Coils

[John L]

What should the secondary coil resistance be for a 3ohm coil?

Will a high energy coil like a Flamethrower or a Bosch blue one,  have a higher secondary reading than say a std Lucus DLB 101?

I have several 3ohm coils with slightly different secondary values, and wondered which value would give the better spark?

John

[RobH]

The resistance of the HT winding will depend on the number of turns used and the gauge of the wire and those will vary from maker to maker. There probably isn't any simple direct correlation between resistance value and spark quality as there are lots of other factors too which will influence that.  Typical values for the HT winding are around  9 to 10 thousand Ohms.

[John L]

Thanks Rob,  The Lucas new one I have 9.12 on both +&-, the new Bosch Blue one 7.68 on both, Bosch one on the car is 8.90 on - and 8.86 on the +, does this mean on this one its getting a bit weak?

John

[RobH]

3 minutes ago, John L said:

does this mean on this one its getting a bit weak?

That is a surprising difference John.  The coil is an auto-transformer so one way round the primary resistance is included and the other it is not but since the primary is only around 3 Ohms it should hardly register on the meter when reading thousands of Ohms, and certainly not show 40 Ohms difference like that.  Strange -  it might just be the meter I suppose but it read the others OK........ 

 

[John L]

Those last readings are for the secondary readings, I suppose they should be the same on the + to the centre as to the - to the centre?

John

[RobH]

The coils are connected like this:

So when you measure from HT to  the - terminal it is just the secondary coil resistance .  When you measure from HT to + it is both coils in series. The difference should just be the resistance of the primary. 

[KiwiTR6]

From memory my Flamethrower measured around 12K when I checked it the other day.

[J R Martin]

Anyone know where I can buy a Bosch Blue coil, they seem to be like hen's teeth at present and I need one for a spare?

Thanks

John

[Mike C]

23 minutes ago, J R Martin said:

Anyone know where I can buy a Bosch Blue coil, they seem to be like hen's teeth at present and I need one for a spare?

Thanks

John

This one?

https://motociclo.com.au/product/ignition-coil-12-v-bosch-blue-guzzi-universal/

[John L]

Martin,  Not knowing where you are based from your profile, here is the reference and a supplier in UK, there may be many, if that helps,

https://www.merlinmotorsport.co.uk/p/bosch-blue-high-energy-coil-0-221-119-027-sport-coil

This is a 3 ohm coil.

John

Edited March 19, 2023 by John L

[Waldi]

Martin,

make sure you select a coil with the correct resistance:

early TR6 (Cp): 3 Ohm, later (CR): 1.5 Ohm; later cars have a resistance in the “feed wire”, which is by-passed during cranking (starting).

If you are not sure which is needed, measure the voltage with the contact switched on (but NOT cranking).

The early cars should have the same as “battery voltage” (say 12.5-13V), the later cars will have around 9 V (or so) measured at the coil.

Waldi

 

Edit: I’m sure Rob will explain it better, I’m a bit cryptic I think.

Edited March 19, 2023 by Waldi