So once you’ve set your trigger offset / base timing, the ignition angle/timing will always match the demanded timing in the ECU right? Unfortunately that’s not always the case. Read on to find out why.
The first thing to understand is that engine angle is not constantly monitored. It is updated several times per engine revolution when the crank or cam angle sensor sees a tooth on the trigger wheel. Between these times, the ECU is guessing the crank position based on the time that 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.
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 the 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 also cause the drivetrain to flex, creating engine speed oscillations.
As an example of how much this can impact ignition timing, an example of an NB MX-5 trigger wheel will be used. These have four teeth on the trigger wheel. Imagine an engine rotating at 3000RPM which sees a brief loss of traction causing the speed to rise by 200RPM. Over a rotational period of 90 degrees, the worst case position error would result in 6 degrees of timing retard. If traction is regained and engine speed falls 200RPM, it would increase timing by 6 degrees. These are worst case examples but they demonstrate how much the ignition timing can be disrupted by a sudden change in speed.
If the trigger wheel is upgraded to a 36 tooth trigger wheel, the speed and position are updated 9x as often compared with a standard 4 tooth MX-5 trigger wheel. The resulting timing error from the same example speed change is less than 1 degree. Not only that but because speed is measured more frequently, speed can change less between trigger points and so speed error will be reduced. It would be unusual to see timing error of more than half a degree with this trigger pattern.
So why didn’t Mazda fit a 36 tooth trigger wheel in the first place? 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 use a 36 tooth trigger wheel. However, even Mazda recognised the benefits of the higher tooth count, which is why they produced a small number of 36 tooth trigger wheels to be used in racing series with aftermarket ECUs.
So if 36 teeth is better than 4, why not more? 60 tooth trigger wheels do exist and are not uncommon. However, this is a case of diminshing returns. A 36 tooth trigger wheel is already a high enough tooth count to bring resolution based timing error down to negligible levels, so a 60 tooth wheel won’t normally provide any significant improvement and uses more processing power.