Aero question. Answer content.

I'll assume most of you know what a CSP Miata looks like. But for the unfamiliar you take a regular Miata and shoehorn 275 section Hoosiers into the wheel wells. Lower it as far as possible, put a splitter on the front and a 10" spoiler on the trunk. Not subtle.

So as I was standing around at out penultimate autocross of the season yesterday thinking about how to go faster (my car doesn't have the aero mods yet) I had a thought. What about the trunk?

Would it be legal/effective to have a prop rod which fixed the trunk open at say 45 degrees to provide that modest downward push you get at autocross speeds? Has this ever been tried before?

I'm no engineer and this is just a guess but it seems like the hole you would punch in the air would negate any increase in performance you would gain.

45deg seems way too much, and I cannot imagine that would be effective in any case unless you build a wrap-around part out of sheet metal that incorporated end plates and a block-off for the gaping hole that is the trunk. With that, you'd get a Kamm tail sort of look; without it, you'd have a lot of air swirling around in a low-pressure area behind you.

45degrees is probably where I would start and go to higher angles from there. The problem you will run into is the leverage is reducing as you pivot it at the hinges and the angle is increasing. The drag is not small, but at autocross speeds it should not be the limiting factor. I would try to get it closer to 60degrees is I were going to try it.
The downforce created will be substantial...

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My brain works differently than most people, but at autocross speeds my #1 priority is my driving style and technique. #2 is the suspension on the car and how it is set up. Tires are a very important part of the equation. Aero is the least of my considerations. My personal thinking is that propping the trunk up wouldn't make a bit of difference on the stopwatch, but I'd try it just to be sure.

That will produce a lot of downforce but also A LOT of drag. You could get the same downforce and just a small fraction of the drag with a big wing. A big ducktail spoiler like the one above would produce more drag than the wing but still less than the trunk being open.

Aero is very clearly prescribed in Street Prepared rules. There's a narrow range of "length" and "height" allowed and propping the trunk open violates both.

I hate to rule nazi it, but it's easier if you follow their rules.

If you're going to do your own thing, build or buy this: http://www.good-win-racing.com/Mazda-Performance-Part/61-1573.html

The issue of just using the trunk is that the air is very dirty right where your "spoiler" will begin. Having the flat right before the spoiler at least gives the air a chance to smooth out a little before hitting it.

ncjay wrote: My brain works differently than most people, but at autocross speeds my #1 priority is my driving style and technique. #2 is the suspension on the car and how it is set up. Tires are a very important part of the equation. Aero is the least of my considerations. My personal thinking is that propping the trunk up wouldn't make a bit of difference on the stopwatch, but I'd try it just to be sure.

At the top level of CSP, driving style, technique and suspension are all givens. It's the guys who pay attention to the other stuff that win. I've had a good look at a top-level CSP Miata, and there's stuff in there that seems trivial. But add up enough trivial changes and you discover a car that's faster than it should be.