12 Volt Coils

[Brian Rupnow]

I am about to build a "shock box" consisting of a 12 volt automotive coil, a 12 volt dry cell battery and a switch, with two electrical leads to go to the internal battery and two to go to the engine and ignition points, with one high tension lead which goes out to the spark plug. This "shock box" will be used to run a number of different i.c. engines.
The only coil I can buy at local automotive stores all have written on the case "Use with external resistor".---Now if I remember correctly, an external resistor allows a full 12 volts to the coil to allow maximum spark for cold engine starts, and then as the external resistor heats up from current flow, the voltage is gradually cut down to 8 volts flowing to the coil, which supposedly let the ignition points last longer. Since none of my small i.c. engines will run for longer than 1/2 an hour at a time, I don't believe I need to bother with the external resistor (Which my auto parts store can't find a source for anyways).---Any opinions on this?---Brian

 

[Noitoen]

The resistor limits the current when the engine stops with the points closed. In a car, if you leave the ignition switch on long enough, with the points closed, you will burn the coil. With a model engine I wouldn't bother. If you want to find a resistor, you can use the one that limits the current to the injector on a Mk3 92/93 Ford Fiesta.

 

[Brian Rupnow]

I just checked it out on an automotive web site. It said that coils which require an external resistor are in reality 9 volt coils. The external resistor lets a full 12 volts flow to these coils for an extra hot spark on start up, but then reduced the voltage to 9 volts when the resistor had warmed up. If you remove the external resistor, the coil will run too hot and self destruct prematurely. A true 12 volt coil does not require an external resistor. Since the coil I purchased is still in its original box, I will return it Monday for a 12 Volt coil . I know the folks I bought it from sell a "Hi Performance" coil that does not require an external resistor, because I have one on my Kerzel engine.

 

[f350ca]

Any original equipment 12v coil that worked with points ran with a resistor. Chrysler used a ceramic cased wire wound one that sat on the firewall, (they should be easy to get, I had one in my 1946 Willies jeep after I switched it to 12v), GM and Ford used a resistive wire in the harness. While the starter was engaged 12v was fed to the coil, when the starter disengaged the resistor was in line. Electronic ignition coils might be 12v though.

 

[Philipintexas]

I've had good results using 12V motorcycle coils for my IC engines. I also use a resistor and this set-up works perfectly, plus it's smaller than most automotive coils. I can get freebies from motorcycle shops.
Another option that works better is to use an automotive relay that turns a standard coil into a "Buzz" coil that produces multiple sparks when the contact points close. This provides much hotter multiple sparks for excellent ignition. It will also "rock your world" if you get in the way!

 

[123RWO]

Instead of trying to roll your own, You could buy a CDI system with battery pack that fits in one hand. See http://www.cncengines.com/ic.html Their system works with points or a hall effect sensor and a 4.8 V nicad pack. The Deans connectors would make it very easy to switch between different engines. They also have a buzz coil box with everything already assembled.

RWO

 

[Brian Rupnow]

RWO--You're a bit late out of the gate. This is for the Atkinson engine which I have been building, and its already kitted out with Chrysler ignition points and condenser. The Webster is also similarly equipped. However, thanks for reading the post and for the suggestion.---Brian

 

[123RWO]

What's the problem, Brian? As I stated in my previous post, the CNC Engines systems are designed to work with points if that's what you have.

RWO

 

[steze48]

Hello all...
I am new to your group and presently building the Webster...
I have purchased a Chinese ATV coil from ebay and have points, condenser, and a 1/4-32 spark plug...
I wired everything up to test the spark and it works...
I placed a piece of wood between the points as not to short the coil too long...
I jumped the points to get a spark a few times and found out that the coil was getting hot fast...
I assume I need a resistor between the hot wire and the coil...
Is the Chrysler ceramic type compatible with this set-up?
I found a wiring diagram for the Chinese atv and they show the CDI box, a hall censor off the flywheel...
The coil is not wired like a points ignition but has the one hot wire from the cdi box to the +side of the coil and the other side to ground...
Will I be able to use this type of coil with points ignition or should I shoot for CDI ignition by purchasing the cdi box and hall censor for this atv?

Steve

 

[Jordan]

I think coils for CDI differ from Kettering type. Some difference in how they saturate, and that CDI makes a spark directly upon being fed current, whereas Kettering fires when current is interrupted. Hope I got that right, but bottom line is: Use appropriate coil, according to whether CDI or Kettering (traditional points) ignition.

Jordan

 

[canadianhorsepower]

Instead of trying to roll your own, You could buy a CDI system with battery pack that fits in one hand. See http://www.cncengines.com/ic.html Their system works with points or a hall effect sensor and a 4.8 V nicad pack. The Deans connectors would make it very easy to switch between different engines. They also have a buzz coil box with everything already assembled.

RWO

I've use these kit's on chainsaws up to 16500 and they are simply perfect

 

[Lew Hartswick]

I think coils for CDI differ from Kettering type. Some difference in how they saturate, and that CDI makes a spark directly upon being fed current, whereas Kettering fires when current is interrupted. Hope I got that right, but bottom line is: Use appropriate coil, according to whether CDI or Kettering (traditional points) ignition.

Jordan

May well be the current methodology but in the 60's I built a Capacitor Discharge box
and ran it on an original conventional system for years. In those days it was a bit more
difficult to do the hardware.
...lew...

 

[steze48]

I think coils for CDI differ from Kettering type. Some difference in how they saturate, and that CDI makes a spark directly upon being fed current, whereas Kettering fires when current is interrupted. Hope I got that right, but bottom line is: Use appropriate coil, according to whether CDI or Kettering (traditional points) ignition.

Jordan

Thanks Jordan...
I did a search on types of coils and found this out...
Live and learn!
I plan on building several engines as I am hooked on this hobby!
I'll set up a CDI system at a later date...

Steve

 

[cfellows]

Instead of trying to roll your own, You could buy a CDI system with battery pack that fits in one hand. See http://www.cncengines.com/ic.html Their system works with points or a hall effect sensor and a 4.8 V nicad pack. The Deans connectors would make it very easy to switch between different engines. They also have a buzz coil box with everything already assembled.

RWO

One issue I've experienced with this product, and I'm not talking about the buzz coil, is that it creates a spark on both the contact and break if you are using points. I've not tried it with a hall effect, but would think the results are the same.

Chuck

 

[steze48]

I went ahead and purchased the CDI box and Hall sensor which they call a stator trigger pick-up coil...
I'll experiment with this set-up but for my Webster I bought the right coil and will make a nice oak box to house the components and mount the engine on top as I have seen on others...
GREAT FUN!!!

Steve