If your car has an automatic choke, disable it. When your car gets hit, the g forces will cause an automatic choke to slam shut, flood the engine, and stall. Then you have to crank and crank and crank to get it going again. I like to install a manual choke, but you can just wire the choke to the full open position if you'd prefer. The car will run rough for a couple of minutes while it warms up, but by the time the derby starts, it should be okay.
Transmissions
If your car has an automatic transmission, disconnect the entire shifter linkage. After a few hits the linkage is sure to fly apart, leaving you without the ability to choose gears. It's a simple task to cut a hole in the floor just to the right of the gas pedal and bolt a 1/2 inch pipe onto the transmission selector. You'll have enough travel in the pipe to get Drive (pipe forward, hitting the dash), Neutral (pipe middle) and Reverse (pipe hitting the seat).
All automatic transmissions cool their oil by using a tank inside the radiator. The steel lines used to get the fluid to the radiator are easily kinked or broken, so you'll want to replace a 2 foot section of each steel line with neoprene gasoline hose to allow for movement of the radiator. Note that this fluid is under fairly high pressure- you can't use plain rubber hose or else it will burst. An even better solution, which I'm going to try this year, is to make your own cooler. A friend of mine donated the complete heater core and blower assembly from a '77 Honda Civic, which I'll bolt onto the back floor and run the transmission fluid through. This way I won't have to worry about having my radiator torn off.
Radiators
Some people will tell you that a radiator isn't necessary in a derby, but I disagree. Even though Chevys are renowned for their ability to run hot, why tempt fate? If you do remove the radiator, be sure to keep the heater core and run a live wire to the heater fan. You'd be surprised how much engine heat one of those suckers can dissipate. Should you decide to keep the radiator, never use a solid fan. Another lesson found out the hard way: if the radiator gets shoved back into the fan, it will snuff the engine. There's really no need to use a radiator fan in the derby anyway, since the sheer volume of water in the cooling system will absorb enough heat to run your car for 20 minutes or more.
Be sure to replace the factory fasteners on the radiator with baling wire or cable ties to hold it in place. This gives the radiator the freedom to move around a little bit and not break.
Two more cooling tips: 1) use a 50/50 mix of antifreeze instead of straight water, since it boils at a significantly higher temperature than plain water, and 2) turn the radiator cap back to its first position so that the cooling system isn't running under pressure, and is thus less likely to blow a hose.
Distributors
If using a small block Chevy or Chrysler, the distributor is located at the very back of the motor right next to the firewall. A couple of good hits in the derby and the engine will shove back far enough for the distributor to hit the firewall and break or get knocked off. At the very least, remove the distributor and bang in that part of the firewall with a sledge hammer before the race. If you're feeling energetic, cut the whole area right out with a torch or chisel.
As mentioned earlier, power is not a big deal. Thus, it won't hurt to retard the spark timing a little bit by turning the distributor clockwise, maybe by about 10 degrees. This has two benefits: the engine will run a smidgeon cooler, and it will be a lot easier to start when hot.
To protect the distributor and wires from getting wet and stalling the engine, I coat everything in sight with a product called "Ignition Protector" (for those of you who live in Canada, get it at Canadian Tire; if you live elsewhere try your local auto parts store). It comes in an aerosol can and dries to form a protective layer after a few minutes. Apply three to four very liberal coats. Don't confuse this product with plain old silicone lubricant, which is something entirely different.
Alternators
Some people doubt whether or not an alternator provides much benefit during a derby, but to me there's no question. You'll have to start your car many times during the course of a derby, often with the engine really hot and/or flooded, and this takes a lot of power. Trust me, you'll be thankful for all the amps you can get your hands on. Plus, there are other reasons to leave the alternator hooked up:
Since you have already removed all OEM wiring, you must learn to wire the alternator from scratch.
Batteries
This is one area in which you don't want to skimp. Get the biggest battery the rules will allow and make sure it can hold a charge. There are few things more frustrating than losing because you didn't have enough battery to start a hot engine.
Most derbies make you relocate the battery from under the hood to the front passenger floor. I'm not entirely convinced that moving 50 pounds of explosive sulfuric acid closer to the driver is a safety advantage, but I don't make the rules. Be sure to bolt it down very securely and cover it with a thick rubber material such as a mud flap or tractor tire inner tube.
Differentials
Most people recommend welding the spider gears in a differential so that both wheels are locked together. (In a typical "open" differential each wheel receives the same amount of torque, causing the wheel with the least amount of traction to spin.) I've never done this, mainly because I don't have the talent or patience to do it right. But for some reason, cars that have their spiders welded tend to exhibit an awful lot of axle tramp when the power is applied. Sometimes these cars will even get stuck all by themselves, which is what this procedure is supposed to prevent. On balance, however, welding is probably still better than not welding, but I wouldn't sweat it either way.
Next: Preparing the Body
