Digital Decisions...What does this have to do with ignition systems? A digital ignition system offers a number of advantages; it makes "decisions" based upon cold-hard facts. If it receives a signal or an "order" to do something, it does so immediately. It can repeat this process time and time again, and it can do so without compromise (in other words, these systems are infinitely repeatable). Speed and accuracy in making these "decisions" are also factors a sophisticated digital ignition system has on its side.
As an example, the MSD Digital 6 Plus ignition system incorporates a high-speed RISC microcontroller to control the ignition's output while constantly analyzing the various inputs such as supply voltage, trigger signals, and rpm. The high-speed controller found inside the box is designed and built so that it can make extremely quick compensation to the output voltage, multiple spark series, timing, and rpm limits while maintaining highly accurate timing signals. The heart of the system is a 15-Megahertz microcontroller (one of the fastest used in ignition systems). What this means is that the internal digital computer analyzes up to 15,000,000 instructions per second. That sort of speed just isn't available in today's analog circuits.
How accurate are the signals? According to MSD, the signals are within 1 degree and 1 percent of the pre-set rpm limits.
By incorporating a digital circuit, ignition manufacturers can, in some cases, physically reduce the size of the ignition box. At the same time, they can increase the power output. When you compare the size of a conventional MSD 6AL and the new MSD Digital-6 Plus, you'll find that the new digital unit is either the same size or physically smaller (and certainly more powerful) than older analog systems.
What's Included?The use of digital circuits allows the manufacturer to include more features into the basic ignition package. Using the MSD Digital-6 Plus as an example, this new system from MSD includes two built-in rev limiters, a single-stage retard, adjustable magnetic pickup compensation, and an LED display that warns of trigger signal problems or a faulty charging system. All of the adjustable features use a simple rotary switch (a small screwdriver is used to adjust the switches) to change the values in 100-rpm increments. The neat part is, these features don't mandate the use of add-on accessories. In turn, this means that your hot rod's wiring harness is less complex and less cluttered.
It's hard to believe, but the MSD Digital-6 Plus is just the tip of the digital ignition iceberg. MSD has a Programmable Digital-7 ignition system that includes features like a PC interface. This allows you to physically program the ignition system for your application. The software allows you to plot timing down to 0.1 degree every 100 rpm, allows you to control the timing on each cylinder, allows for different shift points for each gear, controls three stages of timing retard (useful in nitrous applications), and of course, allows you to custom-tailor the spark curve for the entire run down the quarter-mile.
What Does This Do For You?So what does all of this mean to you? That's easy. Digital controls have allowed ignition component manufacturers to increase the power of their systems, in some cases, to reduce the size of the hardware, and to add countless built-in features. For all intents and purposes, the new wave digital ignition systems give you all of the control that you'll find in a modern computer-controlled EFI system, but without the fuel specifics.
Not only can you have an ignition system that delivers sparks comparable to an arc-welder, you have the option of controlling it precisely. Entering the "new wave" digital world might not be that bad.
In a perfect world there would be no need for ignition advance curves. As soon as the piston in your Mouse motor reaches top dead center and the engine builds maximum compression, you could light the fire. Kaboom. Job done. But this isn't a perfect world. And neither is the case of ignition advance curves.
Why is spark advance needed? Spark is most often introduced into the cylinder prior to the piston reaching TDC. This simply gives the spark sufficient time to light the air-fuel mixture. As engine speed increases, then the time required to bring in the advance increases. Everything else being equal, bringing in the spark sooner creates more cylinder pressure, and consequently, increases low-rpm torque.