[Southcity]

Thanks for clarifying. When a tachometer is connected to the tach connector on an HEI, it's measuring the signal frequency (on/off signal) which then converts it to a voltage to drive the tachometer. It may be that these older style diagnostic tachs are incompatible with HEI, so I understand why you were asking about a filter. Filters generally help "smooth" the signal which may then make these older tachs you have work. I guess you have some options:

1.Buy a used tach that will work with HEI
2.Purchase a filter that should make the tachs you have work (Amazon actually carries some)
3.Permanently install the HEI compatible tach of your choice

Hope this is helpful