Engines with lower compression ratios (10.0:1 and below) usually benefit the most from a functional and properly tuned vacuum-advance unit. The first step is to connect a manifold-vacuum gauge. Drive around with the gauge duct-taped to the windshield and mentally record the readings. If there are at least 5 inches of mercury present under most driving conditions, the engine will probably respond to a vacuum-advance unit. Try connecting to a full-time (manifold) vacuum source and a ported (timed) outlet. The engine will tell you which one it likes better. A manifold-vacuum source will provide advance at idle and then ramp off slowly as load is applied to the engine.
Myth: All HEIs Are Created EqualHEI is a brand name for High Energy Ignition, which General Motors pioneered for the '74 model year. Though other makers had electronic ignition prior to this, namely Chrysler in 1972, HEI stood alone as a brilliant new theory. Designed with transistors that would survive on full battery voltage, HEI did not use resistors to drop the system running voltage. It also incorporated a unique concept called expanding dwell. Unlike the other early electronic-ignition systems that used a fixed dwell time just as breaker points did, HEI would increase the dwell period as engine speed increased.
On a V-8 at idle, the module dwell (on time) would be 5 to 10 degrees and expand to 30 to 35 degrees by 2,500 rpm. The higher input voltage, along with the increased coil saturation (dwell), allowed HEI to fire the spark plug longer and with a larger gap. The increased gap opened a larger ionization window and, since the ignition was very powerful, allowed a longer burn time. Higher ionization voltage with longer burn times equals more power, better fuel economy, and a smoother idle.
Just because an HEI-equipped engine runs does not mean the module is operating properly. To check, simply hook your old breaker points-style dwell meter to the tach terminal on the distributor cap and read the dwell scale at idle and as rpm increases. Cheapo import modules often do not have the proper dwell period, which impacts performance. This simple test should be part of a performance diagnostic routine and performed with every tune-up. Note that if the engine has a CD box, the current draw will cause the HEI module to assume a fixed dwell of around 35 degrees, making this test invalid.
Diagnosing MisfireThe term misfire describes an ignition event in which either the spark has not jumped the gap of the plug but instead went to ground, or a very short burn cycle of the ignition. Many factors can cause a non-ignition-related misfire, especially an excessively lean mixture. When there is not enough fuel present in the chamber, the flame cannot travel across the bore and extinguishes, even though the spark plug is still arcing. This causes a rough idle, lack of power, and poor driveability. When diagnosing a misfire, determine if it is an ignition or a mechanical issue that is not allowing the ignition to work properly.
Myth: Thicker Plug Wires Have Thicker CoresCommon sense suggests that the thicker the spark-plug-wire designation, the more electricity it can transport. That would be true if the dimension used to describe the wire were a measure of the core size. But contrary to what many think, it actually represents the thickness of the insulation. A 7mm wire has thinner insulation than a 10mm design. The core and resistance are manufacturer-specific, so all plug wires are not created equal. The benefit of thicker insulation is less radio frequency interference (RFI) and electromotive interference (EMI) and greater thermal protection.