Drag Race 101: Part IV. Fire in the hold – ignition basics and upgrades.
If there is one factory ignition system that stands above the rest, it is GM’s HEI distributor. In terms of practicality, who else has built a distributor and ignition system packaged into one unit? Installing a complete GM ignition involves nothing more than dropping the unit in, and attaching a single wire. Not only is the HEI system easy to install, but it is also capable of providing sufficient spark—even on a modified engine, well above 6,000 rpm. But, are there ways to upgrade the HEI, and are there better options?
As great as the HEI distributor is, it does have some shortcomings. An HEI in stock form lacks some basic performance “tuning” that is required when using it on a high performance engine. The distributor itself is simply a mechanical device. It’s the coil and module that actually deliver the spark. If you have a lot of modifications to your small block (i.e., compression increase, advanced timing, camshaft overlap), the stock HEI’s spark will actually start to fall off at about 3,500 – 4,000 rpm. Even with this shortfall, it is still capable of jumping a typical plug gap. The problem arises because the spark intensity begins to fall-off, below what is required in a higher-compression performance engine. When your spinning engine reaches around 5,000 to 5,500 rpm, the spark from a stock HEI can become intermittent. If you are looking for an HEI distributor that will deliver good spark for your race engine, you MUST take a look at the coil and the module.
The coil is simple in design, but the function is a mystery to many. The HEI is what is called an inductive ignition. With this type of ignition, 14 volts of electricity is directed to the primary side of the coil. The primary side has wire wrapped around a metal core. When the distributor’s module “closes”, the circuit is complete, and allows this voltage to flow through the primary windings. When the module “opens” the circuit, the electricity no longer flows through the primary windings, and has created a magnetic field around the primary windings. Once the module “opens”, the magnetic energy transfers to the secondary side of the coil. This transfer increases the voltage from 14 to somewhere in the area of 20,000 – 40,000 volts.
The downfall of a stock coil is that the system works the best when there is enough dwell time to allow the primary windings to fully saturate. While is not a problem at 3,000 rpm, when you reach 6,000 rpm, the time of saturation is cut in half. This reduces both the voltage and spark energy that is available at the spark plug. To overcome this lack of available spark, you can upgrade the coil with an aftermarket replacement that can produce more voltage and total spark energy. When choosing an aftermarket coil, there are two designs. The only external difference between the two is that one has red and white power leads, and the other has red and yellow power leads. You will need to know which one of these your distributor has in order to get the appropriate aftermarket coil. Since a stock coil can take a long time to reach saturation, by 4,500 rpm, they begin losing spark energy. Some aftermarket coils are able to reach full saturation more quickly. For instance, DUI’s coil (Davis Unified Ignition), for instance, can charge in half the time that it takes a stock coil. It supplies twice as much spark energy as a stock coil, and can keep making that same spark energy over and over to a much higher rpm.
There are a few choices when choosing an aftermarket coil for your HEI distributor: Davis Unified ignition, MSD, and Jacobs.
When it comes to the control modules in your HEI distributor, there are three types that we need to cover, the four-pin, five-pin, and seven-pin modules. The four-pin module was used on conventional carbureted engines that use conventional, mechanical timing-controls (vacuum and centrifugal advance mechanisms). The five-pin module was introduced in 1978, and was an early attempt at electronic timing control. The five-pin module contains a provision for connecting a knock sensor, so timing can be retarded electronically. The seven-pin module is used on early computer-controlled engines. The seven-pin module contains no mechanical timing control mechanisms as the computer controls ignition timing. If you are looking for high-performance modules for your HEI, Davis Unified Ignitions, Pertronix, and MSD are all great choices.
We need to note; GM cars of the late 1980’s used an HEI distributor with a slim cap. The throttle body injection-equipped engines used a separate ignition coil. These distributors were not stand-alone units because they did not contain mechanical timing controls.
When you have improved your HEI’s ability to deliver a hotter spark at a higher rpm, you will then need to address the ignition timing and distributor advance curve. An engine’s best output is usually achieved when peak cylinder pressure occurs at around 15-degrees after TDC (Top Dead Center). For this to happen, we must actually ignite the air and fuel charge well before that. The compression pressure that is generated by the piston, significantly affects the speed at which the actual burn of the mixture occurs. The higher the cylinder pressure, the faster the burn rate. In reality, this means that as the compression ratio is increased, less ignition advance is required. Keep in mind though, when longer duration (overlap) camshafts are used, the low-speed compression pressure is reduced, so more ignition advance is needed until the dynamic effects take over and start completely filling the cylinders.
Before you can adjust the mechanical advance, you need to set the initial distributor timing. You can only set the initial “static” timing by moving the distributor. To begin, disconnect the vacuum advance (if used), and plug the vacuum line. Make sure that the car is at idle. Now you can set the initial timing to whatever you want. There are a lot of factors that contribute to proper initial timing, so giving you an exact setting is not possible in this article. Most generally, a very slightly modified engine can start at around 12-15 degrees, while a more modified engine can typically start at around 15- 20 degrees of initial timing.
To properly set ignition timing required, you have three system controls that need to be addressed; the mechanical advance, the vacuum advance, and the total advance stop. The following will help you adjust your distributor while at the track. Before you begin, you will need at least a set of distributor springs, and an adjustable vacuum advance. Begin by installing the strongest springs you have on the distributor’s advance mechanism. Stage the car at the staging light. For this adjustment, you’re only interested in the first 60 feet, so don’t worry about the entire quarter mile. Keep tire spin to a minimum. Make a pass and check the 60-foot time. Now, replace only one of the springs with one that has slightly less tension, and make another pass. If the car is faster in the first 60 feet, repeat the process on the other spring by adding a lighter one. Continue this process (one spring at a time), until either detonation is heard or the 60-foot times stop improving.
I doubt that many of you are concerned about the vacuum advance, and it probably isn’t even connected, but to be thorough, we’ll talk about it. A good working vacuum advance is critical if hoping to achieve any kind of fuel economy. Although you may not feel this is of great importance in a race car, having a working vacuum advance helps to clean up the way the engine runs while in the pits, and reduces the possibility of fouled spark plugs. Although an adjustable vacuum advance looks like a good idea, keep in mind that its adjustability is in terms of the amount of advance allowed, not the rate at which it comes in. To set up an adjustable vacuum advance, start with the advance closed, and drive the vehicle. If no detonation is heard during normal, part-throttle driving, then open the screw, and add more advance. Continue with this process until detonation is noticed and then back out some of the timing until the detonation stops.
Finally, if a relatively large cam is used, then the advance needs to come on faster, although little or no additional total timing may be required. The longer the cam duration, the sooner the vacuum canister needs to start pulling in advance. For example, a camshaft with 285-degrees of duration may need the vacuum advance to start applying vacuum at 3 to 5 inches with full vacuum advance in at around 10 to 14 inches.
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