Woaw, my jaw dropped to the floor when I saw so many 3D polygons rendered at a solid 60 fps, with outstanding music playing in the background and very fun game to play!
Congratulations, this is a VERY impressive entry, and it feels like a commercial game!
The game is already complete (and challenging), but I had an idea that maybe you'll like to make it a bit easier: how about making players only bounce/slow down when they hit a wall instead of exploding, unless they have a turbo activated?
That way, you'll a have nice risk/reward: you can go faster, but you risk losing precious seconds if you crash because you'll explode. And if you hit an obstacle without using a turbo, it'll be less frustrating to just being slowed down that exploding right away.
Also, wouldn't it be cool to be able to activate the turbo after the timer reaches 0, but the car isn't slowed down yet? That way, some players may be able to complete a race at the very last second (I lost a couple of time right being the finishing line with a turbo still available). Of course, if the car is stopped, triggering a turbo won't save you because it's too late.
Anyway, these are just random ideas because I spent lot of time crashing into walls :)
This would definitively be worthy of a cartridge release!