Posted by John T on July 10, 2016 at 05:57:03 from (216.249.81.101):
In Reply to: Ignition Coil 12 volt posted by gdtractor on July 09, 2016 at 19:48:30:
Great question, good info below, I will try and give a short sweet not too technical explanation, but that's hard for me being an attorney and engineer lol.
1) When you read all the hype about HIGH VOLTAGE COILS MAYBE 50,000 VOLTS BLAH BLAH BLAH remember that at the same (within coils capacity) conditions the stock coil will fire at "near" the SAME VOLTAGE as "Super High Voltage Coil" as the voltage only rises high enough so current will arc jump the gap and that necessary voltage (maybe 10,000 volts) is a function of THE GAP DISTANCE AND THE MEDIUM (fuel and compression) IN WHICH IT FIRES.
2) However, an after market "high voltage" coil has the "capacity" to achieve a higher (then stock) firing voltage if necessary due to say use of a much wider plug gap, say 0.035 to even 0.060 or so.
3) A High Energy after market coil CAN INDEED CAN DELIVER A HIGHER "ENERGY" SPARK then a stock coil but they dont work well with old points ignition because they "can" draw more current then points can handle and you get premature burning THEREFORE USE OF AN ELEC SWITCH IS THE ANSWER AS THEY "CAN TYPICALLY" HANDLE MORE CURRENT THEN POINTS.
4) Even with a stock coil an elec switch delivers fast more positive current switching then bouncing ringing points so it can help, HOWEVER to get the most bang for the buck afforded by an elec switch, couple it with a high energy high voltage coil and run a wider plug gap AND YOU END UP WITH A HIGHER ENERGY DISCHARGE ACROSS A WIDER PLUG GAP.
5) Unless youre running exotic fuels and super high compression and an extra wide plug gap theres "less need' for an elec system but hey that's your choice, they have advantages as well as disadvantages. But if you invest in an elec switch Id go ahead and opt for a high energy high voltage coil so you get a higher energy spark.
CAUTION be careful of using a high energy coil with old points as if its LV primary resistance is to much less then 3 ohms on a 12 volt system and you draw more then 4 amps of coil current, THE POINTS CAN BURN PREMATURELY.
CAUTION if you start running wider plug gaps and high energy high voltage coils you need good quality cap and rotor and plug wires to withstand higher voltage and current
TO YOUR SPECIFIC QUESTIONS:
a) I am a little confused about what I need for maximum spark.
For more spark energy you need a higher energy coil and a switch that can handle high enough coil current.
b) One person told me to make sure my coil was a 3 ohm coil
If its a 12 volt tractor with points THATS GOOD ADVICE because current is 4 amps which points can handle
c) another recommended an "Accel Super Stock 8104 12 volt" coil, The "more power" Accel coil says it's "Primary resistance is 1.4 ohms and secondary resistance is 9.2 kilo ohms" .. (not sure what this means).
1.4 ohms LV primary resistance is a tad less then Id recommend for points but it will "work"
d) Again ... I am not sure about this "resistance" thing .. the one I had on it said "12 volt use without external resister" ...
Primary ignition resistance is the total of the coils LV primary PLUS any external series ballast resistance.
A coil labeled "12 Volts" or "12 Volts NOT for use with ballast" or "12 Volts NO ballast required" IS JUST WHAT IT SAYS you can use it on a 12 volt tractor as is with no need for an external ballast resistor, its a full true 12 volt coil
CONTRARY to Old Wives tales or what Bubba told you, the typical old farm tractor coil (Not talkin a very few old 30's car coil) DOES NOTTTTTTTTTT HAVE ANY INTERNAL DISCRETE STAND ALONE RESISTOR TUCKED AWAY INSIDE THE CAN. It DOES NOT have an internal "ballast resistor". Its primary resistance is a function of the wires resistance and length.
NO WARRANTY I'm long retired as an engineer and rusty on this and cant cover every imaginable situation nor explain in a paragraph what takes books and years to understand so the above is only a rough short approximate answer TAKE OR LEAVE IT.
THIS IS NOTTTTTTTT TO START NOR ENGAGE IN A POINTS VERSUS ELEC IGNITION AS THAT WASNT YOUR SPECIFIC QUESTION, that's a whole other topic and debate I'm not addressing here.
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