Ignition: COIL...INTERNAL vs EXTERNAL RESISTOR???

[jclary]

OK...Last week, while buying "stuff" to attempt to fire up the 289 on my test stand, I decided to buy new ignition components rather than keep cannibalizing rusty components from my spares. I have an old coil with a ceramic resistor attached to the mounting bracket. Instead, while at the parts store, I asked for a new coil. I was given the choice of an external or internal resistor coil. I bought the internal type. Reason being, it is one less component I have to wire up or fool with. I can exchange it if some of you can give me a good reason I should use the external resistor type.

Therefore, before I mount the coil, wire it up, and put marks on it, I thought I'd come here and ask if anyone can give me a good reason to choose one over the other.

I'm sure that I can return this one, but I hate buying for new, parts that look like they had been used. So, if I get this one dirty, I will keep it rather than return it. I do have a couple of mowers & tractors that I could use it on.

[r1lark]

Are you setting this up as a 6v or 12v engine John? I'm sure you are aware that a 6v engine won't need the resistance.

I believe the shop manuals give the resistance values for the 12 v cars, you can double check to make sure the resistance values of the coil is about right.

But if you put this engine in a car that has a resistance wire built into the harness, you won't be able to use that coil unless you bypass the resistance wire.

[jclary]

Paul, for now, on the test stand, I am going with 12 volts. The starter is a 6 volt starter, but that should work as long as I don't overheat the starter. Currently, there is no wiring harness. I plan to make one. It will consist of nothing more than required to run the engine on the stand. If this engine checks out, my plan is to put it in my 55 truck. It is a truck configured 289. As far as I know, all my six volt coils are "internal" resistor. In the past, when merely replacing components on a car, I've always replaced parts that were already there. In this case, this was a mostly complete engine, with zero electrical components when I bought it. So, this is actually the first time I've had the opportunity to develop my electrical system from scratch. Either way, there will be a resistor in the circuit. Like a lot of these little projects, thinking and talking about it is always easier than doing it. I have already attached an oil pressure gauge, so I can see the pressure while pre-oiling with a dummy distributor shaft run by a drill motor. I have run engines without a generator, but I think I will install one here, if for nothing else but to use it as a belt tensioner for the water pump pulley. While doing that, might as well install a voltage regulator and build a charging circuit too. In addition, a temperature gauge, amp meter, and if I get ambitious...a tachometer.

[11SecAvanti]

Using the non resistive coil allows for direct 12 volts to the coil when the engine is cranking. That is the purpose of the "green wire" from the solenoid. A resistive coil will defeat this by lower the cranking voltage to the coil.

[70Avanti2]

I use a internal resistor coil on a 12 volt system with a 6 volt starter on a small tractor for decades without starting problems.

[gordr]

Your six-volt coils don't have an internal resistor, because they don't need one. They are wound to give their full output on six volts. The move to 12 volts was largely dictated by higher-compression engines that needed more cranking speed. Cranking a cold engine could "pull down" available battery voltage from 12.6 to as little as 10 volts. So they went with coils rated at about 8 volts, and fitted them with an external resistor to limit primary current in normal operation, but bypassed that resistor by means of contacts in the solenoid to give a hotter spark when cranking. Or, more precisely, to give a normal spark while cranking, when it might otherwise be degrade by the starter pulling down system voltage.

There are also non-resistor 12 volt coils that simply have more and finer wire on the primary side, so they allow for proper current flow on a twelve volt circuit without a resistor. The Bosch "Super Blue" coils, beloved by air-cooled VW enthusiasts are so made.

[christophe]

For what it's worth, my only car with an external resistor (pink wire) is the Hawk. All the others (french cars mostly) don't have one and I never encountered starting problems even in cold wheather with the oldest ones. I'd say you're good to go with what you have. Unless its already done, don't forget to change the spark plug wires for new ones. This makes a huge difference.

[345 DeSoto]

Use the internally resisted coil...for the reason you stated. Just don't try to use it with a resistor wire, or ballast resistor. Belt and suspenders doesn't work, in this case...