By the DigiTune team · Updated June 2026
So once you’ve set your trigger offset / base timing, the ignition timing will always match the demanded timing in the ECU, right? Unfortunately that’s not always the case – and the trigger wheel itself is a big part of why. Read on to find out more.
How the ECU tracks crank position
The first thing to understand is that engine angle is not constantly monitored. It is updated several times per engine revolution, each time the crank or cam angle sensor sees a tooth on the trigger wheel. Between these updates, the ECU is guessing the crank position based on the time the last trigger edge was seen and the last measured engine speed. This is simple maths and works very nicely when engine speed is constant.
Why timing drifts when engine speed changes
Where problems tend to occur is when engine speed is not perfectly constant. At cranking speeds, the engine is speeding up and slowing down a lot due to compression forces. Engine speed can also fluctuate if the driven wheels are on the limit of grip or simply hit a large bump in the road. Sudden throttle changes can cause the drivetrain to flex too, creating engine speed oscillations.
An example: the 4-tooth NB MX-5 wheel
To show how much this can affect ignition timing, take the NB MX-5 as an example. Its trigger wheel has just four teeth, so the ECU only updates engine position every 90 degrees of crankshaft rotation. Imagine the engine turning at 3000 RPM when it briefly loses traction, causing the speed to rise by 200 RPM. Over that 90-degree gap between teeth, the worst-case position error works out at around 6 degrees of timing retard. If traction is regained and engine speed drops back by 200 RPM, the same error advances the timing by 6 degrees instead. These are worst-case figures, but they show how much the ignition timing can be disrupted by a sudden change in speed.
Upgrading to a 36-tooth wheel
If the trigger wheel is upgraded to a 36-tooth wheel, the speed and position are updated nine times as often as on the standard 4-tooth MX-5 wheel. The resulting timing error from the same speed change falls to less than 1 degree. On top of that, because speed is measured more frequently, it changes less between trigger points, so the speed error is reduced as well. With this pattern it would be unusual to see a timing error of more than half a degree.
Why didn’t Mazda fit 36 teeth from the factory?
When the MX-5 was originally designed in the late 1980s, ECU processing power was very limited. The ECUs simply couldn’t compute speed and angle measurements fast enough to make use of a 36-tooth trigger wheel. Even so, Mazda recognised the benefits of the higher tooth count, which is why they produced a small number of 36-tooth trigger wheels for racing series running aftermarket ECUs.
So why not 60 teeth, or more?
If 36 teeth is better than 4, why not go further? 60-tooth trigger wheels do exist and aren’t uncommon, but this is a case of diminishing returns. A 36-tooth wheel already brings resolution-based timing error down to negligible levels, so a 60-tooth wheel won’t normally provide any meaningful improvement – and it uses more processing power.
Swapping the factory 4-tooth wheel for a higher-resolution pattern is one of the most effective ways to tighten up ignition timing on a modified MX-5. Our 36-2 MX-5 Trigger Wheel is a direct upgrade for exactly this reason. Once it’s fitted, the next step is to have the timing dialled in properly – take a look at our remote tuning process to see how that works.