OK, here is what I have from the book Four-Stroke Performance Tuning
. Revised edition 1998.
Most road cars use radio suppression cables with a powdered carbon-impregnated rayon cord to conduct spark plug current. With age, the resistance of this type wire increases so lees and less energy is provided to give a good hot spark
These carbon type spark plug wires should be replaced by a high-quality induction-type suppression wire having a metal core to conduct full energy to the plug while having a metal induction spiral wound wire within the cable to provide both noise and induction suppression.
Inadequate suppression could seriously upset the proper functioning of an ECU, and other electronic or radio-type devices - so do not use ordinary copper core or stainless steel core wires.
The use of induction spiral suppression wire will also minimize induction crossfire with a neighboring spark plug wire.
It is recommended that top-quality 8mm silicone cables and boots be used. The majority of race engines will not see voltages much over 20kV. The 8mm Moroso spark plug wires are recommended by the author as being the best, as the spiral core wire has a dielectric insulation strength of well over 50kV and it combines the highest levels of electro-magnetic suppression with the least electrical resistance to ensure maximum ignition energy at the spark plug.
For a typical points type distributor, the recommended plug gap is about .028" with an 11:1 compression. Opening the plug gap and/or increasing compression to 12.5:1 requires a higher voltage - 20kV versus 27kV. The smaller gap of the plug reduces electrical resistance and has a secondary benefit in that it can prevent a crossfire from one cylinder to another within the distributor cap.
With an HEI or electronic distributor, plug gap should be .035-.040". Semi and race engines should use a gap between .028-.032".
With a Transistor HEI, the supply voltage at the coil positive (+) coil should be the same as battery voltage, 12.6 volts. Any reduction in voltage supply will cause a corresponding reduction in coil output. Conversely, an increase in supply voltage will provide an increase in coil output.
The HEI system uses a relatively low amperage draw, BUT, the amperage draw when the module first switches the primary circuit to charge the coil, is very high. Because of this, the HEI must be wired directly to the ignition switch using a heavy #10
gauge wire with no other circuits connected to the wire. So if you are not receiving full battery voltage to the coil, it could be due to to small a gauge of wire.
At higher RPM's, the single coil HEI cannot fully saturate the coil sufficiently through the ignition module to provide a good strong spark to the plug. A stock HEI will provide a good spark just past 5,000 RPM's while a race HEI system will provide a good spark up to about 7,500 RPM's and beyond that the spark becomes weaker and weaker losing HP or can even cause a misfire. (From what I gathered, the just past 5,000 RPM's limit was due to the factory .060" spark plug gaps that were recommended back when the factory HEI came out. Wider gaps required more spark energy to fire them and there comes a point where the RPM limits are affected by spark gap and the stock coil/energy to fire them. Not saying you could not spin the engine above 5,000 RPM's, but the spark got weaker)
Many HEI systems will accept a higher supply of voltage and thus provide more secondary spark to fire the plugs at higher engine speeds, or fire a bigger plug gap or exotic fuels. A six-cell battery puts out 12.6 volts and can work up to 7,500 RPM's, while a seven-cell battery puts out 14.7 volts and can work up around 8,200 RPM's. This is for the HEI only as most electrics should not be operated above 13 volts. Always check with the manufacturer to ensure your HEI will handle higher voltage, otherwise you may overload part of the electronics system.