So you're wondering what's the big deal with "digital," are you? Unless you've been hiding under a rock for the past decade, almost everything you hear about is new, improved, and digital. The "old stuff" was analog. And guess what when it comes to adding this technology to your hot ride, there's more available than just CD players. One of the most recent areas to go digital is electronic-ignition systems. So what does it mean, and why are digital systems better than older analog electronics? Before looking at digital ignition systems in particular, let's take a look at the differences between analog and digital circuits. Here's the technical definition of both (drawn from an electronics dictionary):
Digital Circuit: This is a circuit in which the output currents or voltages are interpreted as having one of several (often two) values, depending on which of a corresponding number of ranges they fall into. Such circuits implement logical operations or operations on representations of discrete numbers, often in binary form.
Analog Circuit: This is a circuit in which the output voltage and current values are considered significant over a continuum. Analog circuits may be used for such purposes as amplifying signals.
Proportionate Technologies?There's more to consider than a simple definition, though: When you take a look at a computer, you'll find that it is basically a device capable of performing a series of arithmetic, or logical, operations. A computer is distinguished from an adding machine (a good example is a simple hand-held electronic calculator or even an ancient abacus) by being able to store a computer program (this allows it to repeat operations and make "logical" decisions) and to store and retrieve data without intervention from you.
Technically speaking, computers are classified as analog or digital. An analog computer operates on continuously varying data. Meanwhile, a digital computer (such as today's common PC) performs operations on what is called "discrete data." An analog computer represents data as physical quantities and operates on the data by manipulating the quantities. In a complex analog computer, continuously varying data is converted into varying electrical quantities and the relationship of the data is determined by establishing an equivalent relationship, or "analog," among the electrical quantities.
While analog computers are commonly found in such forms as electrical watt-hour meters, they largely have become obsolete for general-purpose mathematical computations and data storage thanks to digital computers. Inside a digital computer, data is expressed in binary notation (or "numeration"). Essentially, this is a series of on-off conditions that represent the digits 1 and 0. A series of eight consecutive binary digits, or bits, is called a byte and allows 256 on-off combinations. Each byte can represent one of up to 256 alphanumeric characters. Mathematics and comparative operations can be performed on data represented in this way and the result stored for later use. Digital computers are used for things like desktop publishing, scientific research, data-processing...and now, race car ignition systems.
A Less Intense Explanation...As you can see, the definitions of analog and digital are definitely hard on the brain, and if you're not into computers or electronics, they can certainly tend to be geek-speak. Here's the simple answer: The best way to understand the differences between digital and analog is to think of the differences between a good old-fashioned wristwatch with a sweep second hand and the LCD readout on something like your VCR. Both display time, but the wristwatch is constantly varying. Simply stated, as the second hand sweeps around the face of the wristwatch, the information that is displayed is constantly changing.
Next, regarding the clock on your VCR, it's really presenting the same information, but in what's called a "discrete demonstration." In other words, it displays one piece of information at a time.
An analog system is like the good old-fashioned wristwatch with a second hand. It attempts to recreate the information as it actually happens. The digital system takes the information and represents it as a series of changes, or "bits," that are represented in code by zeros and ones.
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.
There's a trade-off though: As the engine speed increases (particularly in high gear), there is a loss of some top-end power. That's why high-gear spark retard systems show performance increases. There is a point, however, where you can dial in too much advance into the ignition system. Too much advance and the engine will detonate. This is caused when the "explosion" in the engine is timed too early.
Initial advance is the base timing that is dialed into the engine before the centrifugal advance begins. How much initial do you need? It depends. For example, it's interesting to note that as the altitude increases, so does the need for additional spark advance. Why? More advance helps to compensate for the lack of oxygen. There are other factors as well. Check out the following chart from MSD:
If you take a look at this chart, you can see that the variables can change throughout the range of the engine operation. MSD notes that the timing mechanism of the distributor must make timing changes based upon these factors. It's also easy to see that there is no one perfect curve. Each engine will be different and consequently, each curve will be different.
Generally speaking, there are two types of advance curves commonly used in a distributor; centrifugal and vacuum. OEM Chevy distributors and aftermarket distributors used on street cars usually incorporate both systems. Both systems function independently of one another. Centrifugal advance is based upon a set of governor weights and springs, which in turn are controlled by engine rpm. Centrifugal force moves the weights outward against the tension of the springs. This causes the spark timing to advance.
Vacuum advance arrangements are more complex. All vacuum advance units operate on a system where the diaphragm reacts to the difference between atmospheric pressure and induction pressure. The way they operate is different. Early, pre-emission Chevy vacuum advance units were typically linked to a manifold vacuum source. This meant that the vacuum was most often taken from a location below the carburetor throttle body. During idle and part-throttle operation, manifold vacuum is high. This advances the ignition timing under those conditions, and obviously, improves fuel economy. When the engine is operated at wide-open throttle, manifold vacuum is low. This means the vacuum mechanism does not advance ignition timing. As a result, there is no chance of detonation (or pinging).
