RE: Science Olympiad
I can offer a few things. First, the design of your car is blocking a significant portion of the front prop's thrust. If the rules allow you to build it wider so the two can be side by side I think that would help a bit by itself. As for the electric stuff, you want the lowest pitch with the biggest diameter you can use. That gives you more static thrust (actual push when the vehicle is not moving) therefore will accelerate the car better. You said you are limited to 9v, but how are you measuring that? A 2 cell Li-ion is going to be 7.2v at roughly mid discharge, while a 3 cell is going to be 10.8. The batteries' voltage will sag under load and so that 3 cell may very well be at 9v during the actual run, but the rules may not allow you to measure it that way since the resting voltage will be a lot higher. Are you using a regulator? Assuming that 9v rule is being enforced, you want to pick the motor that will turn the props you choose at the highest RPM without going over their watt limit (watts are volts x amps). The manufacturer will have data on that, so that part is pretty simple. Given that you are making short runs, you can get away with staying right at the limit or going a little over.
Pusher props are not necessary as on most motors you can mount the prop facing the right direction and reverse the motor by switching the wires. Obviously check with the manufacturer on that to be sure though.
As for your questions, Kv is how many rpm a given motor will turn per volt. So if you pick a 1000Kv motor and put 9v on it, you'll get 9000 rpm if the motor is running at 100% efficiency (that's assuming you are actually putting 9v on it under load). It doesn't matter what load is on the motor. It will draw whatever amps it has to to turn 9k rpm assuming it is being fed with 9v from the battery. The Kv is determined by, among other things, the turns in the motor winding. Car guys talk about turns more than plane guys do because they often used to tweak their motors by rewinding them, but Kv is a more useful term and really all you need to know. Gearboxes are sometimes used because many motors are more power efficient if they have more turns and therefore turn more RPM (higher Kv rating) but in some applications that makes the watts go to high. A gearbox reduces the load when using a big prop letting you keep the efficiency of the higher Kv but slow everything down so nothing goes poof.