Coil on Plug Coils

I get a lot of questions as to what coils are suitable for use with the Rae-San  ignitions – and while some guidance in the forma of 2.5 to 5 ohms coils are suitable – I decided to do some investgation or readily available coil choices .

As part of this exercise i was looking for a standard type of coil fitment that I could recommend  – and also  looking for some suitable Coil On Plug (Stick Coils) that migth provide a sensible option – as some of these can be had quite inexpensively.

This page documents the Coil On Plug Results

You can find the Cinventional coil investigations investigations on this page – Conventional Coils

Tested Coils

Murray CDI Cops

Borrowed from CXPhreak to provide a comparison – these are essentially a CDI coil on top of a COP boot. They sit up fairly high on the valve cover.

”’ssrrThe parameters of interest are  resistance, charging time -in relation to the dwell time, spark strength and heat generated in both the Rae-San module and the coil itself.

Resistance

The resistance is a factor in that it determines the current drawn by the coil when it reaches saturation –
Once saturation is reached any addition dwell time is wasted as the coil is not storing any additional energy and must dissipate it as heat. As an example a 3 ohm coil with saturate at near 5Amps – and this is a power of nearly 75W into the coil – for no gain.

Dwell / Time to Charge

The time to charge the coils can vary greatly – as they have  widely varying inductance’s (L). as a result the L/R time constant – the time taken to roughly reach 2/3 of the max for the coil – can vary as well. We want to aim for the dwell time to be about 1.5 – 2.0 times the L/R time constant so as to avoid wasting current and generating excessive heat.

Spark Voltage

Without a high voltage probe this is a bit hard to measure directly – but I will grab a spark tester to be able to measure the gap jumped.
Typically the coils run about  100:1 turn ratio – so an estimate of the spark voltage an be made from the amplitude and duration of the spike on the drive side.

Spark Duration

This can be observed from the trace – and is directly related to how much energy is stored in the coil – the more energy the longer it takes to dissipate – note that a factor in this is the amount of resistance in the spark plug and cap – as this determines the current. No resistance will shorten the spark – making the spark current higher  – I recommend always having a single resistor  – either in the plug or in the cap – this gives the intended spark duration and generates less electrical noise – both back into the bikes own electrical system and to vehicles that you might be next to.

Coil Heat

This is the single biggest factor in the lifetime of the coils. They get hot – and then they sit above a hot engine – so they get even hotter. Any excess dwell time results in excess current – which results in excess heat.
At low RPM we don’t care so much – as the coil isn’t firing as often so overall a longer dwell here doesn’t add up to much.
At higher RPM we care A LOT. if you are running sustained operation at 6000 rpm say – that’s 100 fires a second ( on a wasted spark system)- every mS of extra dwell time  once near saturation becomes 0.1S coil operation and another 7.5W of heat in the coil.

Module Heat

The MOSFETs used in the Rae-San Ignitions are good – but not perfect – they have a resistance om about 1/4 ohm – so at saturation of say a 5A coil – they each dissipate 6W for the short time -if we are driving say a COP coil of 1.2ohm  – this rises to 25W for short peaks. Neither of these is a problem, but it is important to check whether additional heatsinking when mounting the module needs to be considered.

Test Results

Murray’s CDI Cop

Conclusion

A Version 3 module with GY6s was used to drive these with no issues, very little heat generation in plugs and good sparks.

38Kv output and good duration – like a oem coil on a plug. just a bit bulky.

Coil On Plug Conclusions

TAI Coil on Plugs are usually not operated as wasted spark except for CDI COPS. For the TAI COPs they need very tight control on the dwell to limit the heat generated at higher RPM. Compared to a non-wasted spark system – the COP fires twice as often – effectively seeing double the RPM – so this could be the equivalent of up to 20000 RPM if it were non-wasted speark system.

CDI Coil on plugs dont store energy internally so don’t have anywhere near as much self heating and are commonly used on high reving two and 4 stroke engines running wasted spark.

As part of this exercise the need to drive High currents for COPs in the 1.2 ohm range allowed assessment of some areas where the board layout for the Rae-San V4 module could be improved – these were incorporated in a new revision and bench tested as version 4.2.

Also the software was updated to allow for another timing mode to provide tighter control of the dwell  – this is rolled into the standard software.

If you want to use a COP – CDI is the recommended mode.

If you must use a TAI cop – then the dwell needs to be controlled down to the 2mS mark to avoid the COP cooking itself in sustained cruise or  high RPM running.

Users wishing to use a TAI style COP will need to contact me for the 2ms firmware to be loaded.