Thursday, June 24, 2010

Distributorless Ignition In A Classic Muscle Car

Distributorless Ignition

Multiply your V-8’s firepower

by Frank Bohanan / photography by Frank Bohanan

Most gearheads will agree that, when it comes to musclecars, “more is better.” That’s often true, at least in some cases.

One very good example of this is in terms of engine performance (can you ever really have enough?) or, in the case of this article, the extra spark energy that gets it for you. This extra juice comes by being freed of the inherent limitations of an ignition distributor and a single coil. The Compu-Tronix Distributorless Ignition System (DIS) completely eliminates the distributor and, in the case of a V-8 engine, allows you to use four coils instead of just one. This greatly increases the available spark energy, especially at higher rpm, plus it also allows more of the energy generated to actually make it to the spark plugs.

In the standard ignition system of most V-8 musclecars the distributor performs the function of directing the spark to the appropriate spark plug as determined by the firing order of the engine. The problem is there is only one ignition coil so the spark energy has to go from the coil to the center of the distributor cap, down to the “button” that is in contact with the rotor, across the rotor, and then jump from the spinning rotor tip across an air gap to the outer terminals on the distributor cap before it is then sent down the spark plug wire where it will ultimately then jump the spark plug gap and, hopefully, ignite the fuel/air mixture. Whew! There’s a lot more involved than most people realize.

The article featured on this page is from the May 2010 issue of Musclecar Enthusiast Magazine.

There are also plenty of opportunities for things to go wrong. Since only one coil is used there is only so much time available for it to generate a spark for each cylinder. Less time equals less total spark energy available for each firing event. Throw in the difficulty the spark has in going across the sliding contact at the distributor cap button, the air gap at the spinning rotor tip, and the spark plug gap itself and you can see why misfires occur. It’s even worse when parts get worn; spark plug wire resistance increases with age (especially in the case of OEM carbon core wires) and you get ozone and/or carbon deposits that can cause arcing inside the distributor cap.


If you’re on top of things in terms of maintenance you can eliminate some of these potential problems from getting too serious but you’ll never be able to overcome the inherent limitations in the design in terms of coil charging or “dwell” time and having more than one air gap to jump for each cylinder. There’s got to be a better way and, clearly, we know there are virtually no new cars that use ignition distributors anymore. There’s no way they could pass emission laws if they did.

Compu-Tronix has taken this new car technology and found a way to adapt it to your old V-8 musclecar. They offer a number of kits that fit older V-8s from various manufacturers. These kits come with a module that drops right in to replace the OEM distributor, two coil packs which have two coils each (four coils total) and the necessary wiring to hook it all up. They also include a set of high quality spiral-core spark plug wires that you cut to length yourself based on where you mount the coils. The latter are necessary because this setup produces such high spark energy that they wanted to avoid arcing or other such problems which would occur if inferior wires were used with this higher energy system.

The advantages of this system come in many forms. First of all, there is no distributor cap so there is no sliding carbon button contact and no spinning rotor air gaps to deal with. The spark only has to jump the air gaps at the spark plugs themselves, nowhere else. Again, this is also made easier by the lower resistance, high quality spiral-wound wires that come with the kit.

Most importantly, however, is the use of four coils instead of just one. This gives each coil much more time or “dwell” to generate as much spark energy as possible for each firing event. Since this system uses the “waste spark” firing method there are actually twice as many firing events per cylinder as usual: One during the normal power stroke and the other “waste” spark which occurs during the exhaust stroke. The latter really isn’t wasted in the true sense since the cylinders are wired in pairs such that when one cylinder is on its exhaust stroke the other cylinder on the coil pack is having its power stroke. The spent exhaust gas is a surprisingly good conductor so about 99 percent of the total firing energy actually makes it to the plug that needs to fire the mixture.

In practice

The system certainly worked well for us. We noticed a very significant improvement in idle quality, especially when the engine was cold. The owner of the car (a ’69 Mustang with a moderately modified, carbureted 351W) even said it now idled better cold than it used to warm and we hadn’t even fully dialed the timing in yet. He later reported the throttle response and overall performance were better as well. While we can’t vouch for the claimed improvements at really high rpm, it only makes sense they should be realized due to the greater coil charging time and the elimination of the spinning rotor and extra air gaps.

The system was also very easy to install although there are a few things you need to watch out for. In the following photos we’ve shown how easy it is to install plus we’ve also provided a few suggestions we used in our installation to help save a little time. For a cost of about $600 for the whole system it would be hard to beat the improvements in performance, drivability and reliability this system provided us. There may even be a small fuel economy benefit if you don’t keep using the extra performance all of the time. The bottom line is this is a fine example of where “more is better” in terms of the number of ignition coils you can use but it’s also a case of “less is more” since you have fewer air gaps for the sparks to jump plus the distributor, cap, rotor, and so forth are eliminated.

