I think billios has it right (though not in the sense that "more drag => more downforce"). Spoilers on aircraft wings create drag and kill lift. IMO that's exactly the case here. Top down might be a configuration with more drag and less lift (doubt it's making downforce, but same relative effect)

I do have two degrees in aerospace engineering, FWIW (not that that makes me right).

I don't agree with the driver comfort theory. For one thing, I'm more comfortable at the track with the top up (my faster primary track cars have been coupes).

I would still bet at most tracks that top up is faster, certainly true for miatae, anyway.
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I'm not buying it and I didn't gather that, that was what billios was trying to convey. This is all pure conjecture until an S2000 gets tested in a windtunnel top up and top down. The results of this "test" are dependent on the driver of the car. When we're talking about tenths of a second over a minute+ lap, the differences are on the order of 1%. Now these guys might be good, but I'm more prone to believe raw data from a calibrated wind tunnel.

You are more comfortable with the top up, so that means you will be slower with the top down, yes? Does that mean that all S2000s are or should be faster with the top up?

Personally, I don't care whether drag, or CG, or anything else is "better" with the top up or down. Being comfortable on track far outweighs a 0.1% change in CG height or a 0.01 reduction in C[sub]D[/sub]. However, it cannot be concluded from that video that all S2000s are faster with the top down, nor anything about the drag coefficient.

 

Are you saying that you don't buy the results, 0.5 seconds faster on 5/6 cars? 1/2 a second is not so small over a minute plus lap,

Of course. But when most drivers are a decent amount faster in one configuration over the other, that might be indicating something that is real.

I've worked in a wind tunnel, and that would be great to have such a test done. But without having a handy wind tunnel to test in for reasonable $$$, I find it very easy to believe that you could get less lift (effectively more downforce) with the top down, in addition to more drag.

Stock S2000 generates lift, it's like a really crappy wing. Drop the top, and it most likely becomes an even CRAPPIER wing, generating more drag and less lift. I bet...

In any case, without wind tunnel data, the only *real* differences I see are c.g. height (tiny difference) and aerodynamics (relatively bigger difference). There's pretty good data on the car having less drag with the top up. So any aero benefit must be from less lift.

Again, wind tunnel data would be nice to have of course.

I don't think that would be the case. Perhaps I should say that top-up is the configuration I'm more *used* to. I think I would be faster in the configuration that feels more confidence-inspiring on the track. A reduction in rear lift would pay double-dividends at the track, particularly for the novice, as in addition to having slightly more rear grip, you have a car that feels slightly more stable.

No. It of course will depend heavily on the track and other mods to the car.

I almost always run top-up, following the lead of the Spec Miata guys, in the interests of minimizing lap times for time trials (If weather's nice I will drop the top for open track after the TT for fun). However, given these Tsukuba results, I will try to quantify the difference between top-up and top-down at NHMS next time I take it there.

Of course not. "All S2000s" includes a lot of winged and diffusered cars, vastly different power/weight ratios, and vastly different suspension setups. It is likely highly track-dependent as well.
Others report higher trap speeds and max straightaway speeds at faster tracks with the top up, I think it's a pretty good bet that top-up is the lower-drag configuration by a decent amount. Suggesting that any aero advantage at Tsukuba must be either downforce (or, rather, reduced lift) or better aero stability.

I'm not stating anything categorically, just saying what I *think* could be the case. That is, that top-down might less rear lift on a stockish car.

Hey, lookie what I found: http://www.corvettefever.com/techart...g/viewall.html
They tested a bunch of Corvettes, including C6 coupe and convertible.
>>>As you might expect, the ragtop fared worse in the comparison. CDA for the convertible came in at 7.41 with passengers, compared to the hardtop's 6.99... The most interesting part of the comparison came when we compared the lift numbers. Although slight, the convertible actually turned in lower lift numbers.<<<
I think that was with the convertible's top *up*, though. But still, the inferior configuration for drag was superior for reduced lift in this case.
FWIW...

 

OK, I just actually looked at the results in the video.

It is interesting to note that the one outlier is the only MR car, which has its windshield well forward. The rest are classic FR 2-seat roadsters, with the windshield well aft.

Lap time advantage with top down:
AMG SLK55: 0.74 sec
S2000: 0.55 sec
Z 'vert: 0.32 sec
Z4 vert: 1.15 sec
Boxster: -0.06 sec (slower)
Crossfire: 0.87 sec

The FR cars averaged 0.73 seconds quicker with the top down. That's a significant amount over a ~1:08 lap.
The Boxster car was pretty much as quick with the top down as with it up.

The FR car with the smallest advantage with the top down is the one that has the least "cab-rearward" design (always kinda hated that about the 350Z, windshield so far forward it almost looks FF!).

Interesting...

 

Maybethetrack temp was 3 degrees warmer.

 

Maybe the track was 3 degrees cooler. Maybe there was a vortex creating a tailwind all the way around the track. Or a headwind.

May be...

 

Fap-fap-fap...

 

Actually the fastest is top down, windows up, radio blasting dub step.

But seriously, it really depends on the top speed, in AutoX I doubt it matters, top up helps only on tracks with long straights or at 25 Hours of Thunderhill IMO.

 

At the end of the front straight of Texas World Speedway, I consistently hit 129 mph indicated with the top up.
I almost exclusively track with the top up.
However during a particular track day one session I did was with the top down.
For that one session, I was not able to get into the 120s.
I have no lap time data, however.
YMMV