[spuggy]

If you get a tach repaired or reworked by John Bell (jbell959 on this forum, highly recommended by many - including me - for his excellent work), he installs an updated tach driver board (from Bob Ashlock, the same as the adapter) into the tach as a matter of course (pretty sure).

My stock factory tach worked with my MoTeC without any issue, my custom tach needed a 1 KOhm pullup to 12V - with that, it worked perfectly. My wrench, a certified Motec dealer/installer/tuner, said "meh, some need a pullup, some don't".

If I understand it correctly, old tachs were part of the points/coil circuit. So expect a considerably more substantial pulse than the later ones (which expect a trigger output from the CDI instead).

And some research I did indicated that this is a very common problem for older cars/instruments, not just Porsche, when going EFI.

Many report success with wiring the coil of just about any small automotive relay you may have lying around in-line with the tach input - eg between the tach signal input and the TD terminal on the tach. The pulse the coil outputs from the collapsing EMF is apparently more like what the older instruments want to see.