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Technical stuff => Electrical => Topic started by: JohnFol on April 30, 2021, 04:59:40 PM

Title: Ignition Coil - replacements
Post by: JohnFol on April 30, 2021, 04:59:40 PM
Afternoon all, I am working through the electrics on a carb spyder and found the ignition coil insulator had a crack. A bit of gentle "encouragement" and I'm in the market for a replacement. Philosophy is to use more modern equivalents rather than source a 40 year old "authentic" coil

Any recommendations or I'll just do the generic Halfords shop

Title: Re: Ignition Coil - replacements
Post by: Nigel on April 30, 2021, 10:09:22 PM
Hello John,

There's possibly a difference between a coil for points and condenser, and one
for electronic ignition. I would seek numbers from your existing unit
and attempt a replacement from your local motor factors.

I use Halfords for, well, not much.


Title: Re: Ignition Coil - replacements
Post by: peteracs on April 30, 2021, 10:55:54 PM
Hi John

I did a bit of Googling and this came from an article on Xweb forum and appears a sensible write up.


“To generate a spark, the coil's primary winding current is interrupted by the points, or by an electronic switch. This causes a collapse in the magnetic field which in turn generates a high voltage in the secondary winding. In order to perform well at high revs, the coil can't have too high a primary resistance because the current takes a finite length of time to build back up after it's interrupted to be enough to generate a good spark the next time it's interrupted. This is known as "dwell" time. Points have a fixed dwell angle, controlled by the points gap that you set. This means that at low revs the dwell time is long and at high revs the dwell time is short. Thus for points the dwell angle is a compromise between adequate sparks at high revs, and overheating the coil at low revs. Along with this, a 12V points type coil needs a resistance of around 3 ohms to give good performance without overheating. The big advantage of electronic ignition is that it can provide the correct dwell time to suit the revs, thus not overheating the coil at low revs and still giving a good spark at high revs. Therefore electronic type coils can have much lower resistance, typically 1 to 1.5 ohms, giving a good high peak current and a fat spark at all revs.

The upshot is if you use a points type coil with electronic ignition you are not getting the best out of the system. If you use an electronic type coil with points you will overheat it and burn the points. If you use a too low resistance coil for your electronic system, you will overstress it and possibly release its smoke”

Title: Re: Ignition Coil - replacements
Post by: WestonE on May 01, 2021, 08:12:41 AM
Hi John

These are Bosch coils for electronic ignition PN Years ago I spoke to Bosch technical and they told me it had been superseded by My Lancia technical book gives the coil resistance figures as 1.2 to 1.6 Ohms Primary winding at 20 degrees c. secondary winding 6000 to 10000 Ohms. They are significantly different to a coil for points including the earlier points Marelli item. You can look at later dry coils if the resistance figures are close. The IE Dry type is primary 0.42 to 0.46 secondary 7560 to 9240 Marelli BAE 500B so not suitable.



Title: Re: Ignition Coil - replacements
Post by: smithymc on May 01, 2021, 08:58:11 AM
On points a generic coil will be fine I would say. If you convert standard distributor to contactless like I did, a better coil is recommended. I got both from H and H Ignition in Brierley Hill, West Midlands. Coil not expensive as I recall.


Title: Re: Ignition Coil - replacements
Post by: JohnFol on May 12, 2021, 09:26:22 AM
For those interested I spoke with Ignition Car Parts as it was getting complex. They recommended this part as it's a direct replacement for the factory fit BOSCH 0 221 122 031 coil (assuming mine was the original) (