[Rev]

You can get the reproduction of the original "Rally Pack" for the '66 from Dallas Mustang, but it's going to be very expensive. It includes a clock and a tach that hangs off of both sides of the steering column. Unless your car is a concourse show car where originality is important, I would just get an inexpesive electronic tach from Auto Meter or Mallory for $50 or so.

It's true you can have detonation that you don't hear, especially at wide open throttle with loud pipes. Most people I know just keep advancing the spark until they hear it knocking or pinging and then retard it a couple of degrees from there.

Thunderbolt gave an excellent description of how spark advance works. Reread that post carefully and then get a book that goes into more detail about how to time the spark for best performance. HOW TO BUILD HORSEPOWER by David Vizard is an excellent book that explains a lot about high performance street engines (a little Chevy oriented), but still informative.

Rev