Here is my slowest MX5. But don't be fooled, it's not slow. My other 2 MX5's are turbo charged, this one isn't. And yes I know it's a turbo forum. There are some things that still apply to building a budget friendly track/race car that others might like.
It started out as a cheap and nasty standard silver 2003 MX5. We have the sport model only here in Australia.
I did the basic things- full service with new seals, belts, hoses, water pump, etc. B.C coilovers, big front sway bar, lightweight flywheel h.d clutch, Torsen diff. Then had a cage welded in and stripped and painted the cage and interior.
http://i65.tinypic.com/29mu39w.jpg

The first round of the state sprint series was on the following week, so I assembled the car over the next few nights. Besides throwing on a spare fastback, it's was just put together to use and test.
http://i65.tinypic.com/wrklm9.jpg

Then I threw in some good brake pads and fluid. Got the wheel alignment done, threw it back on the trailer and took it to Phillip Island. The car went really well and won its class. Very happy with the result and time considering it was just hacked together. It was quicker than all the MX5 club competitors that had an event there the week before. So it's got pace.
Stole a pic to show it on the track.
http://i65.tinypic.com/16kcppu.png

Then I pulled it back apart to paint the outside black, and put the carbon fibre doors, fastback and trunk on. Vented hood, wing, splitter, etc.
http://i67.tinypic.com/14ux5pu.jpg

I made some vac infuse carbon fibre doors. The pic shows the alloy window frame in place. And my new door "handle". You might need to zoom in to see it. I also trimmed all the excess weight from the door latch and the hinges.
http://i63.tinypic.com/16aa6ia.jpg

Video of me vs a new Porsche going into turn 1 at 122mp/h and not lifting off the gas.

Same corner, but I put the very worn tire on the left rear. So didn't grip as well.

I cut out one piece windows, painted a border in them to hide the alloy frame and stuck them in. Then drilled a hole in the window so the door can be opened from outside. Did some trimming and got the door skins to line up and bonded them on. Now the doors are solid with no flex. I will weigh them, but at a guess I would say ~ 8 pounds complete with hinges.
http://i68.tinypic.com/oh0dnq.jpg

The paint is industrial 2k. It's very tough paint. I am doing the whole outside with it as well. Doesn't give a professional finish, but it will be fine for a race car.

Painted and assembled. I am waiting on a new fastback so I can paint it black to match. But it's ready to use again.
http://i63.tinypic.com/116tdt5.jpg

I will paint a new fastback roof and trunk and that will finish basic paint job. Then I will do the stickers and stripes. Add aero, paint the other wheels. Then race the thing.
http://i67.tinypic.com/1znoaxd.jpg

I have one if the first production fastbacks on the car now. Next is to paint one black and leave it on.
http://i63.tinypic.com/28a6ae0.jpg
http://i67.tinypic.com/qslhmv.jpg
http://i67.tinypic.com/dot9u0.jpg

its a blast to drive on the streets even without a turbo. Weighing under 2000lbs probably helps. When it gets its final fastback I need to get some ducts in the back side windows and most likely one or two electric inline blowers. The Windows don't open at all.

The car is loaded up and ready to go. I think I will struggle to get a 1:36 with the RS4's and tall diff ratio. But as always will give it my best shot. I bought some alloy flexible duct so I can make it a bit cooler inside while driving. Will just poke it out the side window and point the other end at me.

http://i67.tinypic.com/wjxhd3.jpg

Cheers. Here is an on track pic.
http://i64.tinypic.com/1611gxt.jpg

Quick note on the track day from last Monday- I went up with Hankook RS4's, and a pair of old R specs just incase. I really needed 4, but I could only bring the 2, plus the other 2 are really bad. After sliding around and doing a 1:38.6 on the RS4's, I bolted the old R specs on the front. Then set up the car for understeer (Max wing angle, stiffen up the front,etc) knowing the front would have way more grip, and to try and balance out the car. It dropped 2 full seconds to a 1:36.6 with only 2 R spec tyres. Thats half a second off the outright MX5 club record. And 6th fastest on the day against some pretty wild cars. So pretty happy with that.

And a video.. I left my new camera at home, and only had this one that has a badly scratched lens.

Not sure how to fix the quality of the pic. Maybe click on it.

The side mounts for the wing are made because I am not a fan of regular wing mounts. Even with swan neck you are still disturbing air leading to the wing. Plus you have end plates byway, so why not make them bigger and use them to hold the wing? To make them is pretty simple, you just use a solid board in the right spot, and laminate a bunch of carbon fibre or fibreglass over it and down to the car. This is my new updated version, it's for a full 65" wide wing.
http://i66.tinypic.com/23le643.jpg

The front is starting to take shape.

http://i64.tinypic.com/2qtcq3b.png

I painted the bumper, uprights, wheels, skirts and a few other bits.

http://i66.tinypic.com/72qg78.jpg
http://i65.tinypic.com/2yoo3dw.jpg

I will soon see how much downforce the front has when it gets to the track. The splitter has a much larger surface area than the bumper overhang above it. And the wing is pretty big, so it will need downforce on the front to even out the aero balance. I think it will work fine. The car tends to oversteer at Phillip Island, except for one fairly high speed corner that goes over a crest. So I will be able to tell straight away if it's not working.
I have been told this design won't wok for many reasons. Another is the air will be pushed out the sides and won't stay attached down the sides of the car. Air can not stay attatched at the wheel well opening. The air is very disturbed coming out from there.
At the end of the day, lap times don't lie.

So much still to do, but the skirts are on, just reusing the old ones. Mirrors are on, uprights are on, some stickers are on. Wheels have tyres, and there on..

http://i65.tinypic.com/1418jgw.png

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I made a flat floor between the chassis rails from the front splitter all the way back to a small rear diffuser. The side skirts make what look like tunnels down each side between the chassis rail and the skirt. I have braced it all, and it seems solid enough. Even making everything out of lightweight carbon fibre or this alloy sheet, it still adds a considerable amount of weight to the car. I am hoping the drag reduction helps and overcomes the added weight.
http://i66.tinypic.com/maxn2d.png

Having the bracing in the ideal spot, going diagonally from the top is best. But it means a lot of drag and disturbed air in front of the wing. This is an experiment, and if it works, I can make everything stronger and do away with the braces. But there is a lot more time and money involved in doing it better. This is a relatively low cost set up. It actually has cost me less than $100 all up. The cloth and resin was old stuff I had spare, plywood was left over. All I had to buy was $60 worth of alloy sheet.

I haven't updated this in a while. I did the State Time Attack round. The car went well, but the aero had too much drag for a stock 1.8. It did go half a second faster, but top speeds where a lot lower. It was fun holding it flat around the high speed corners.
I went back with the smaller wing and normal front aero. But it rained all day, so didn't get to see if it wa ps an improvement.
We made a video of the new production Fastbacks for those that receive them in big boxes. Enjoy-

And a pic from a hillclimb I did a couple of weeks ago.
http://i63.tinypic.com/21wfte.jpg

The big overhang of the splitter was too flexible, so the whole car shook at high speed. Plywood is not an ideal material, it's not stiff enough. Next one will be honeycomb or expanded alloy.
The guy that did the video makes it look semi professional. I noticed it's got over 10,000 views last time I looked.