12 volt coil on 9n

I am looking at building my own kit. I think I can do it fairly cheap. What I was wondering is if you need the napa coil or is it the same to use the square can 12 volt coil thats included in kits. I heard its better to use the 12 volt coil than a six volt with resistor is this true. I'm reading mixed reviews on using a resistor with 12 volt coils like you do with a 6 volt, do you still need to and why? Does it look better to use the square can 12 volt coil.
Thanks in advance.
G.Bott

Bottsie........because of the "weirdness" of the 4-nipple frontmount distributor, its always eiziest to use the squarecan ignition coil. Either 6-volt or 12-volt squarecan ignition coil NEEDS the MANDATORY "infamous ballast resistor". Understand???

Due to the LAWS of electricity, you can pull a cheap trick and use a 12-to-6v converting resistor (2.5-ohms, 50watts; $5 cheap) with the 6-volt coil stedda buying a $40 12v-squarecan ignition coil.

Iff'n yer handy, you can make a converting adapter to convert the squarecan coil base to mount the more robust 12v roundcan coil. Ya pays yer money, and makes yer choices. .......Dell, a 12v advocate for the right reasons

Go by your local junk yard and get a alternator for a 80's model GM vehicle that uses a S10 by Delco. Here's a diagram using the 6V coil. David..............

when using a 6v coil on 6v system , the resister drops voltage at coil to approximately 3v or 1/2 , so is it right to think you can use a 12v coil on 6v system without a resister

I explained this in my post below to jeffthompson. Maybe you missed it.

The answer to your question is a practical application of Ohm's Law.

Technology & materials being what they were in the 30's, that square coil would melt if it ran on much more than 4 amps for any length of time. (see tip # 38 for an example). In order to get a hot spark at the same time the starter was drawing max current from the battery, a ballast resistor was added in the ignition circuit. What that did was add about .3 ohms of resistance in the circuit, added to the 1.5 ohms of the coil. That got you 3.5 amps or so at start up. As the voltage increased when the engine was running to about 7.5 volts, the resistor heated up, adding more resistance in the circuit. 1.0 ohms hot, plus 1.5 ohms of the coil got you down to 3 amps or so to keep from melting the coil. The same rule (actually, Ohm's Law) applies to a 12v circuit. I= E/R. Current equals voltage divided by resistance.

Now, do the math on a 12v system.

First, no matter what coil you use, you must use the OEM ballast resistor.

14.5 volts, 1 ohm on the ballast resistor & 3 ohms on the coil.

14.5v divided by 4 ohms gets you 3.6 amps. That's probably ok. But, herein lies the problem: what if the alleged 12v coil is only 2 ohms (and some are). 14.5v divided by 3 ohms gets you 4.83 amps. Guess what that will do to your points & coil?

You can use a 6v coil & add more external resistors. You can use a 12v coil & add fewer. Or, if you are lucky, none at all.
50 Tips

Boler.......back to school for you. The original 6-volt squarecan ignition coil is actually a 3-volt coil, therefore the "infamous ballast resistor" drops the tractor's 6-volt power to the designed operating voltage of 3-volts. For convenience, we refer to the original coil as a 6-volt coil.

Your question as I understand it, could you use the new modern 12v-squarecan coil WITHOUT the mandatory ballast resistor in a 6-volt system? Umm?.....a qualified yes. Since the summ of the OEM 6-volt coil resistance (1.2-ohm) and the nominal resistance of the "infamous ballast resistor" (1.3-ohm) is 2.5-ohms, the same as the reported resistance of the new modern squarecan 12-volt coil. It should work, for a while.

Given the less than stellar performance of the modern 12-volt squarecan ignition coil, I don't think it would last very long before it barffs its internal insulative tar.

The unfortunate thing about BOTH squarecan ignition coils, NEITHER are adequately marked as to operating voltage or part number. Yet they can engrave the country of origin. (frown)

Since this is an academic discussion, and NOT a recommendation, I would NOT use the modern 12-volt squarecan ignition in a 6-volt system. .......respectfully, Dell, yer self-appointed sparkie-meister

You're sure now about that?

Sure about what? Ohm's Law? The math? What is it you are questioning?

I was asking if you could use a 12v coil on a 6 volt system without using a resistor, 1/2 the voltage should equal 1/2 the current then if the 12v coil was only 2 ohms wouldnt the amps using 6 volts be less than 4? Just a point of view??

The issue is the resistance value of the coil as well as it's ability to w/stand the heat generated by the current. A round coil w/ oil is going to w/stand the heat much better than the tar/resin filled square coil. All coils are not equal even if the resistance is.

Soooo........doing the math & ignoring the heat:

7.5v, 2 ohms = 3.75a

7.5 volts, 2.5 ohms = 3a

But, what is the heat? And, unless you measure the coil, you won't know if it's 2 or 2.5 ohms.
50 Tips

My thoughts are that the heat in the coil is produced over and above ambient by the current passing through the low voltage winding so if 4 amps is OK when using a resistor why isnt less than 4 amps OK without a resistor?

Because the external resistor is dissipating heat outside of the coil. As the coil heats up, it's resistance actually increases. The resistance of copper wire increases by 4% for each 18* F. increase in temp. A 2.5 ohm coil at 50* isn't going to be 2.5 ohms at 150*. It's going to be closer to 3 ohms. If the coil is 1.5 ohms w/an external ballast resistor, the 1.5 ohm coil at 150* is going to be 1.8 ohms. Or something like that....I'm doing the math in my head!

(quoted from post at 21:56:56 07/31/09) Because the external resistor is dissipating heat outside of the coil. As the coil heats up, it's resistance actually increases. The resistance of copper wire increases by 4% for each 18* F. increase in temp. A 2.5 ohm coil at 50* isn't going to be 2.5 ohms at 150*. It's going to be closer to 3 ohms. If the coil is 1.5 ohms w/an external ballast resistor, the 1.5 ohm coil at 150* is going to be 1.8 ohms. Or something like that....I'm doing the math in my head!

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Thats why mine still has a 6v everything, including a 6v Positive ground, one wire Chevy alternator.

(quoted from post at 22:02:35 07/31/09)

(quoted from post at 21:56:56 07/31/09) Because the external resistor is dissipating heat outside of the coil. As the coil heats up, it's resistance actually increases. The resistance of copper wire increases by 4% for each 18* F. increase in temp. A 2.5 ohm coil at 50* isn't going to be 2.5 ohms at 150*. It's going to be closer to 3 ohms. If the coil is 1.5 ohms w/an external ballast resistor, the 1.5 ohm coil at 150* is going to be 1.8 ohms. Or something like that....I'm doing the math in my head!

Click to expand...

Thats why mine still has a 6v everything, including a 6v Positive ground, one wire Chevy alternator.

Click to expand...

Ifn I ever do decide to go 12v.

It will be with a coil conversion.

http://www.theviperr.com/hobo_dnn/Default.aspx?tabid=69

OK Nuff on this for now

OK Nuff on this for now

(quoted from post at 23:50:13 07/31/09) OK Nuff on this for now

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Who died and left you to decide what is enough?!!?!?!??!!?!?

(Just asking)