I have an 87 SC and just got a 145 Speedo out of a Z28 of the same year. I know there is a port on the box for the tach wire to go in at, and someone else told me that there may be a resistor on the firewall where the old tach went into the coil. My question is can I just run a straight wire (of course with a fuseable link or something for protection) from the back of the cluster to the box or do i have to use factory wiring.

 

Click here and read MSD's instructions for GM ignitions:
http://msdignition.com/downld1.htm

The tach port on the MSD is for an aftermarket or MSD tachometer, not your factory tach in the dash. If you hook up the unit according to their directions, your dash tach should work as normal. I bought their harness and everything is great.

A search of the Electronics or Tech/General forum might reveal more topics on this subject.

 

The tach terminal on the box is for factory tachs as well.