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MG MGB Technical - Which coil should be used with Mallory Unilite?
|I've installed a Mallory Unilite distributor with my Lucas Sport Coil, however, the Moss catalog says this coil should not be used with electronic ignition systems. The wiring diagram for the Unilite shows a ballasted system ignition coil.|
|the Lucas sport coil I have has resistance built in so an external resistor is not needed. Mallories I do believe need resistance. Use a VOM to measure the coil resistance if you're not sure.|
|I installed the matching Mallory dizzy to match the Unilite. I found the model through their tech support (it probably still shows on a search of their bbs). It has the built in resistance required for the proper opperation. It was a much easier route than introduction a ballast...at least to non electrical MG nut.|
If you have trouble finding the model, shoot me an email and I'll pop the hood.
|A standard 12v coil has a primary resistance of about 3 ohms, and a 6v coil about 1.5 ohms. A Lucas 12v Sport coil I have measures about 2.4 ohms. The 6v coil on an MGB must be used with an external ballast which from the factory was a length of resistance wire contained within the loom. Whether or not the 12v coil contains any resistance is immaterial.|
The reason for changing from a 12v coil to a 6v with external ballast is that during cranking the system voltage drops to 10v or less and hence ignition HT voltage suffers accordingly. The 6v system allows the ballast to be bypassed during cranking, which connects the full system voltage to the coil, and so you get *more* HT energy duruing cranking, when you need it most, than when running.
Ordinarily electronic ignition should not need a 6v coil as the voltage pulse to the coil is largely independant of input voltage. This voltage is in the order of a couple of hundred volts so giving very much higher HT spark energy. If you connect a high voltage pulse to the primary of a 6v or sport coil you may get problems with HT breakdown.
However that is with full HV electronic ignition systems. Electronic triggers like the pertronix switch 12v to the coil just the same as points so the same reasoning does not apply. But although the Pertronix can control a 6v coil people often have to run an extra wire in from the ignition to power the Pertronix, and the ballasted supply on the coil +ve somethimes causes problems.
Since Unilite systems seem to be available with external electronic modules I suspect the internal module is just a 'pointless' trigger like the Pertronix, and indeed Pertronix supply an Ignitor for the Unilite.
It may well be that the the Unilite, and the Pertronix, are not capable of switching the higher currents of the 12v sport coil. Indeed one set of instructions I have read states that on no account should a 6v coil be used *without* a ballast or damage will be caused to the electronics.
|If I were buying a replacement electronic distributor I would buy a DUI. When you order one they ask you to complete a form listing what is in the engine and how it is to be used. The distributor is put on a machine and curved for your engine. I know it's high at $425. The Moss unilite lists for $499.99 plus you need a coil, then you you need a rolling road to set it up. http://www.performancedistributors.com/mgdui.htm|
I have not tried one and I'm not associated with Performance Distributors, but it looks to be the best solution for getting a unit to match you engine specs. Clifton
This thread was discussed between 07/05/2004 and 08/05/2004
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