HEI not only needs full 12 volts, but it also needs a heavy-gauge wire from the ignition switch to the distributor. Use at least a 12 gauge wire, and 10 is better. The way the HEI system develops the 50KV+ firing voltage capability is by flowing a pretty heavy primary current through the ignition module. The heavy wire is essential to having sufficient current flow for the system to work properly. A clean, properly-gapped spark plug will fire at about 10-12 KV, so a HEI has nearly 5 times the voltage capability for good ignition. On a dyno demonstration I have cut the ground electrode completely off a few spark plugs without noticeably affecting the performance when using a HEI distributor.
Jerry
