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.