In the mid-to-late-'60s, vacuum advance mechanisms changed to suit emission requirements. The vacuum source was changed from the manifold to the carburetor venturi. This is called "spark-ported vacuum." Spark-ported vacuum is lowest at idle, then increases as the throttle is opened. This is completely opposite to manifold vacuum. At idle, a spark-ported vacuum system has no vacuum advance (in contrast, a manifold vacuum advance might have as much as 12 degrees extra timing).
In a dedicated racing application, the first thing that gets "modified" is the vacuum advance. It gets removed. Typically, a vacuum advance system can increase the total timing to 50 degrees (or more) advanced under certain circumstances. A good example is a car cruising at 65 mph. The increase in vacuum advance can improve the fuel economy (by significant margins), without knocking or pinging. In a race car, there's little interest in fuel economy and besides, there is little or no part-throttle (or high-vacuum) operation. The real catch is the moveable breaker plate. With the vacuum advance system hooked up, there is a chance the plate can move, which in turn can create inconsistent spark timing. In the accompanying photos, we'll show you how the vacuum advance is disconnected on a MSD distributor.
When the initial timing and the centrifugal advance are added together, you come up with total timing.
As an example, if your engine has 12 degrees of initial (dialed into the distributor by way of the timing marks on the harmonic damper) and it has another 25 degrees of timing in the centrifugal, then the total timing is 37 degrees. Some people also factor the vacuum advance into this figure as well. Assuming that the vacuum advance mechanism adds another 12 degrees, you have 49 degrees total in the system.
The results of tinkering with the advance curve on any engine (race or street) can be remarkable. For example, many stock production line Delco distributors were set up to bring the advance all in at engine speeds of 4,000 rpm or more. Bringing the curve in sooner can result in startling performance improvements. MSD describes the process: "The function of the advance curve is to match the ignition timing to the burning rate of the fuel and the speed (rpm) of the engine. Any factor that changes the burning rate of the fuel or the engine speed can cause a need for an ignition timing change. Refer to the previous chart for some of the factors that will affect engine timing."
MSD also offers the following tips on selecting an advance curve:* Use as much initial advance as possible without encountering excessive starter load or engine kickback.* Start the centrifugal advance just above the idle rpm.* The starting point of the centrifugal advance curve is controlled by the installed length and tension of the spring.* How quickly the centrifugal advance (slope) comes in is controlled by the spring stiffness. The stiffer the spring, the slower the advance curve.* The amount of advance is controlled by the advance bushing. The bigger the bushing, the smaller the amount of advance.
Here are a couple of extra curve tips we've discovered over the years:* Automatic transmission cars (particularly foot brake, non-trans brake applications) need a quicker-but-shorter curve than stick shift cars, however, the total timing should still be the same.* Automatic transmission cars (particularly foot brake, non-trans brake applications) can use more initial advance than stick shift cars.* By using a separate starter and ignition switch, you can overcome adverse starter load by spinning the engine first, then clicking on the ignition switch.
So far so good. The very best recurves are those that use your vehicle as a test bed. All that is required is a degreed damper (or a timing tape), a timing light, a reliable tach, a note pad, and a bit of patience. The main idea when recurving a distributor is to bring the curve in as quick as possible without the engine detonating. In other words, play with the springs until you reach the optimum curve for your application. Some cars may require one very light spring and a heavy spring; certain combinations will require a pair of medium springs; others can get away with a pair of light springs. We've even seen Delco applications that required but one spring. The other weight was used "springless."
When experimenting with the curve, set the initial timing and make a note of it. Then record the spring combination in the distributor. Increase the engine speed, and log the speed at which the curve begins along with the speed at which the curve ends (where the curve is "all in"). You can also graph the results by checking the timing at 200-rpm intervals (correlating the advance shown on the harmonic damper to the engine speed). Test the results and begin again. Trial and error plays a major role in the selection of a proper timing curve.
What you have to obtain is good throttle response along with detonation-free timing. As mentioned earlier, some powerplants will "like" more initial timing than others, while some combinations will want more total timing. In any case, you can adjust where the advance starts, the rate of advance (slope), as well as the total amount of advance. Take the time to sort through the timing maze and be certain that you record all changes in the notebook. It will become a valuable guide when setting up the curve for your particular combination.
As you can see, in the world of ignition timing is not perfect. Even when Chevys are seemingly equal, they might need a different curve. It's all a matter of trial and error.
For a closer look at the mechanics of ignition advance curves, check out the following photos.