Having developed a set of modifications to the then new ZZ series crate engine large cap HEI's some years ago, when the fine folks at Limeworks Street Rods in Whittier, California were installing these engines into their built from scratch cars, and seeing performance, and other issues such as over heating from a wrong ignition curve with the engines as delivered from GMPP, I developed a series of modifications to those large, coil in cap HEI's to bring the performance back to the level it should have been.
I offer a service to do those modifications to these ZZ distributors. I install a metal, positive stop to the vacuum advance that sets the total number of degrees of vacuum advance, so the vacuum advance can be connected to a full manifold vacuum source below the carburetor, and not over time the engine. This is as it should have been in the first place for those engines. This allows the mechanical advance rate to be "sped up" to the rate it should have been for those engines. When GMPP did the setup, they sourced the vacuum advance to ported vacuum, clearly, to do that, they slowed down the mechanical advance. These modifications return the mechanical advance speed to where it should be, and tailor the vacuum advance to work with full manifold vacuum, as it should.
Main shaft end play is also set, as most of these distributors have too much end play.
I have found these specifications work well not only on ZZ crate engines, but most stock to mild/moderate performance Chevrolet engines.
For more information, and pricing on this service, please either call, or send an email requesting "ZZ distributor modifications", using the firstname.lastname@example.org address. Please ask for "ZZ HEI modifications information".
A pictures and information package can also be sent on the modifications, should you wish to do them your self. These instructions and images are sent as one package, free of charge.