An explanation of the ballast resistor Every coil has a characteristic called the time constant T. T is the time it takes to fill the coil windings. No matter what you do T comes out around .005 second. The equation is T=L/R where R is the resistance of the primary coil wire and L is inductance. Inductance is a measure of energy stored in a magnetic field and is related to the number of turns of primary wire. More primary wire turns increases L but R then goes up too. Increased R reduces the current you can draw. Also for same size coil, increased turns means the wire gets thinner which means even more R. In this case the coil makes less power despite higher L. Going the other way, fewer primary wire turns reduces L which is the power storage, so no free lunch here. Coil size and number of turns was settled and optimized by 1930 so messing with any parameter trades in a bad direction. If you go to a physically larger coil, R and L go up together and then you do have more energy. Some Mallory coils went this way. Rule 1: the only way to get “more spark “ is with a larger bigger heavier coil. But more spark is not the real problem. The real problem is that .005 second coil time constant needed to fill the coil. Take an 8 cylinder engine running at 6,000 RPM. The math says you need a spark every .002 second. But with the coil time constant of .005 second, the coil does not get full at that speed so the spark starts fading. What to do? If you could increase R in the equation T=L/R you decrease T and the coil would be full at high RPM. When 12 Volt systems came in, some very good engineers added a special resistor to increase R. This is the ballast resistor we know and love. With the additional R from the ballast resistor at high RPM the coil time constant decreased and the coil is full for spark needed at high RPM. There is more to the ballast resistor. It uses a special resistance wire. It is iron wire which, when cold, has low R. This is great for high RPM. At idle, each spark still only needs .005 seconds and the points are now closed far too long (.025 second). The coil current goes way too high. The solution -- our ballast resistor’s iron wire gets very hot at idle and R increases to limit coil draw. The ballast resistor is a clever little device. By controlling dwell by adding electronics (HEI or Pertronix etc ) you can shorten that dwell at idle by delaying the start of coil fill (but a ballast does same thing). But over 5,000 RPM or so —all else equal — ignition by points or by electronics, they are close to the same spark. For a while, Chrysler bypassed the ballast resistor during cranking. The thought was this gave a stronger spark when battery volts dropped during cranking. Other manufacturers did not follow suite and their cars start just fine. An even more important consideration is that electronic ignitions will lose 1 volt in the transistor switch while points will lose nothing. The energy in the spark goes as the square of the coil supply volts. When the engine is running, you have 13.6 volts in the system. For a distributor using points we have 13.6v squared = 184. Electronic with the 1 volt loss would be 12.6v which squared is 158. You get 17% more spark from points! As far as reliability, you walk when Pertronix or MSD punts. With a matchbook and screwdriver I can repair points and drive on. But points do have to be set correctly and in a tight distributor and a good capacitor. Points are fine for 20,000 miles at least after initial wear. All this was driven home one day with a 300B engine on the dyno. We swapped in a stock B distributor after the MSD failed and we got the same 380 HP right to 6000 RPM. The stock B distributor, with the higher dwell from dual points, gets the coil closer to full charge at high RPM. That is why our cars went 140 MPH. In my opinion, 85% of the problems blamed on points are the capacitor or from trying to set point gap without a dwell meter. |