Sep 16 2013

# Monday Mailbag: What Are Ignition Voltage, Millijoules & Resistance?

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Got questions?

We’ve got the answers—Mondays when the Summit Racing tech department tackles your automotive-related conundrums. This week, we provide an introduction to ignition terminology, including voltage, millijoules, and resistance.

J.M. Halethorpe, MD

Q: I am putting together a hot rod and need help putting together a good ignition system. I see all of these different boxes and coils and it is really confusing. The car has 360 cubic inch engine with a two-barrel carburetor and stock camshaft. The engine won’t see over 5,000 rpm and is tuned leaner than stockthat is why I need a good ignition.

Can you explain to me how to choose the right ignition coil? Also, can you explain what voltage, secondary current in amps, millijoules, and primary coil resistance are?

A: Let’s start with a quick lesson in ignition systems. The reason you want high voltage is to ionize the gap between the spark plug’s center and ground electrodes. That creates a path for high current (measured in amps) to travel across that electrode gap and light the air/fuel mixture in the cylinder head combustion chambers. Primary resistance is the measure of a coil’s ability to move spark energy to the plugs. The lower the resistance, the more power the coil can make. Lower resistance also builds more heat in the coil, reducing the length of time the coil can produce energy without damage. That is why very low resistance coils are used in drag racing and are not the ticket for extended street use.

For applications like your 360, a coil that produces 25,000 to 35,000 volts usually works well. Engines with very high compression (above 9.5:1 or so) or very lean air/fuel mixtures will require higher voltage to ionize the spark plug gap. But big voltage numbers are only part of the equation. With a stock-type points or electronic ignition system, the voltage will only rise as high as needed to ionize the plug gap. For example, you may have a coil that puts out 60,000 volts, but it may only go up to 28,000 volts before the spark jumps that plug gap.

Millijoules (mj), a measurement of heat, is a good way to determine a coil’s ability to light the air/fuel mixture, but you have to pay close attention. Some coil makes list output as millijoules per spark, while others list it as millijoules per sequence. For example, Company A’s coil is listed at 300 mj per spark. That is six sparks per sequence, or 1,800 mj per sequence. Company B’s coil is listed at 2,400 mj per sequence. That translates to 12 sparks per sequence and 200 mj per spark.

What does that tell you? Company A’s coil provides a hotter spark, but less spark per sequence. It also tells you that Company B’s coil delivers more of a “multiple spark,” which aids combustion at engine speeds to around 3,000 rpm. That improves driveability and fuel economy. Above 3,000 rpm, there isn’t enough time to have multiple sparks, so a hotter spark becomes more important.

For your 360, we’d recommend something like an MSD 6AL ignition box, which delivers the multiple sparks, an MSD Blaster HVC coil rated at 42,000 volts, Taylor Spiro-Pro ignition wires, and platinum-electrode spark plugs. While they are more expensive, the platinum plugs need less voltage to jump the electrode gap. We would open that gap as wide as your engine will tolerate to give you a better chance of lighting your lean air/fuel mixture more consistently.050- to .060-inch should be close.