air, and without closing it off aka some form of under tray/diffuser, its an open parachute" but that's not true. Like I said the car felt exactly the same with or without the rear bumper cover, top speed, lap time and high speed handling where aerodynamics are most important. So the bumper doesn't "act as a parachute." I'm just trying to save everyone some effort on their track car. If you like how the bumper vents look then cool, do it. And the raw aluminum diffuser posted above looks very cool too.
x2 and having the concerns about trapping heat around the diff / fuel tank
Last edited by miamirice; Jan 17, 2017 at 03:04 AM.
