The Path Of Least Resistance

Cap-A-DaptSept. - Oct. 2015 Motor State Performance Report Article By Jim Kaekel, Jr.

Ignition problems including misfire and crossfire have made more than a few performance enthusiasts literally pull their hair out over the years. Although most modern cars employ coil-on-plug ignitions, the vast majority of high performance autos found at the local car show or race track still utilize a distributor type ignition, often enhanced with a high output, capacitive discharge unit. A capacitive discharge unit offers significantly more spark energy, however, the increased spark energy requires that the secondary side of the ignition system (coil wire, distributor cap, rotor, spark plug wires and spark plugs) be kept in top condition to prevent occurrences of misfire or crossfire.

Both misfire and crossfire occur due to the fact that "voltage always takes the path of least resistance", commonly known as Ohm's law. Misfire occurs when secondary voltage does not jump the plug gap, due to a fouled plug, faulty spark plug wire, cap or rotor. Crossfire occurs when a spark plug fires out of sequence, typically caused by a bad plug wire, cap or rotor. For example, spark plug wires in poor condition may allow secondary voltage to jump from one wire to another and cause a crossfire, or a distributor cap with condensation present may be the culprit. One symptom of crossfire is starter "kickback", a condition that occurs where the starter experiences sudden changes in engine cranking, creating havoc between the starter and flywheel.

The enemies of ignition system components are many, including heat, vibration, moisture, dirt, contaminants, oxidation and ionization, however, most ignition issues can be eliminated before they even start with careful component selection, proper installation and a little preventative maintenance. Joe Pando of MSD Performance states that "people tend to be pretty good about changing their engine oil and filter, but tend to overlook the ignition."

SPARK PLUG WIRES should be carefully routed using separators and kept clear of the exhaust manifolds or headers. Periodic maintenance involves keeping the wires clean and inspecting their condition. The plug wire terminals at both the spark plug wire and distributor end should be scrutinized as well, ensuring that they are in good condition and free from corrosion. One sign of trouble is the terminals taking on a blue/green appearance, evidence that they have been subject to oxidation. To truly evaluate plug wire condition, they should be checked for resistance using an Ohm meter. To perform the test, first determine the specified resistance from the wire manufacturer's catalog, listed as per-foot. For example, Moroso Ultra 40 wires offer 40 ohms per foot, while MSD 8.5mm Super Conductor wires are 40-50 ohms per foot. Follow by measuring the length of the wire and then checking it's resistance using a quality Ohm meter. Wires that show excessive resistance likely have a break in the conductor and should be immediately replaced.

There are several products available that can add protection to plug wires that are subject to extreme under hood temperatures, such as turbocharged applications. Plug wires may be covered with Pro-Heat Guard (MSD3411), a special sleeving constructed of a thick glass woven core that includes a silicone rubber coating. Spark plug boots that route close to exhaust headers may be protected using Pro-Boot Guard (MSD3412), a fiberglass woven sleeve that is coated with a specially-compounded silicone rubber. Accel offers a solution to the high temperature issue with Extreme 9000 spark plug wire sets. The wires feature 600 deg.F rated double silicone wires and ceramic spark plug boots that withstand temperatures up to 2000 deg.F. When installing plug wires, dielectric grease (MSD Spark Guard #8804) may be used to help prevent voltage leaks, protect against E.M.I. (Electromagnetic Interference), and ease plug boot removal.

SPARK PLUGS should be carefully chosen for proper type and heat range. A plug which is too hot or cold for the application can be prone to fouling, detonation or misfire. If it's a new engine and you're unsure of what plugs to use, consult the engine builder or cylinder head manufacturer. Plugs should be properly gapped prior to installation. Although it's widely known that a larger plug gap may provide a bit more performance, larger plug gaps also place more load on the corresponding ignition components.

The DISTRIBUTOR CAP is often subject to cracking, carbon tracking, oxidation and ionization, all causes of misfire and crossfire. One of the most important spots to check is the carbon contact, located in the inside center of the cap, which wears over time, requiring that the cap be replaced. Caps that include blue/green corrosion on the terminals, or a black, powdery substance, referred to as carbon tracking, should be cleaned or discarded. Ionization is a condition where the air inside the cap become electronically charged, creating spark scatter or crossfire. Many of these issues are caused by condensation building up inside the cap, which can be minimized, or even eliminated by using a vented distributor cap. If a vented cap isn't available, 1/4" diameter holes may be drilled into the side of the cap, adjacent to the rotor. Small-sized distributor caps are more susceptible to crossfire since they have less space between terminals, and therefore, high performance or racing applications would be better suited to using a large, Ford style cap when possible. The Ford style cap features more area between the terminals, lessening the chance of crossfire. Special Cap-A-Dapt kits, available from MSD Performance, include a cap, rotor and special cap adapter that allow installing a Ford style cap on a GM or Chrysler distributor. Kits are offered with fixed (MSD8445) or adjustable (MSD8420) rotors.

The ROTOR should be periodically inspected as well for signs of wear and corrosion, particularly at the rotor tip. Racing applications would be best suited to an adjustable rotor, a two-piece unit that allows setting precise rotor tip-to-distributor terminal alignment, commonly referred to as rotor phasing. To properly phase a rotor, proceed by drilling a large hole at the #1 cylinder terminal in an old, yet suitable distributor cap. After installing the adjustable rotor and modified cap, continue by hooking up a timing light, starting the engine and then aiming the light at the hole in the cap. Look closely and note the rotor-to-terminal alignment. Shut off the engine and adjust the rotor as required to achieve perfect rotor tip-to-terminal alignment. Finish by re-checking rotor tip-to-terminal alignment, confirming that it is correct.

One of the most overlooked, yet critical components of the ignition system is the COIL WIRE. Since the coil wire transfers voltage from the primary side to the secondary side of the ignition, it is subject to significant loading. A typical V-8 engine coil wire, for example, carries eight times more load than a single spark plug wire and should be replaced regularly, according to Pando. Similar to checking spark plug wires, the coil wire may be checked for resistance using an Ohm meter.

Ensuring that the secondary side ignition components are in tip-top shape will certainly help keep you're voltage on the right path, free from misfire or crossfire, and you'll feel like you just had a refresher course in Ohm's law to boot.

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