As chip, dont know if still available but years ago turbo technics had a diaphragm type vacuum two way advance unit that advanced under vacuum and retarded the ignition when on boost, when i did a escort cvh in 93, for some reason had to use a bosch type dizzy and the advance linkcage rod had to be cut to give the required amount of ignition retard. i think the old audi's and saab had this advance unit. i was on the limit of det @ 12psi.

Regards Dave R.