Ignition coil & ballast resistor choice

[usagoal]

So I made a foolish mistake by leaving my ignition on and burning out my coil & pertronix unit. I replaced both parts and rewired the ignition power so it only stays on when the fuel pump is running. Anyway, I bought another pertronix unit and an Accel super stock coil #8140. I never used a ballast resistor in my ignition system as I was not 100% sure if I needed it up until now. From what I know, a ballast resist limits the current to the coil. This helps from wrecking the coil. There was some info I've read about certain coils not needing a ballast resistor depending on the coil resistance (mine is 1.4 ohms on the primary). Accel recommends a 0.8 ohm ballast resistor with this coil. A few questions still remain regarding coils, resistors, etc: What kind of current can this coil handle? I know it depends on the total resistance of the circuit (ballast + primary of coil) and the voltage my alternator is putting out at any given moment (13.5-14.2v). It makes sense that I can use a variety of ballast resistor values and depending on the coil's current/voltage needs, will dictate how well the coil operates. I just don't know what coils "like" in general. I've done all sorts of reading on this subject and everybody has a different opinion. I am looking at using a different ballast resistor than what Accel recommends. This way if it ever fails, then I can run to the local parts store and purchase one cheaper than what Accel charges. Wells only offers a handful of resistors in .5-.6ohms, 1.2 ohms, and a few more that are higher. What do you guys think I can/should do? Any options not mentioned above I can use (other than switching over to direct coil control, I want to stay with the dizzy for now). Thanks.

[roarin_mouse]

If you're using Megasquirt to control your ignition, you shouldn't need a ballast resistor. If your system is configured correctly, the coil shouldn't charge unless the engine is turning over. I'm not familiar with your setup and how the Pertronix behaves when the engine is stopped and the key is on but here are the formulae for the coil current:

Static: I = V/R. You must know the maximum allowable current for the primary to choose a ballast resistor. The "R" term should be the coil resistance + the ballast resistance. Hence; R_ballast = (12/Imax)- Rprimary. For example if your coil's primary is rated at 6 Amps and your primary resistance is 1.2 Ohms, the ballast resistor must be no less than 0.8 Ohms.

Dynamically: I = (V/(Rpri+Rballast)) (1 - e^(-t/tau). Where t is your dwell time, tau = Lpri/(Rprimary+Rballast), e = 2.718.

So, once you've selected a ballast resistor (from the static equation), you must now solve for dwell time to ensure that you put enough energy into the coil before firing.

[usagoal]

The pertonix acts just like points, but it's magnetic (similar to a hall sensor). I just grounds the coil and allows it to fire. I'm not using MS to control timing, I'm letting the distributor do that. And the big question is what current the coil is rated at. I have not find that info yet, but I'll keep looking. I've done exactly what you were saying with the static calculations for the ballast resistor value. I'm guesing if Accel requires a .8 ohm resistor, that's probably the minimum I can use. I could probably go higher but end up with less voltage to the coil. I did find a .8 ohm resistor at Tractor Supply (of all places!) so I'll see how that works. At least the store is local in case I need another one. Thanks for the reply!