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Posted By Discussion Topic: coil resistor? or not?     fenbach 07-31-2021 @ 9:25 PM reply profile send p.m. Member Posts: 227 Joined: Dec 2009 i want to experiment with a coil adapter and modern coil on a '37 distributor. the instructions with the adapter kit say to bypass the original coil resistor. no instructions with the coil itself. do modern [6 volt] coils have a resistor built in? or need one? for that matter, does it matter? my coil is a Duralast C809. thanks. ford38v8 07-31-2021 @ 9:50 PM reply profile send p.m. Senior Posts: 2764 Joined: Oct 2009 All the experiments have been done, and the results are in: Your best shot is to put 11A counterweights in your '37 distributor and run a Skip Haney rebuilt original coil with the original resistor under the dash. And yes, it does matter if you run a resistor or not, unless you want to change your points every time you turn around. Alan This message was edited by ford38v8 on 7-31-21 @ 9:51 PM carcrazy 07-31-2021 @ 11:13 PM reply profile send p.m. Senior Posts: 1668 Joined: Oct 2009 If you run a modern 6 volt coil with the coil adapter on your '37 distributor, you do not need to use the resister as the coil has a built in resister. You will want to bypass the resister located under the instrument panel in your car. mikefamig 08-01-2021 @ 5:55 AM reply profile send p.m. Member Posts: 1 Joined: Dec 2014 Your goal is to deliver the correct amount of current to the distributor points. The amount of current is determined by Ohm's Law "E=I*R" or "Volts = amperage x resistance". Your voltage is fixed at approx 6 volts and your resistance is determined by the resistance of the primary windings in your ignition coil plus any ballast resistor in the circuit. I do not know the correct specs for the 37 Ford but I copied the following explanation from The Jalopy Journal forum: Heres what we know . its a flathead using the old style distributor with remote coil and a six volt battery. Heres what the factory bullitens say from April 1938: The normal resistance of the ford V8 primary circuit is 1.00 to 1.35 ohms. If the resistance of the primary is less than normal , then more current will pass thru the primary and shorten the life of the breaker points. So if we know what the resistance spec is actually then we can calculate the actual amps needed for proper spark. 1.35 ohms divided into 6 volts = 4.4 amps or the spec current for this system. This 1.35 could be a .35 resistor and a 1.0 ohm coil or any combo of the two. IE= No resistor would just need a coil of 1.35 ( i would suggest a 1.5 to lower contact current) If a resistor is used it would need to be watts=volts x amps so 6 volts x 5 amps would = 30 watts..... Ok now that we have the numbers correct ( whew that took a few of us) Lets fix this car......... Get a oil filled coil of 1.5 ohms etc hook it up direct and start the engine and drive it !!!!!! No resistor needed !!!!!!!!! Hope this helps, Mike. JayChicago 08-01-2021 @ 7:19 AM reply profile send p.m. Member Posts: 484 Joined: Jan 2016 MikeFamig: Good info there. Thanks for doing the research and posting. fenbach: You should use an ohmmeter between the two posts on that Duralast C809 coil to verify that it's primary winding is about 1.5 ohm. Then it should be used without a resistor. You can bypass the resistor under the dash. Original coils are close to 1.0 ohm, and need the resistor which is about 0.4 ohm. BTW, I agree with ford38v8 that the best, most reliable thing is a rebuilt Skip Haney coil. He uses modern technology in the internal windings. Have proven to be very reliable. Adding that adaptor plate to use a modern coil just adds another source of potential problems. supereal 08-03-2021 @ 10:31 AM reply profile send p.m. Senior Posts: 6819 Joined: Oct 2009 The resistor in the coil circuit is a kind of voltage control to lessen the damage to the points and coil as the engine speed increases and the generator output goes up. It is a "nichrome"coil, such as was used in old toasters. As the voltage goes up. the coil heats and reduces the output. If you have a modern coil, or a coil that has been rewound, the resistor become less necessary. The original Ford coils use a kind of waxed paper to insulate the windings. Any prolonged higher temperature and/or voltage can cause shorts between the windings and either reduce output, or cause failure, kirkstad 08-04-2021 @ 9:32 AM reply profile send p.m. New Member Posts: 192 Joined: Jul 2017 When I sent the original coil from my 36 to Skip Haney, I ran an adapter with a 6 volt Napa coil, it worked great. The Napa coil was marked that it had an internal resister so I bypassed the under dash resister. I am now running my rebuilt coil and the car runs like new. I'm not sure that all 6 volt coils have an internal resister like a Model A's for example, but I'm not sure. Frank fenbach 08-31-2021 @ 7:27 PM reply profile send p.m. Member Posts: 227 Joined: Dec 2009 i'm stumped. i tried both my duralast coil and then a 6-volt NAPA echlin coil with my coil adapter, and the engine misses at RPMs higher than idle. but if I hot wire in my skip haney coil with little jumper wires, it runs fine. the NAPA coil says no resistor needed, by the way, and i did bypass the resistor. i connected the "+" side of the coil to the distributor and the "-" side to the ignition. hope somebody has a clue. ford38v8 08-31-2021 @ 11:23 PM reply profile send p.m. Senior Posts: 2764 Joined: Oct 2009 Yes, here’s your clue: take off the adapter and toss it. Install the skip Haney coil powered through the ballast resistor. Don’t forget the condenser. Alan juergen 09-02-2021 @ 8:32 AM reply profile send p.m. Member Posts: 258 Joined: Jan 2010 Back to your question of do modern coils have a resistor built in: all coils have resistance. some also use an external resistor. When the industry shifted to 12 volts, most (all?) went to a 4 pronged starter solenoid. When the starter was engaged, 12 volts went directly to the coil. When the ignition was on an external resistor or resistance wire was used to limit the coil voltage to about 9 volts to extend point life. This is similar to what 32-48 designs using a resistor which increased resistance as it got hot. After 1949 Ford used the solenoid solution instead of the thermistor solution to provide max voltage during start and reduced voltage during run. Using a coil with the 1.5 ohms results in less voltage during starter running which is already dragging down the battery voltage. Use the original Ford coil (including the external post 40 type which needs an external resistance) and keep the relevant external resistor circuitry for engine running.

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