just got 1989 iroc running but tach reads way off.With scan tool hooked up factory tach reads 1500 rpm and scan tool reads 800 rpm.The only thing i added to ignition is MSD blaster coil.Thanks in advance

 

From what I have read about the blaster coils some of them need a tach adaptor for the tach to read correctly there is also something called a tach signal spliter and it prevents the tach from getting two impulse signals which sounds like what may be happening since the reading is about double I would check on both hope this gets you on the right path