Windows down/windowless aero tweaks
 

I run with sanctioning bodies that require me to run with the windows down. Currently, I have no windows at all due to the fact that I'm running a pin-on door skin on one side, and a swing out door skin on the other, and there is no structure to attach windows to even if I wanted to.

That said, has anyone done any A/B windows up/down testing, or tried any aero tricks up and around the A pillar to mitigate any windows-down drag/ill effects?

I think back to my college days of driving my Pontiac Grand Am GT with the windows down in the rain - when it had those polycarbonate wind deflectors on the door, you were bone dry in the cabin at speed. But when the double-stick tape gave up the ghost and I drove without them, you were drenched with the windows down.

Watching with interest. I have seen people use spacers to create a small gap between the hard top and the chassis to reduce the parachute effect, although I don't know if the results have been quantified. We always look at wind tunnel pictures from the side of the car with flow going over and under; however, I imagine that a wind tunnel picture looking down from above a windowless Miata would look like a disaster, based on the amount of buffeting and turbulence I feel on the track.

I have a lightweight skin top and we are also supposed to run with windows down I noticed a lot of buffering with the top, removed the rear window and it helped a lot, I’ve also seen people have round cutouts in those lexan

I remember reading some results based on highway mpg along a set stretch of road. Best of the bunch with windows up was hardtop. Best with windows down was soft top with the rear unzipped. He did not test hard top with class removed...

I've played around with this a little. I first got interested in this back when my NB hardtop was actually a street car. I noticed specific speeds where there was a crap ton of buffeting in the cabin from the pressure reversions. The air would hit the B-pillar of the hard top and pour in, which causes the cabin to be high pressure.
If you just put a spoiler on the leading edge of the door along the A-pillar about an inch wide, you can redirect the air just enough to get the buffeting to stop, which would reduce pressure in the cabin. It's not allowed in NASA TT6, so I haven't investigated any further. Also another good option is to drill holes in the back window along the bottom. This has the added benefit of helping keep flow attached over the trunk lid, which usually separates mid way down the rear window. So there's the added benefit of reduced drag, and also more efficiency from the rear wang. This is also specifically called out in the rules.

I seem to remember some Spec Miata guys getting busted for shimming the Frankenstein bolts enough to have a gap at the back.

In my case (FC RX7), I have a rear hatch which has to stay put, so my options are pretty much limited to tweaking fenders and A pillars if anything.

For tricks, you'd want to make the leading edge of the opening very smooth with a sharp cutoff. Then you'd make the leading edge of the B pillar rounded. Think NASCAR style. A quarter window flush with the A-Pillar (not sunk in like a stock Miata widow) would decrease drag as well.

https://cimg8.ibsrv.net/gimg/www.mia...6662085b71.png
https://cimg2.ibsrv.net/gimg/www.mia...8202988791.png

Clearance issues may prevent me from adding a quarter window, but good call on the B pillar! That I can do easily.

Looking at Occum Racer's page, its crazy to see how much the factory hardtop "sticks" out into the air stream. The SuperMiata S1/S2 group had a discussion on adding b-pillar spats a few months or so ago, but I don't know if anyone actually tested the theory.

I would like to see testing on window rear window vents. I've see a lot of speculation on if they work and where they should be located. Some people seem to believe that once the top is "pressurized", the drag is minimal because it creates a "bubble". I could see this because air in a Miata cabin with the hardtop is pretty stagnate stock. Others say to vent the rear window. Spec Miata used to create gaps on the lower edge, but this was likely more because of they were skirting the rules and didn't want to get caught. Older race cars would vent at the top of the rear window (911's and original GT350R's for examples). If/when I swap to a lexan rear window, I was going to use some snap vents in various configurations to see what was better.

There's a post around here somewhere, I think in the DIY aero thread, where someone does some tuft testing on small, circular rear window vents. The tuft testing very clearly showed it was sucking air in, not venting it out.

If I had a hardtop with a Lexan window, I'd move the window down and create a gap at the top edge.

An interesting idea is guide vanes along the B-pillar. While this wouldn't do anything for the parachute effect, it might fill in the gap to the sides of the hardtop. This is a long paper, but they used guide vanes to reduce drag by up to 18% on a SUV.
https://www.researchgate.net/publica...al_Guide_Vanes

If the rules allowed it, it would be cool to make the guide vanes internal to the hard top. So it looks the same from the outside, but there are slots on the inside that exit at the rear. Get all that interior cockpit air working for you.

If you look at Chris Cassidy's 914 class project, configuration #29 has slots in the B pillar. This reduced drag by about 13%.
http://www.cassidy-online.com/porsch...ids/index.html