For a cost of about $600 for the whole system it would be hard to beat the improvements in performance, drivability and reliability this system provided us.

1 Ready for an upgrade: The engine we installed the DIS kit on is a typical carbureted small-block V-8, in this case a Ford 351W. The engine was moderately modified with a cam, aluminum heads and headers, etc., so it clearly was in need of a better ignition system.

2 Parts report: The Compu-Tronix kit includes not only the DIS module which replaces the distributor, but the coil packs, some split-loom to cover the wires, some terminals and a full set of low resistance, spiral-wound spark plug wires needed to handle the hotter sparks the DIS system produces. The system is completely bolt-on.

3 We’re No. 1: After you’ve removed the distributor cap and wires you need to rotate the engine so that the rotor points roughly where the distributor cap terminal for cylinder No. 1 was. Just hold the cap over the distributor with the spring clips and cap indexing peg lined up and then mark where No. 1 is. Bump the starter or use a breaker bar and socket on the crank pulley nut to rotate the engine until the rotor points at it.

4 Disassembly: Remove the coil and its bracket but make note of which wire is the hot (+12v) lead for the ignition. Also make note of any tachometer wire you may have been using. The distributor can then be removed after the retaining bolt and clamp are loosened. Be careful not to let any loose dirt or debris get into the engine.

5 Ring thing: The o-ring must be removed from the distributor and transferred onto the new DIS module. Be careful not to damage it; lube it with oil or silicone spray (WD-40, or similar) before you put it back into the engine. If the o-ring is hard or breaks as it comes out just get a new one. Note our use of a bronze gear for our roller cam.

6 Nice fit: The DIS module drops right in where the distributor was. There’s no real need to worry about which way it faces, as you’ll soon see. Just make sure the wires have some slack and are able to reach where they need to go. The lower profile of the DIS module is actually an advantage since it easily clears shock tower braces, etc.

7 Wiring: The shorter red wire gets connected to the ignition on/”hot” (+12v) lead while the purple wire connects to the tach. We soldered the wires instead of using wire connectors, plus we zip-tied them to keep them out of the way. Make sure you leave enough slack so the DIS module can be rotated as far as will be needed.

8 Power source: The longer red wire is supposed to go to the positive terminal of the battery but we decided to just jump over to the alternator instead. It’s connected directly to the battery positive by a very thick cable so the relatively minimal current draw from the DIS module won’t matter. It also had a convenient stud to which we could attach our ring terminal. Remember to splice in the supplied 20A fuse to protect the DIS.

9 Test light: The instructions tell you how to set the position of the DIS module so the engine will start. This is just temporary to get the engine running so you can then set the final timing with a timing light. It basically involves turning the module and watching the LED go on and off. Make sure the coil packs are not connected.

10 Coil packs: The coil packs can be mounted just about anywhere including the valve covers, the firewall, shock towers, wherever. A template is provided to help drill the holes for sheet metal screws. We wanted to keep the wires as short and even as possible so we mounted the coils to the factory “export brace” with strong wire ties. Note that mounting the coils upside down requires you reposition the spark plug wires since the diagram in the instructions assumes the coils will be upright.

11 Graph it: This diagram shows how much more total spark energy the DIS system provides versus a standard single coil system with a distributor. The spark also doesn’t drop off at higher rpm and is much more stable.

12 High wire act: Once the coil packs have been mounted you can then run the appropriate wire connectors to them. We shorted and re-soldered the wires to make things look a bit neater and to ensure the wire bundles would still fit into the split-loom. Keep the wires away from any high heat or moving parts like the throttle linkage, etc.

13 Plug wires: Determine the routing of the spark plug wires from the coil packs to the spark plugs based on the firing order of the engine and the diagram provided in the instructions. Then cut the wires and crimp on the terminals as also shown in the instructions. After each terminal is on, spray a little WD-40/silicone into each plug boot and push the terminal/wire in, making sure they line up properly with the boot.

14 Spark and go: The finished installation looks similar to this: Each coil pack will have two wires that go to the near bank and two going to the far bank. Be sure to leave enough slack for engine movement. Clearly, we were more concerned about function than looks or else we could have mounted the coils to the firewall and run all the wires more discreetly from behind. Start the engine and adjust the timing to the desired total advance with the engine above 3,200 rpm. If you use a timing light with a dial-back feature, you must set it to 0 degrees or it will provide incorrect readings.

The article featured on this page is from the May 2010 issue of Musclecar Enthusiast Magazine.
Click here to read the free digital edition of Musclecar Enthusiast


  1. $600 and you have to reuse an old oring??

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