I was approached by a chap in Cape Town who wants to race an '87 XJS (with the HE engine - I know, not the best choice) to help him solve his ignition problems.
He did not understand the merits of the Lucas dual coil ignition system, replaced with something else (throwing away the HE system) and discovered that now he could no longer rev past 4000rpm. This is because few ignition modules will rev a v8 past 6000rpm which translates to 4000rpm on a v12.
Plan A was to find an HE ignition system but we could not locate one in time for this weekend's race;
Plan B was to use Lumenition which runs on my E-type sweetly to 6000rpm but none available locally;
Plan C was to use MSD 6A, not knowing if it would handle a V12 at 6000rpm - which he did - and now it revs well beyond 4000rpm.
The MSD requires either a 12V square wave or a magnetic pickup to trigger from and in this case he is triggering it from the HE distributor which has a magnetic pickup.
So, just for the record, an HE dissy (which will also give you vacuum advance and the benefits thereof) combined with an MSD 6A, can also be an ignition solution for a Jag V12.
Talk about the E-Type Series 3