I would think the primary leed would go to the dis depending on what type of tach your useing.On BB chevy motors we run down the track 1/4 mile we degree the tach with an optical eye yanked out from a corvette installed on the harmonic balancer so the ignition system is self contained.
If you have no way to install a tach on that system a optical eye from a coprvette will work for a tach but you have to degree it as you would degree a cam for true Rpm.When we have 800 to 1500 hp a ignition controlled tach is useless and untrue as most ignition controll tachs really are at any hp.We use a optical eye from a vette but I know the jegs and summit sells a system that is non ignition system also and equivelant.I would think though there must be a ignition system plug.I am just getting into buicks and tiny motors so I have no idea what your looking at.I can not even find the battery in my car yet(have not spent the time) and I think I stumped the techs with my tranny change on this site but yes you can hook up your tach no matter what.The way I suggest is for real rpm regulation but your not going to see those false high numbers to impresss yourself and others-You will see the true value from the crank.
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