Pros and Cons of Miata's Distributorless Ignition

Distributorless ignition has been widely used in today's cars, but the one used in Miata is quite unique and has its pros and cons. While most distributorless ignition system carries one coil per cylinder, Miata's ignition system carries one coil per two cylinders. Without distributor, how does each coil distribute spark energy to two cylinders at the right time?

Each coil takes care of a pair of cylinders that are out-of-phase by 360 crankshaft degree (or 180 camshaft degree). Then, Miata's ECU (Engine Control Unit) can fire both cylinders, say 1 and 4, at TDC at the same time. That means each spark plug is fired two times in each 4-stroke cycle, namely the end of compression stroke and the end of exhaust stroke.

At first I found it hard to believe that firing a spark plug at the end of exhaust stroke (or the start of intake stroke) would NOT affect air-flow path of exhaust gas or intake gas. But if that's the fact, I take it for granted. Then, there's this 14-18 degree BTDC ignition timing practice that proved to dramatically affect engine performance. Back in 1985, I already knew that ignition timing advance helps low-end torque. I've tried it in my 1/10 RC car, my '80 civic, my '85 Toyota Corolla GTS, and my '90 Miata. I have to admit that my '90 Miata has the greatest response to the change of ignition timing comparing to the others..

Twin-spark ignition is probably the reason why Miata gives great response to ignition timing change. In addition to the well-known effect of advancing ignition timing prior to combustion stroke, the effect of advancing ignition timing at the end of exhaust stroke in the mean time contributes to low-end torque as well. During exhaust stroke, the piston goes upward and pushes exhaust gas out of the cylinder through the exhaust port. If there is any unburnt hydrocarbon remains in the exhaust gas, the spark at the end of exhaust stroke helps accelerate the gas out of the exhaust port somewhat. Since the exhaust valves are closing towards the end of exhaust stroke, the later the spark ignites, the harder the gas can escape. Also, the later the spark, the higher the possibility of disturbing intake gas pattern.

So, the advantages of Miata's twin-spark distributorless ignition seem obvious:

  • needs less mechanical parts than regular distributored ignition.
  • cleans exhaust gas by burning unburnt hydrocarbon.
  • helps accelerate exhaust gas out of exhaust port somewhat.
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    However, there are disadvantages to the "twin-spark" feature as well:

  • spark plugs burn twice more often thus wear out faster.
  • higher load on coils and reduced frequency response, i.e. reduced output at high engine speed.
  • higher combustion chamber temperature.
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    It is the last point mentioned that brings in the temperature spiking problem mentioned in my next topic.

    Next: ConeKeeping the Hot Miata Engine Cool

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