For years, the debate has raged on short tracks throughout the country-to alternate or not. The alternator itself has undergone some changes. Of course, race cars have changed. So should you use an alternator? There is not a quick and easy answer.
The alternator is only one part of the electrical system in a race car. This system begins with the battery. Batteries come in sizes best judged by cold cranking amp (CCA) capacity. Only secondly should batteries be judged by their physical size. A battery of about 600 CCA should start your Street Stock. There is a catch to something this easy. If your 12-volt battery has the capacity to start your engine, the question becomes this: Will it do it all night and light the fire in your ignition, too?
There are several options. One can be a larger, heavier, higher CCA battery. This can have the capacity to still properly fire your ignition at the end of the night. Of course, that depends on how many times you start the engine and how many other electrical items are drawing on the battery. An important item about your battery is its reserve capacity. This is in the dealer's book. It may or may not be on the battery. This will tell you how quickly the voltage will bounce back after the starting load stops. Knowing this information can help you get your dollar's worth in batteries.
The second thing to consider is whether or not to use an alternator. I have raced both with and without a charging system. There can be some compromises to either.
Let's get back to the battery for a moment. I'll pass along some information from an interview I had with Gale Kimbrough, technical services manager for Interstate Batteries. Your battery should check to be at least 12.00 volts on an accurate digital voltmeter after the night's racing. He considers a battery to be fully charged at a level of 12.66 volts to 12.75 volts. A battery at 11.90 volts at no load is considered to be a fully discharged battery. Remember, this is no load.
Obviously, you want to have adequate voltage to the ignition at the end of the night's feature race. One way to achieve this is with an overly large battery. It will have the reserve capacity to start the engine and then come right back to the necessary voltage for ignition. The downside is weight (unless you need it). If you are using battery weight for assisting the weight distribution, you will need to mount the battery to the rear. This means long cables of sufficient size. I like at least a No. 4 welding cable. Buy it from a welding supply for about $1.00 per foot. Be sure to tie it down properly, because you don't want something heavy like this flopping around in the car with you.
Now, enter the alternator. With an alternator, you can use a smaller, lighter battery. A small battery, such as the Performance Distributors DynaBatt, weighs only 11 pounds, yet it will crank a high-compression race engine. However, it won't do this many times unless there is an alternator to begin the recharging process. It is the alternator that keeps a high enough charge in the battery to properly fire the ignition. Some electronic ignitions require as much as 10.5 volts to operate properly. A 12-volt battery can easily drop below this point after repeated starter use.
I spoke with Marvin Grebow at Pertronix about ignition voltage. He said the HEI will run at about 10 volts, but substantially more voltage is required for best performance.
Alternators come in several sizes-big, small, and in between. A fullsize GM Delco one-wire alternator weighs 13 pounds. Most of these will have an output of 65 amps at 13.5 volts. A smaller unit, such as the Performance Distributors alternator, weighs 6 pounds with an output of 50 amps. Either of these are of sufficient size to keep the battery charged while racing. An electronic ignition will use somewhere between 5 and 10 amps. If you are using a good electric fan, such as the Flex-a-lite 3300, it will take 18 amps to pull it. The Flex-a-lite puts out 3,300 cfm at 0 static pressure. Even if you are using radios, the 50-amp alternator should be sufficient.
Another thing to pay attention to with the use of an alternator is the rpm. An alternator turning slowly doesn't generate any power-or does it? It won't generate any power until the alternator reaches the rpm of excitement. In the Delco one-wire unit this speed is 6,800 rpm. This means if you use a pulley combination where the engine speed doesn't drive the alternator to 6,800 rpm, it won't charge. Did you ever wonder why GM put such large pulleys on the crankshaft? All is not that bad. Once the alternator reaches 6,800, it can be slowed and the alternator will still charge (even at idle). Therefore, if you have an engine turning 6,500 rpm with one-to-one ratio pulleys on your alternator, you now know why the battery isn't getting charged during race night. Just remember, we are talking about alternator rpm, not engine rpm.
In the case of the six-pound Performance Distributors alternator, the excitement rpm is only 4,900. This means it can be turned slower. This can reduce belt friction. Use one of these to reduce weight on the front of your race car. By the way, alternators usually have a 12,000-rpm limit. Again, we are speaking of alternator rpm.
Alternators usually have an output of 14.4 volts, which provides plenty of voltage for an electronic ignition.
If you are using the battery only for starting, either the DynaBatt or a conventional battery of about 600 CCA will do the job (with a weekly charge). No, I didn't forget about the ignition. When an alternator is not used by the end of the night's racing, the ignition can get shortchanged. The cure, in this case, is an 18-volt battery such as the VIP. The 18-volt is used only for the ignition. Many ignitions, such as the GM HEI units, do their best work at something over 12 volts. Even if an 18-volt battery is run all night, it will still have more than enough voltage to ensure that the ignition will not suffer.
ConclusionSeparation of battery sources, one for ignition and one for starting duties, can ensure adequate voltage for one night of racing without an alternator. Battery voltage should be checked with a digital voltmeter. If it discharges too fast when sitting idle, it may not be reliable on the race car. This problem can easily be misdiagnosed as a starter problem. A larger battery also ensures against low voltage. Do this if you need ballast weight. Still, you will get only about 12 volts to the ignition. An alternator providing 14.4 volts or a VIP battery transmitting 18 volts offers the voltage necessary for top performance.
Use an alternator when extra current drain on the electrical system (via electric fan, radios, and so on) is expected. Using an alternator allows the use of a small, light battery such as the DynaBatt. Be aware of the pulley ratios that come with an alternator.
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Watch That BatteryBatteries can be sneaky. A charged battery can show a full charge when checked, but batteries usually fail slowly and lose the capacity to hold a charge for a period of time. You need to have access to an accurate digital DC voltmeter. Check the battery every day for several days. A good battery will lose very little charge, maybe from a full charge of 12.75 to 12.73 in a week. A battery that is going away might drop to 12.25 in the same period of time. Although this battery is not discharged at this point, it has lost much of its capacity and is dying. This is the same as having a bucket with a cup full of water in it. When you pour it on your best friend, he will get wet-but not as wet as if the bucket were full. How mad he gets is a measure of how good a friend he is. Many starters have been replaced when the problem was a battery that was not able to take or hold enough charge. For dependability, if a battery loses more than 2 percent of its charge in a week, I would not trust it. The same battery might last a while longer in your daily driver because it is being charged every day.-Sleepy Gomez