jester_s1

FWIW, I think this is a really poorly designed contest. Power systems are going to be all over the place, and with such a short track differences between vehicles aren't going to make much difference. It's all about which kid's dad can get the most oomph out of his motor for that first 1/4 second without flipping the car sideways. Nevertheless, I'll offer what I can to help him win.

Given the rules as your are giving them to us, the biggest 3 blade prop that will fit on the car with the lowest pitch they make it in is the way to go. Prop style may also be a consideration. There are wide blade props that are designed to optimize static thrust. They are made for "3D" flying where planes spend a lot of time hanging relatively motionless in the air more like helicopters. I've never shopped for a 3 blade 3D type prop so I couldn't even tell you if they are made though.

To answer your questions as you've asked them: 7" prop is fine if that's the biggest prop you can fit on the car. If you are able to go bigger, then do so. Go with a 3 blade if you can since that moves more air, making the prop pull like it is bigger than it really is, although the vast majority of appropriate props for your project will be 2 blade. Don't go over 4 pitch unless you can't get one that low in the diameter you need. The lowest I've ever seen was a 3, and 3.75 is sometimes used for props designed to do 3D flying. For RPM, read the following paragraph.

To properly design this power system, start with the prop. Let's say you wind up with an APC 9x4.5 prop just for argument's sake. Since there are no amp limits and no battery capacity limits, your only RPM limit is on how many watts the motor can take before it burns up. So in choosing a motor, you start with its watt limit, so let's say that's 200 watts. Your battery is at 8v resting so it will probably give you 6.5-7v under load (I'm going to suggest you consider going to a large capacity 2 cell lipo for this. It will be 8.4v hot off the charger and won't sag as much under load). 200 /7 = 28 amps you can safely draw. Then you look at the prop specs the manufacturer gives you to find out how many amps the various different Kv ratings will draw with the prop you've chosen. So say you have a choice between a 1300kv motor and a 1500kv motor, with the 1300 drawing 24 amps with that prop and the 1500 drawing 30 amps. Go with the 1500kv motor because as I said above you can fudge a little on max watts if you're not going to run the motor wide open without adequate cooling for a long period of time. The last thing to buy then is the battery. The slower you discharge any battery the more it can maintain its voltage, so go with the largest capacity that you can. If the battery counts as part of the vehicle weight that's a major plus. A good rule of thumb for high performance setups is to not go above 10 times the battery's capacity in your max discharge unless you spend money on the really good lipo batteries. So in this example, you'd need a minimum of 2800 mah, with absolutely no penalty for going higher other than the cost.

So to summarize, choose your prop, then your motor, then your battery. While we're on the topic, use good thick wire and the highest amp rated connectors you can get. Many of us like the Deans ultra plug for connections like this. If the rules allow it, use a Deans plug as your switch rather than installing a separate one. Few switches can handle the amps your going to be pulling without dropping considerable voltage.