Duraspark, need a resistor or not?

[Rory428]

I have been running factory Duraspark ignitions for decades, both factory installed, and as a conversion from points. On my FE powered Fairmont drag car, I ran a recurved junkyard Duraspark for over 30 years, with a MSD 6T ignition box, and on my 74 F350, I added another junkyard Duraspark with a factory blue Duraspark ignition module, and in over 25 years, never had an issue with either. Currently, I have a 428 CJ in my 59 2 door sedan, with yet another (recurved) junkyard Duraspark distributor, along with a factory Duraspark blue module. I have been driving the car for about 5000 miles over the past 3 years, and all seemed fine. However, last weekend, a racing buddy from 4000 miles away, came for a visit, so I let him run my 59 at our local dragstrip, while I ran my Fairmont. This was the first time racing the 59, with the fresh 428, and it seemed kinda weak, and he said that above 4500 RPM, it seemed kinda lazy. (on the dyno, pear HP was at 5400). It wasn`t misfiring, just didn`t pull as well as I thought it should. Anyhow, as an experiment, I bypassed the 59s factory firewall mounted external ballast resistor (no idea of the amount of resistance), and ran full 12 volts to the ignition. Next pass, the car picked up 4 tenths of a second, and he said it ran much stronger, so we left the resistor bypassed, and made several more 1/4 mile passes, and then drove it home, and I have been driving it some since seems fine.  However, after doing some googling, I am seeing conflicting information. I have seen some opinions where they say to run 12 volts, other say it needs a resistance of 1.10 ohms, some say 0.7 ohms. Just wondering if anybody actually knows for certain. I have heard that there was "Duraspark 1", and "Duraspark 2" versions, supposedly the small cap models were the 1, and the large 2 piece cap were the 2. Since the only FEs with factory Durasparks, were used on 75 and 76 trucks, and all those that I have seen had the small cap, I ASSume the truck FEs were all Duraspark 1. So... to make a long story a bit shorter, I am wondering if running 12 volts will be detrimental to a long life of my ignition, or if I need to run a certain amount of resistance. The engine runs stronger at 12 volts, but don`t want to fry anything, as 2 hour drives to various car meets is not uncommon. I have heard that later (60s-70s Fords) used a pink resistance wire, at least on a points application, but no idea what factory Duraspark vehicles used.

[Stangman]

I could be wrong but I always thought that if it was electronic ignition you got rid of the resistor wire.
I found out the hard way when I put a Mallory unilite in my FE years ago. Car would start for a second and die couldn’t figure it out. My uncle asked me if I took the resistor wire out and replaced it. Well changed the wire and walla.

[Katz427]

Yes, the duraspark should have about 6.5 to7 volts, running, and 12 volts at start. The only, exception was the MSD SVO duraspark ignition, which ran on 12 volts. These were sold through Ford Motorsport too qualify as an approved ignition , for NASCAR. Today you can use a Mallory, or Accel ignition Superbox that runs on 12 volts if that is what the engine likes.

[GerryP]

If you have a spare Duraspark module, you can always test the theory that 12V will damage the module.  I don't think so but a spare module isn't very expensive.  I looked it up out of curiosity and you're right; there are strong opinions on resistor and no resistor.  Odd one there for sure.  You would think this would be known.  But, if you just get rid of the Duraspark box and convert to an HEI module, you have to run 12V to the coil and HEI module.  You need to use 12AGW wire to the coil and module to handle the 5.5amp draw and a .45ohm coil to take advantage of the HEI's ability to maintain the coil at 5.5amps and to vary dwell.  The conversion takes very little time.

[MeanGene]

I am setting up a Duraspark with MSD 6AL (today) on my test/ break in stand, on a customer's CJ. I have it set up with a ballast resister and use a dependable points dist for cam break ins and such, and am building a small platform for the MSD box, coming off the upper trans bolts (C5TA bell) so wiring will be pretty simple. I can just bypass the BR to get full switched 12V to the MSD. My thought for use in a car with the resister wire would be to just trigger one of those little box relays with the stock resister wire so the relay could feed 12V to the MSD. Sound right?

[galaxiex]

I am setting up a Duraspark with MSD 6AL (today) on my test/ break in stand, on a customer's CJ. I have it set up with a ballast resister and use a dependable points dist for cam break ins and such, and am building a small platform for the MSD box, coming off the upper trans bolts (C5TA bell) so wiring will be pretty simple. I can just bypass the BR to get full switched 12V to the MSD. My thought for use in a car with the resister wire would be to just trigger one of those little box relays with the stock resister wire so the relay could feed 12V to the MSD. Sound right?

