Resistance Will Be Futile

   The ballast resistor assembly in the TR8 does not appear to develop many problems, but it certainly seems to cause them. It is so complicated and inscrutable that it just begs to be hacked out of the circuit whenever the ignition system comes under suspicion. Don't be in such a hurry to simplify things; the ignition system in a TR8 is exactly that - a system. The ballast assembly was carefully designed to solve problems that cropped up in the brave new world of 1970s high-tech automotive electronics.

   The system was so closely tuned to its environment that injected and carburetted cars use different coils, ballasts and distributors that only look alike.

   1. The ballast. The resistors that are connected to the "tach" and "ECU" signal pins reduce the amount of noise radiated into the wiring harness by the coil. The values aren't critical; the higher the resistance, the quieter things are. Up to a point, of course - too much and your signal gets lost.

   The other resistors are the actual coil ballast. Back in the '70s, high current/high voltage transistors were expensive. Even a small increase in current capacity could cost a lot more. The output transistor in the Lucas distributor can't reliably handle enough current to allow complete bypassing of the ballast while cranking, so a resistor is placed in parallel instead. Now, why the EFI ballast uses three resistors (0.2, 0.2 and 1.0 ohms) instead of two (0.58 and 1.0 ohms) that give the same resistance is something I can't explain.

   2. The coil. The reason for using a different coil for the carbed and injected cars is a little less of a mystery. I figure that the ignition system for the carbed cars was designed and put into production first and later found to be inadequate for use

with EFI. One would think that the more accurate fuel control provided by EFI would make the coil's job easier instead of harder. Not so. That extra accuracy allows the EFI cars to run leaner than the carbed cars in many circumstances. A lean mixture gives better mileage and can help emissions but is harder to ignite. The spark plug gap was increased on the injected cars, perhaps causing some mis-firing, so a "hotter" coil and lower resistance ballast were specified to fix it.

   3. The distributor. Once the coil and ballast were changed, the barely adequate distributor output transistor had to be upgraded to handle it. At least, that's my theory. There's also an obvious difference in advance curves. The vacuum advance capsules are different and the plug that mates with the ballast is keyed differently to prevent mix-ups. More information about the distributor is here.


   If you are going to revamp the ignition with anything short of something like an MSD system, you can probably leave the ballast assembly and its right side wiring as it is. If you install a Mallory dual-point distributor, just leave the red/black wire disconnected. If you replace the diz with a Mallory Unilite or its guts with a Crane, Luminition or Pertronix, you can make the 12V connection to the red/black wire. If you use a Unilite distributor, I recommend you install the optional power line filter, too. If you change to an unballasted "sport" coil, just shift the coil positive wire from the far forward left side ballast terminal to the center (carb) or rear (FI) one.

If you are not replacing the distributor or its electronics, make sure the coil/ballast combination has at least as much resistance as the original. The stock coil for both carbed and injected cars has a primary winding resistance of at least 1.2 ohms. The Accel 8140 coil seems to be a popular replacement.

   If you are looking for an exact replacement for the EFI ballast, check out a Jaguar XJS. I have heard that certain model years used the same ballast as the TR8.

   One last note: Leave the ballast bolted down at all times – it helps transfer heat away. The ballast has to dissipate up to about 30 watts, so it can get pretty warm.