Minimizing spark scatter in the Vintage race engine

Minimizing spark scatter in the Vintage race engine

By Sam Logan
Photography by Moore Good Ink & V&B Engines

Remedy-for-torsional-vibesOften Vintage racing engines exhibit excessive spark scatter caused by torsional vibration in the distributor drive system. To correct it Virkler & Bartlett adds a miniature flywheel to the system. They mold a series of rubber couplers with a range of Shore A hardness, which allows them to tune the system. Note rubber coupler glued within steel ring.




How to get the best from a Vintage engine ignition system

King 16-T distributor machine, checking distributor,

Aided by the latest King 16-T distributor machine Virkler & Bartlett check the distributor’s performance, its parts, including bearings and point’s cam, as well as coil, condenser and plug wires. Particularly useful for complex Vintage engines with two distributors and four sets of points, its advanced technology not only allows them to analyze and tune the ignition system but also to break-in all the ignition components on the machine rather than on the engine.

Chatham, Virginia:Vintage racers are often forced to live with points-and-coil ignition. But the most successful know the shortcomings of the ignition system and have it corrected.

For the past forty years or so electronic ignition has been the standard, but most Historic race cars produced before the 1970s are equipped with something other.

Unfailingly, coil-and-points ignition systems work best when optimized mechanically and electrically. But how is it achieved? Vintage racers seem to run a little faster each year, and as compression ratios and engine speeds creep up, deficiencies in points-and-coil ignition systems can precipitate the perfect storm of performance problems.

Bob Bartlett, adjusts advance curves,

Bob Bartlett verifies spark advance for each cylinder and adjusts advance curves. Today’s Vintage engines are often run at higher RPM and power levels, putting additional demands on the points-and-coil ignition systems.

Just over one hundred years ago, the brilliant engineer Charles Kettering invented the ubiquitous battery-powered “points-and-coil” ignition system that first appeared on the 1910 Cadillac. Remarkably, it was used in most cars until the mid-1970s.

An engine-driven mechanical cam operated a set of breaker points, switching electrical current to the coil which converted it to high voltage required to fire spark plugs. A rotor within the distributor routed high-voltage impulses to the correct spark plug. The condenser had the dual function of extending the life of the points by quenching the arc across the points and forming a resonant circuit with the coil that boosts peak voltage.

Old timers remember tune-up kits consisting of spark plugs, points, rotor, and condenser, which were duly installed at 12,000-mile intervals. Invariably, engine performance deteriorated as tune-up time approached.

The eight major elements in the points-and-coil ignition system:
Common problems and some of the solutions

Race engine builders Virkler & Bartlett have become an authority on point-and-coil ignition. They use a King distributor test machine that allows them to examine and make adjustments to the entire ignition system. Their engine dynamometer includes a video system with timing light, allowing them to observe ignition timing across the range of engine speeds.

• Spark Scatter – refers to any random and unwanted variation in ignition timing at a constant engine speed. Mechanical components are examined first. The entire drive system from the crankshaft to the distributor must be sound and the distributor bearings free of play and points unworn. The distributor shaft must be straight and the advance mechanism clean and lubricated.

• Advance Curve – Mostly older race engines use a mechanical advance system, which consists of centrifugal weights and springs. V&B check the advance curve with their King distributor machine and adjust if necessary.

• Dwell – the number of degrees the points remain closed is critical to ignition system performance. It is a function of the point’s gap and point’s cam design. Sometimes V&B observes misfire or phantom, unwanted ignition events occurring with misadjusted dwell.

• Points – The rubbing block of the point’s assembly wears-in after installation and consequently effects dwell. It’s important to examine dwell and re-lubricate the point’s cam after wear-in. V&B executes this on the distributor machine. Further, they discovered it’s best to use Mallory Cam Lube Grease. It’s made for the purpose.

• Wires – Even vintage racers use radios, data acquisition systems and other electronic gear. Solid conductor wires are not compatible with electronics, but some of the better RFI suppressed wire will still deliver a hot spark. Keep ignition wires clean; lacquer thinners works well for the purpose.

• Condensers – As reliable as bricks in the old days, regrettably, today’s off-the-shelf condensers are prone to failure. As a result V&B supply specialty high-reliability condensers. The symptoms of a bad condenser are reduced life expectancy of contact points, high-speed misfiring, low-speed back firing and missing, increased spark scatter or some combination of the four. Some condenser problems are thermally linked. Sometimes V&B use a heat gun and freezing spray for diagnostic purposes.

• Coils – As a general rule, 3-ohm coils work well at lower engine speeds and with fewer cylinders. Low impedance coils of 1.5 ohm or less, shorten point life and require more current, but work better at higher engine speeds, increased compression ratio and more cylinders. Always use a coil designed for points and use a ballast resistor if required. Like the condenser, some coil problems are thermally linked.

• Spark Plugs – Avoid exotic metal plugs such as platinum. The humbler metals in standard plugs have lower ionization voltage—meaning it’s easier to set up an arc with them. Don’t use giant spark plug gaps prevalent on today’s new cars; consider 0.025” as a good starting point. Old-time racers would often start and warm-up their cars with a hotter set of plugs to reduce fouling then replace them with a colder set for the race. Finally, electrical arcs like to propagate from sharp corners. So don’t race with an old set of spark plugs with rounded center electrode.

Call (434) 432-4409
to speak to V&B’s Bob Bartlett personally

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