A coil for use on ballasted systems is nominally a 6v coil, and *must* be
used with a ballast as otherwise it will burn points and could overstress
other types of trigger. A 12v coil for use on non-ballasted systems should
be used without a ballast, using a ballast is not harmful but will reduce
spark output.
6v coils are nominally 1.5 ohms primary resistance, as is a typical loom
ballast used on MGBs, making 3 ohms in all. A 12v coil is nominally 3 ohms
resistance i.e. much the same primary current when running. A 'sport' coil
in my possession measures about 2.4 ohms and is also for use at 12v i.e.
unballasted systems. From the factory chrome bumper MGBs had 12v coils and
no ballast, rubber bumper cars had 6v coils and loom ballast.
With points it is easy to check whether you have any ballast in circuit or
not. With the points closed measure the voltage on the coil +ve/SW, and if
it measures 12v there is no ballast and needs a 12v coil, there could be
either a 12v or a 6v coil fitted. If is measures 6v it has a ballast and a
6v coil which is correct. If 9v then probably a ballast and a 12v coil
which is incorrect.
The ballasted system is better is with the original starter motor the
ballast is bypassed during cranking to boost the voltage to the coil which
results in *greater* spark energy then when running, i.e. the opposite to
what happens when cranking with a 12v ignition system.
PaulH.
----- Original Message -----
> I have been trying to get advice on what ignition coil I should us on my
> MGB GT boasting a Rover V8 3.5L with Lucas Opus distributor and Lumenition
> 'points'.
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