I’m having trouble with the Lucas coil on my 1950 XK120, I Got this new Mallory coil with my car, I measured the primary coil resistance and it was 1.8 ohms vs the standard coil I was using which had 3.7 ohms. Can I use it, or is the resistance too low? Thanks
I checked the specs of the coil.
Max Volts 40,000.
peak current 54 mA
Primary resistance 1.36 Ohms
Secondary resistance 10.37k Ohms.
Spark duration of 3.4ms
Is the ignition still points?
Are you running points? If so, then I believe you’re gonna want the ballast resister too MSD / Mallary Part # 8214 .08ohms resister.
On my two XK’s which are electronic ignition and Fuel Injected I swapped out the coils with MSD Blaster 2 coils & MSD HEI amplifiers & refreshed the condenser - everything else is stock.
I also ditched the ballast resister without a single issue, but I’m rethinking that decision anyway.
The MSD Blaster 2 coil
primary resistance is .07
secondary resistance is 4.5k
max voltage is 45,000
peak current 140mA
spark duration 350uS
I ran MSD Blaster 2 coil #8202 (with the stock ballast) in my '74 XJ6 (points) with out any issues. AROUND TOWN. I did eventually replace the entire ignition system distributor and all, not because of the coil cooking the points.
The distributor bushings were worn out and center shaft would flop around in a big circle over 2200 rpms. Played hell on the timing and the points. Got fed up and replaced it with a Pertronix system. I did have to use the Pertronix coil because the blaster coil would cook the Pertronix ignition module.
The swap was cheaper than repairing the OE distributor.
What kind of HT Leads are you running?
My thoughts on HT Leads…
Some of these higher output coils can cook (crystalize) the carbon in a carbon core wire, causing premature HT lead extra resistance (in the wire), which equates a weaker spark.
Best choice for points
If you don’t have a radio or care about EMI / RFI noise or about those next to you in an intersection, then your best bet is a solid core HT lead wire.
The trade off to low wire resistance & EMI / RFI suppression is a spiral core wire. Decent noise suppression still low resistance - equating to a better spark.
Points ignition
If it were me, I’d use the coil and recommended ballast resister. After the initial install, I’d monitor the coil temp make sure the coil isn’t overheating.
Oh, if the ignition was swapped out with an electronic variant, then use the manufacturers suggested coil and HT Leads. a mismatched coil can cook an ignition module.
Hope this helps
Mark
I’m running a stock ignition setup with points, I’m not sure about the HT leads, I purchased them from a usual jag supplier. Thank you, I’ll look into a ballast resistor, but any risk running it, and watching for overheating coil?
The XK120 with the original points system normally does not have a ballast resistor.
My coil died on the road once and I burned my hand trying to fix it.
I bought one from a nearby auto parts store, Standard Motor Products Intermotor UC-15, which works fine and does not need a ballast resistor.
Its $17 at Rock Auto or $16 on Amazon.
If your coil is still mounted on the block under the front carb like mine was, you might want to consider getting a later type mounting bracket and mounting it above the carb.
The coil is better mounted on the body, rather than the engine, as there is less vibration.
@Rob_Reilly I moved mine to top of the carburetor too. I ordered a new Lucas sports coil drop SNG for $24, thought it was a good price , but really love the look of Mallory coil, all chromed! It came with my car with a brand new Mallory twin points distributor, I still went with the stock rebuilt distributor, any experience with Mallory twin points distributor?
You may wish to consider heat dissipation in the coil. Coils have an internal resistance and sometimes this resistance is designed to be used with an external ballast resistor.
When the coil is powered and the points are closed the heating power dissipated in the coil goes as the square of the voltage across the coil primary windings and internal resistor divided by the internal resistance. If the voltage across the primary windings and internal resistor is 12 volts then the heating power dissipated in the coil while points are closed is 144/R watts where R is the internal coil resistance. For R =1 ohm, that would be 144 watts, for R =1.5 ohms, it is 96 watts, for R = 3 ohms it is 48 watts, and for R = 4.5 ohms it is 32 watts.
