Conventional 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 form 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 results of the Conventional Coil Tests

You can find the COP investigations on this page – Coil on Plug

Tested Coils

AB7030 Coil

Ordered from wemoto and listed as a gl500 coil –

WRONG – CDI COIL

Note that while the coil has two lug terminals they are at 90 degress to each other – this might be a gereral indication on CDI coils

OEM HondaTAI coil

Original CX TAI 12V coil for reference

The 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 additional 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 will 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 roughtly 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 1.2 ohm  – 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.

Testing Results

Summary

AB7030 Coil

AB7030 as 12V coil

AB7030 as CDI Coil

Conclusion

NOT RECOMMENDED as  a 12V – this really seems to be a CDI coil although it works as a TAI style coil. If run at 3.5ms rather than the higher dwell time of 5ms – the heat in the coil is manageable  –

If run as a CDI coil – its quite GOOD.

Spark :  —
Cost    : $$
Mount: Stock
Leads:  Stock

OEM Honda TAI Coil

Conclusion

Reference Coil from a cx650 – 3.0 ohm. at 5ms dwell gets warm but not hot  from sustained 6600 Rpm and 28KV+ out. At 3.5 mS dwell run colloer with only a very slight drop in output – works with either dwell.