July - August 2000 Motor State Performnace Report Article By Jim Kaekel, Jr.
Thanks to recent product releases by MSD, Accel, Crane, Holley and Mallory, the age of digital ignition systems has arrived and the days of searching for RPM chips are over! The racer can now turn a rotary switch, rather than plugging in various chips, to adjust RPM limit on most modern ignition boxes.
The majority of popular ignition boxes currently on the market are capacitive discharge (CD) which are much more effective in performance or racing applications than inductive units.
According to Silver Gomez of Autotronic Controls Corp., this is because capacitive discharge provides higher voltage levels at the primary side of coil, resulting in higher secondary output levels at the plug wires. Inductive ignitions, with lower voltage levels, are not as effective, particularly above 5,000 RPM.
Evaluating which box is the right one for any particular application is no easy chore. Proper selection is a matter of doing some homework and narrowing choices to the boxes that include the most desirable features in terms of output, amperage draw, coil and distributor compatibility and, of course, budget restraints. Price, however, should not override more long range considerations like spark duration specs, and primary and secondary voltage output.
Racing/engine building legend Smokey Yunick adds that duration and high amperage should be the most important features to look for in an ignition box.
Longer spark duration, measured in degrees of crank rotation, encourages more complete combustion because there is more time available for the air/fuel mixture to burn. While high amperage provides more spark energy, levels above 20 amperes can be fatal, and, for that reason alone, ignition manufacturers are reluctant to increase amps to this plateau.
The energy output of an ignition box is measured in millejoules (a joule is the measure of work done when one ampere is passed through a resistance of one ohm for one second), which basically tells how hot the spark is as it jumps the plug gap and ignites the fuel mixture.
Compatibility with current distributor and coil is important. All manufacturers supply an instruction sheet with information on what coil specifications should be for ideal compatibility. If the present coil doesn't measure up, there are recommendations on which ones will do the job.
CD ignition boxes are available with rev limiters having one, two, or three steps as well as start, high speed, and multi-step retards for nearly any application.
A single rev limiting function is solely for engine protection, preventing the engine from revving beyond a selected RPM limit in the case of a missed shift, loss of traction, or a driveline failure.
A two-step rev limiter uses the second RPM step to limit launch RPM, while a three-step's additional rev limit is used to limit RPM during a burn-out. Two- and three-step rev limiters may be easily set and wired to activate when the trans-brake or line lock is engaged.
A start retard function in an ignition box allows easier starting of high compression, stout cammed race engines with a lot of advance or with the advance locked out, by automatically retarding ignition timing when the starter is engaged. Timing returns to the normal mode after the engine starts. High speed and multi-step retards allow the ignition timing to be retarded when a gear change is made or the moment a nitrous system is activated.
The majority of popular ignition boxes are relatively trouble free. The most common problems (our MSD manufacturer rep calls them "alleged defects") are usually due to improper installation by the user.
Always exercise care in making good wiring connections, using correctly sized wire, particularly if a wire must be extended from the factory harness. Ground connections must be clean and made directly to the engine block. Solid core plug wires, due to their high RFI, should be avoided because they create havoc with CD ignition units.
Say "good-bye" to RPM chips and "hello" to the digital age! The better you understand this technology. the better you can guide your racing customers in their purchases, and subsequently increase your own sales!