The alternator must have a...
The alternator must have a good ground all the way back to the battery.
Alternator output, both rated and sustained, must be measured and recorded.
The alternator must have a good ground all the way back to the battery. It must be able to carry the maximum rated alternator current output.
The alternator to solenoid wire must be large enough to carry the maximum output of the alternator to the starter solenoid or other distribution point.
The alternator must be able to supply enough energy to power all the power-consuming devices in the car. In addition, the alternator must have enough reserve output to recharge the battery. Remember that the alternator does not supply its full output at low speed.
The battery supplies the energy to start the car and deliver any energy that the alternator cannot. If, while running under caution, your electrical load is 100 amps (not an unusually high number if you have a helmet air conditioner and a heavy duty electric radiator fan) and your "140 amp" alternator is only putting out 60 amps, the battery must supply the other 40 amps.
The rated output of the alternator is the amount of power that it can produce safely for a short period of time. Sustained output is normally much lower. If you disassemble the alternator and it is blackened and looks burned, it is a sure sign that it has been overworked.
If you run a second belt off...
If you run a second belt off of the water pump for the alternator, then the water pump pulley is used to calculate the alternator drive ratio.
After cranking the engine to get oil pressure up and then starting the engine, the battery may have already lost 50 percent of its capacity. After a number of caution or parade laps, the battery may run out of juice even though the alternator is "working properly."
Pulley diameter determines alternator speed and accordingly limits alternator output. If you run one belt to operate the water pump and the alternator, then the crank pulley is part of the equation in determining alternator speed. If you run a second belt off of the water pump for the alternator, then the water pump pulley is used to calculate the alternator drive ratio. The alternator manufacturers have data showing the output at different alternator speeds. The engine builder or car owner must determine what size pulleys to run so that the alternator supplies enough power at both caution and race speeds. Care must be taken to avoid turning a stock alternator too fast since it could self-destruct.
Self-exciting (single wire) alternators typically do not start charging until about 3,500 alternator rpm. They also can draw power even when the engine is off. If the engine is going to remain off for more than a few minutes, be sure to shut off the master switch to prevent a power drain and potential damage to the alternator.
Many alternators that are operated in a "single wire" mode are not designed for such operation and merely have had a jumper connected to activate the unit. If the unit has not been properly internally modified, then an external gated sense wire will need to be installed. This will supply a differential voltage to the regulator to permit proper regulation of the output.
Some alternators have an internal fan that draws fresh air in from the rear of the unit. This internal fan is designed to work in conjunction with the external fan that is normally mounted behind the pulley. Generating electricity also generates a lot of heat that must be dissipated if the alternator is going to survive.
Tachometers and performance are tied together as tightly as a teenager and a cell phone. Many times in the past, the tach was used to show only an increase or decrease at a given point on the track or some other relative reading. Today, tach readings need to be more accurate. The engine builder or tuner wants to be sure that the rpm limit he has set is not exceeded. The driver wants to see as much rpm as possible. In practice and testing, if the tach is reading 300 rpm low, the car is running well and the engine builder is happy, then everyone is happy. But what happens for the race when you are issued an ignition panel with an rpm limiter that is "accurate?" If the limit set by the box is supposed to be 8,000 rpm and your tach was 300 rpm low, now you can only turn 7,700 rpm by your tach. Guess what? Now your times are 4/10 slower than you tested and you did not bring another gear. Get your tach checked and calibrated!