Okay, so I had some odd aerodynamic behavior with my S after looping it at Road America and ripping off my rear bumper cover and taking off my trunk decklid & mini-wing.

First off, here is a picture of the car for the purposes of knowing the hood and wing I'm running. Ignore the rest of the stuff (rollbar, sidestrakes, wheels & tires) because that is not what I was running. I'm not sure what they are exactly, but I think they're both Seibon dry carbon vented hood and wing of questionable value.

pic of car before I bought it.

So I am comparing it to my previous S2K at Road America but with stock hood, decklid and front clip (same suspension, KWv3 on both).

My original car got a little light at the rear at the end of the straights and wasn't as stable (I'd get a double-rebound) as I'd like in the kink.

This car felt slightly less light in the rear at speed, and was more stable in the kink, but not some huge difference.

Then I looped it and crashed the rear into a tire wall, tearing off the rear bumper and deranging the trunk a bit such that I had to remove it, however the car was mechanically fine and square.

I went out 2 more times after losing the rear bumper, decklid and wing; once with the top up and once with it down.

The strange thing is that with the top up, the rear was VERY light, which I pretty much expected having both lost my wing (minor) and having the trunk area completely open (I figured this would have a bigger impact).

However the run with the top down, figuring I'd get more air just passing through the cabin and into the trunk more easily, the car felt MORE stable than it did with even the decklid and little wing.

So I'm wondering, have I stumbled upon some sort of ghetto diffuser-ish sort of behavior? As I look at it, as this is a track day only car, I could forsee taking it a step further and cutting out the toolkit tub and fabricating some directional vanes to let air expand from under the car and up through the trunk hole. I know there is a bunch of turbulent air, and I don't really have a measure of what this did to my top speed, but still....

Thoughts? Discuss.....

-Steve

 

I definitely find this interesting, and have a couple of random questions / thoughts.

First of all, with the top up / top down situation, you have the soft top in this car? Do you also have the trunk lining and plastic soft top tray in? I'm wondering what the effect would be if you had NO top, NO plastic tray, and NO trunk lining, thus creating a very large opening from behind the factory interior pieces to the trunk area, right out the normal trunk opening. Going a step further, what if you also had the interior bits behind the seats out?

Also, if you did want to cut out the toolkit tub, I would imagine you could always easily replace it by welding either it back in, or just a flat piece of whatever (steel, alu, CF, etc.), so that might be something worth experimenting with.

Very fascinating indeed, though.

 

To answer your question, I have almost no interior pieces in save carpeting, dash, and the plastic around the interior of the windshield.

I have race seats, no interior behind the seats, the OEM roll hoops are not there (Hard Dog Bar), and nothing is in the trunk.

I do have a soft top in place, so it was in the way a bit, but otherwise I did indeed have a gaping pass-through of air from the passenger compartment through and out the trunk opening.

I also find this fascinating, and your right that anything I did to the big useless space for the toolkit could be un-done. But this is (definitely now) a track only car, so I really don't care about that unless whatever i do turns out to have a negative effect.

I'm now just wondering what the next step would be. It's not realistic to make a CFD model with any accuracy to this situation, so everything I did would be very trial-and-error, though I'm sure there are smart and experienced folk on this forum who have some good ideas and a good methodical way to go about it.

I also wonder how this would play with a BAW if I chose to add one.

 

K, having that gaping pass-through definitely makes your findings make more sense to me.

A potential next step (and this may be more work than its worth) would be to create some sort of fiberglass inner "shell" for that entire passage, from the interior all the way out to the trunk opening, in an effort to smooth out the flow through there. It's also possible that'd be completely useless.

Probably the easiest next step (since your interior is gutted so much already) would be to see how the effect differed (if any) with removing the soft top, too.

 

you haven't done too many soft-top removals, have you

I doubt anything to smooth the air out in that area would have any valuable effect as the cabin with the top down is already a mess of turbulent air.

I was thinking more of either:

1. creating a 'ramp' to block off the back of the trunk by the tail lights, directing the airflow that is passing through more easily and quickly helping to create a high pressure area in the trunk for [potentially] some real downforce.

2. Opening up the vertical, forward part of the toolkit tub and similarly directing it up and out from underneath the car, allowing for expansion of the under-car air at the rear which would result in a lowering the pressure under the rear of the car, giving it a sort of diffuser-ish effect.

They could both the done and work together in concert.

I' just wondering if anybody out there has done anything like this, or have the 'real racers' focus been on keeping the exterior airflow as smooth as can be (and perhaps needing the trunk space for a fuel cell?).

 

I've done two soft-top swaps, but I've never driven / been in a car without one and no CR tonneau cover.

I'm not sure I have any more thoughts, but I'd definitely like to see where this goes.

 

It certainly makes sense, as the cooler air slams in it's going to push against, and down. Forcing the rear down a little bit, as well as simulating weight (without actually adding weight).

Food for thought: I wonder what the effect would be of slitting the trunk in a way that it still latches? This would cause a diffuser like effect, but actually using the body/chassis as its own diffuser. i.e:

 

Interesting thoughts.

I'm going with more of an aerodynamic principle / Bernoulli effect concept of high & low pressure air, but the end result is similar.

I think that with the top up, the air in the trunk stayed pretty stationary or [very] low speed w/respect to the high speed air passing over the top and underneath the car. The low speed air will be a low pressure area, and allow the high pressure air under the rear of the car to cause lift.

I like your idea of venting the hood in concert with the 'ghetto diffuser' I was describing. It is certainly a cleaner solution for a variety of reasons. I may even choose to actually true-up my rear bodywork based on that (I was thinking of leaving it as-is since this is just a track dog) and play around with the concept with a spare trunk.

 

[QUOTE=whiteflash,Oct 24 2010, 09:48 PM]I wonder what the effect would be of slitting the trunk in a way that it still latches?

 

Well I think there is already plenty of airflow underneath the car looking for an easy escape, hence the 'ghetto diffuser' moniker.

Also the discussion has thus far focused on the use case with the top down (or off), so there is plenty of airflow from the cabin, even though a more proper and tidy solution would have a top in place.