Did GM offer a tach drive HEI distributor?

[Mark E.]

I know Corvette went to an electric tach and HEI in 1975, but was a tach drive HEI available over the counter?

[Duke W.]

...not that I'm aware of, but they are available from the aftermarket... easy Web search.

Diuke

[Tom R.]

An interesting question, but then how would they do that? HEI also drives the tachometer, which incorporates a tach board integrated with other circuitry. I mean, we see electromechanical all the time, well, through C5, but how to produce and assemble in a pre-1975 vehicle? Perhaps mechanical continues to drive the tach while HEI manages spark distribution, of course, in a more highly efficient mechanism.

[Mark E.]

Tom,

That's right. For pre-1975, aftermarket HEI tach drive distributors accommodate a cable for the tach while providing an electronicly triggered ignition.

I asked the question because I want an HEI tach drive as a toy, and in my experience GM HEI distributors are much more reliable than aftermarket ones. So I wonered if GM made one, perhaps as an over the counter performance part.

[Duke W.]

The most common HEI failure is the module and since aftermarket HEIs use the same module, all of which are now made by non-GM sources, I don't see how the failure rates would be different between current aftermarket systems and an original HEI system with a current aftermarket module.

I had the original module on my Cosworth Vega's HEI fail in the eighties, and two other failures. The wires from the pickup coil to the module failed since they are continuously exercised by the vacuum advance, just like the breaker plate ground wire on the single point system. Also, the CV has an external coil and the two-wire harness to the external coil failed right where they exit the distributor housing.

A significant cause of these failures was likely the intense second order unbalanced vertical shaking force generated by inline fours that amounts to a sinusoidal +/- 15g @ 233 Hz force at 7000 revs on my CV, and it spent at least 5000 miles hot lapping Riverside and Willow Springs over a 15 year period from the early eighties to the late nineties. Properly balanced cruciform crank V8s don't have any significant first or second order shaking forces or rocking couples below fourth order, so they are a much less hostile physical environment for mechanical and electronic parts attached directly to the engine.

Duke

[Mark E.]

Thanks Duke. Good point that no new modules are made by GM. I haven't checked availability bit maybe install an NOS module and coil in a new aftermarket HEI tach drive distributor?

I'm not a big fan of retrofitting electronic ignition, BUT Corvette suppliers are selling nice looking HEI tach drives for a bit more than $100 complete with cap and rotor. Tempting.