Coils with internal resistances less than 3 ohms typically have an external ballast resistor added, perhaps 1.5 ohms, along with a cutoff circuit employed during starting to aid in spark when the battery has a large current draw to the starter. That external ballast resistor also aids in reducing the current flow while points are closed and keeps the coil operating cooler.
Except for specialized applications, I have the impression most roadcar coil ignitions run with R between 3 and 5 ohms whether externally ballasted or not. For example, Lucas B.12 45012 coil is designed to have about 4 ohms internal resistance, no external ballast resistor, carries 2.5-2.88 amps current engine stopped and points closed, and averages 1.05-1.5 amps when engine is running. Engine at 2.88 amps and 12 volts is dissipating 34 watts and at 1.5 amps is 18 watts.
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While we’re on coils: can anyone tell me the specs of my Lucas Sports coil HS12 from 1950? I am using it with solid copper wire ignition leads, stock points, and Lucas plug suppressors 738113A (which were an option when the car was new). Should this ignition setup perform adequately?
Sport coils not using an external ballast resistor commonly are found with internal resistance of 2.5-3.2 ohms. There is a page at http://www.bobine.nl/jaguar/13-electrical/lucas-sports-coil/ which describes Lucas sport coils.
Sport coils helped on race cars running at high rpms where the coil secondary, spark voltage tends to taper down. The sport coil makes a higher potential spark voltage due to higher current in the primary because of lower internal resistance. If the car is not run at the highest rpms like in racing, then the spark plug will not likely benefit from the potential higher output voltage of the sport coil. Spark plugs break down when the needed spark voltage is reached and additional higher voltage possible on an open circuit coil output is not achieved. The cylinder spark occurs at the same voltage for a regular coil or a sport coil. Unless running at highest rpms, perhaps a sport coil will help make improve ignition on really dirty spark plugs. And a sport coil would help if the capacitor/condenser is not correct value in the distributor. A correct capacitor momentarily bumps the primary voltage in the spark circuit from around 12 volts up to 200-300 volts which boosts the output voltage considerably.
Yes, using a sport coil on your engine should work fine as long as the heat dissipation is not a problem.
Thanks, Roger. I was wondering if the suppressors would be adversely affecting the strength of the spark? However, Jaguar themselves fitted them, especially to export cars, so I’ve read, so there shouldn’t be a problem, I would think. I read on bobine that they are rated at 18 k ohms (sorry, can’t write the Greek Omega symbol).
A resistance of 18 kilo-ohms on the line for, or in, the spark plug should be fine. For example, Belden 734601 resistive spark plug wire which I use with a Lucas Model B.12 coil has 5,000 ohms per foot. A three foot wire is 15,000 ohms and a four foot wire is 20,000 ohms. I don’t worry about a 5,000 ohm difference between wires in this range of resistance.
Resistor and suppressor spark plugs have been in use for automobiles approaching 100 years. The goal is to knock down the radio frequency interference (RFI) produced by the spark. Letting the voltage rise to spark level and then limiting the peak current flowing by a resistor or suppressor works very well for most automotive applications. A 20 decibel RFI power reduction is a helpful factor of 100 less interference power. That RFI suppression helps remove unwanted RFI effects in the surrounding environment, particularly in modern automobile electronics.
There are a couple of subtleties when measuring resistance in coils, ballast resistors, and resistor or suppressor spark plugs and wires. A resistance measurement with typical benchtop meters may not provide a very accurate number towards the bottom of the range, say less than 10 ohms. And not all resistor/suppressor wires or plugs are purely resistive in their action and the resistance measure may not reflect the suppression capability.
Many thanks, Roger. I’m amazed at the expertise some people display on this forum! I think I largely get the gist of the technicalities you describe, and glad to know that my suppressors are not having any adverse effects.