MG Sports Cars

Related to my vapour lock potential listed below, I have started looking at my coil. It has seemed to be getting very hot, too hot to touch. I am ruinning Pertronix 1 with matching Flamethrower coil in a D&D Buick distributor. Coil has a resistance of 1.5 ohm. Pertronix claims its fine but they also state that max current should be 8 amps. Trouble is my voltage at the coil when motor is running was 14.5v. So current is 9.3, which could be too much.
Based on this I added a ballast resistor as a trial, 1.2 ohms. Coil voltage dropped to 10.5 and car has not died yet. Car does seem richer though so re set idle mixture with vacuum gauge.

So am I masking a potential failing coil by using a ballast? Should I replace with another pertronix coil (800 miles only but many years old). Get a lower ohm and higher voltage coil and run my new ballast resistor?

Thanks,
Mike

If you have a 1.5 ohm coil, a typical start/run ignition circuit uses a 1.5 ohm ballast in the 'run' circuit ('start' circuit is bypassed).
Or, alternatively, use a 3 ohm coil w/o a ballast - this type coil circuitry works fine but is not as forgiving if system voltage starts to drop.
Here's an article by Jeff Schlemmer specifically for Pertronix components:
[www.mgexperience.net]

FWIW - I use a 1.5 ohm coil and 1.5 ohm ballast and a Crane electronic points module and the Crane instructions say to retain the ballast....



Edited 4 time(s). Last edit at 08/29/2012 10:38AM by ex-tyke.

Graham,

Thanks for the link. Think he is talking about stock mg's though. I know the 3 ohms work but is it optimal for Petronixs on a V8?

Not sure that # of cylinders changes anything.
Here's installation info from the Pertronix website that may help:

[www.pertronix.com]

[www.pertronix.com]

There are several youtube video clip tutorials (courtesy of Moss Motors) of coil useage/resistance measurement/troubleshooting etc - here's a couple but there are others.
[www.youtube.com]
[www.youtube.com]



Edited 2 time(s). Last edit at 08/30/2012 05:32PM by ex-tyke.

With a V8 we have a shorter time to charge between fires. Now max current that the coil can get is a function of the resistance of the coil, lower resistance giving more current hence energy for the spark and better combustion if needed for a given condition.
Now charge time up to a given current is T=L*I/V, so I guess the big question is what is the inductance of the two coils? Will a 1.5 ohm be the same as 3 ohm? And what current/energy do we need at a given RPM. A V8 at 5000 rpm only has 0.003s to charge and fire! So we likely can only get 4 amps in 0.002 s leaving 0.001s for the firing of coil. Is 0.056 J enough energy (J=0.5*L*I*I).

3 ohm coils are specified for 4 & 6 cyl cars as they have too much time to charge and would create more energy than needed at lower rpm and get too hot. They were also used in point V8 as that all the points could take. Pertronix does specify 1.5 ohm for V8 cars with the Pertonix I module and no ballast.

Another problem is Pertronix 1 has no control over the dwell, so you don't get the true advantage of lower ohm coil and the increased energy available at lower RPM. Now this may be giving too much energy for the spark at lower rpm that just goes up as heat in the coil hence my issue.

Where are the Electrical Engg's in the group hiding?

Maybe time for pertronix III and the 0.3 ohm matching coil with its adaptive control of dwell, RPM limit etc.

Back to my original thought, my Flamthrower coil is likely failing and I have put a bandaid on it with the ballast resistor.



Edited 2 time(s). Last edit at 08/31/2012 09:47AM by mstemp.

Mike, If you can get a copy of Rick Astley's excellent book on MGB Electrical Systems, he delves at length on ignition systems/coil selection and electrical theory - I would recommend it!
Here's a short excerpt on the subject of 1.5 vs 3 ohm coils as found in late and early MGB's.

"An ignition coil with less turns exhibits both lower resistance (R) and lower inductance (L). The time (t) the coil takes to charge with magnetic flux is proportional to: L/R. If, as in a typical low resistance coil, the number of turns were halved both L & R would be approximately halved too and there would be no benefit to such a coil; both the numerator and denominator of the equation having halved, the time (t) would remain the same. However, if we add then some resistance, in the form of an external ballast resistor, to double the resistance (R) to it’s original value, the time (t) will be halved. At high engine speeds, when the coil has little time to charge, such an arrangement can result in a more reliable ignition.
Why not just make the coil using a higher resistance wire rather than use an external ballast resistor? [as in early MGB ….ex-tyke] That could be done but the amount of heat generated in the coil would be difficult to dissipate and it could be short lived. There is another benefit too. Having an external ballast resistor [late MGB. .. ex-tyke] gives the option of using it when wanted but of selectively taking it out of circuit at other times."

As far as electrical values of a std non-ballasted coil vs a ballasted coil, Rick has a chart that shows resistance values of >3 Ω
vs 1.3-2 Ω and Inductance values of >14mH vs >8mH respectively.

So called 'Performance coils' for non ballasted vs ballasted have values of 2-3 Ω and 0.5-1.2 Ω and inductance values of 10 to 13Mh and 5 to 7mH respectively

To get the full story of coil selection (and performance coils aren't necessarily better), you'd have to read the book



Edited 1 time(s). Last edit at 08/31/2012 10:07AM by ex-tyke.

Graham,

Good info but I think there might be a flaw in his data. Max energy is now decreased out of the coil, so yes you might get faster charging but to a lower total energy. Guess depends on what the motor needs.

I see we can get Megajolt for $160 plus Pick n Pull coil packs, EDIS etc. Maybe thats the way to solve all these single coil compromises!



Edited 1 time(s). Last edit at 08/31/2012 02:31PM by mstemp.

I really like the Ford EDIS system. As a package that is pre-engineered and relatively easy to adapt to any engine it is both very reliable and highly effective. There are two issues. You need to mount the trigger wheel and pick-up. Then it uses a sawtooth wave form to input timing advance from an external source. MegaSquirt provides that waveform. But it will run the engine without it, just not as efficiently.

With megasquirt or megajolt you do have the capability to run the coil packs directly and advance is internal. The newer versions can even drive individual coil-on-plug systems. They can be triggered from a distributor but a crank mounted trigger wheel is much better and tends to be more reliable as well.

Jim