Sure the relay will work.
Just be aware sometimes you need a diode in the relay coil (trigger) circuit.
Otherwise the relay gets "stuck on" and the ignition won't shut off.


Had that happen on an old chebby small block that I converted to an HEI distributor.


Used a relay to power the HEI distributor, and it wouldn't shut off until I put a diode in the relay coil circuit.

[Rory428]

To add to the confusion, I have seen wiring schematics for the Duraspark system, that shows no resistor at all on the Duraspark 1, and a 1.05-1.10 ohm resistance for the Duraspark 2 system. I suppose that I could use the current blue Duraspark module as a "sacrificial lamb" ,since I do have several of them, from a wrecking yard binge purchase years ago at a local wrecking yard. So if one does fail from running 12 volts, I can replace it in a few minutes. Just wondering when the timeline for the change from Duraspark 1 to Duraspark 2 took place. I would ASSume since the last FE PU truck was made in 1976, it would be the Duraspark 1, but that is guess my guess. I don`t really want to use a non Duraspark module, as they have been so reliable, and I have a nice location where I mounted the blue box, where the voltage regulator used to be , from the original generator, a MSD box won`t fit nicely there, and although my 30+ year old MSD 6T has been trouble free, I had 2 later 6ALs fail in my 85 Mustang Stocker, so not thrilled with MSDs reliability in recent years. As for the HEI module, that is a hard NO, there will be ZERO number of GM parts on my Fords.

[Chuck]

The duraspark 1 came out in the early seventies and was short lived. No resistance wire to the coil. The spark box would turn off coil power if the distributor wasn't turning or you weren't cranking so it wouldn't burn it up with the key on. Duraspark II does use a resistor wire for run that is bypassed for cranking. Power to the box itself is key on battery voltage. It is always a good idea to run a good supply through a relay rather than through the key switch and of course make sure it is also powered while cranking.

[BigBlueIron]

I have never used a resistor on a Duraspark box. Maybe to the coil depending on what coil I'm using.


I recently did a complete harness from american autowire for a 78 F100, while they did provide a resistor for the coil, the box receives 12v. I didn't use the resistor for the coil either, just got a coil that didn't need one to keep the wiring cleaner.

My understanding of the Duraspark, the two wires that receive power both 12v, one is key hot the other is hot while cranking, either hooked to the I terminal of the solenoid or even on the same large terminal of starter wire. This wire advances timing, something like 10 degrees for starting.
I have one I keep around with a simplified harness to start random engines and just use alligator clips and I can never which wire is which. They will run fine either way. I took timing light to it once to verify this hooking one up at a time and starting the engine but don't remember how much it was.

This is based on Duraspark II or blue strain. I could be wrong but I don't think the distributor cares, as mentioned the 75-76 truck Duraspark distributor is the one I always use, no matter what box i.e. MSD, Mallory or Duraspark.

[HarleyJack17]

Chuck is right and so is BigBlueIron.
The module does not care. The box and coil does normally. The D2 does use a resistor wire, and a 12v wire for cranking only, and a different coil. Most aftermarket boxes, HEI, or stand alone(Pertronix) require a 12V coil and 12V full switched power to operate correctly. Good grounds are a must too.

[427Fastback]

I have always run my Duraspark conversions on 12V.I have never had a problem with any of them.I always use the blue strain box.The last one I did the owner supplied a painless wireing harness...It came with a resistor.My 68 has been duraspark since Ford Motorsports introduced the conversion harness in the early 80's.Both my small block and later the 427 used it and both engines would cleanly run past 6000rpm..I too have always wondered about the resistor and have never found a good answer...

[Henrysnephew]

   Many years ago Mr Davis (of DUI Ignitions) wrote a tech article in one of the car mags about Duraspark conversions.  He claimed that running system voltage to the coil (no resistor) would overheat the control box to the eventual point of failure.  He recommended a 1.4 ohm resistor for "normal" driving but installing a toggle switch to bypass the resistor only when maximum ignition output was called for.
   I have done this on several street/strip builds over the years and it seemed to work out fine - best of both worlds.
Randy M
   

[GerryP]

To me, Randy, that is baffling.  Could Ford not come up with a technological solution to allow the system to function on full battery voltage?  That just seems daft and it is hard for me to believe the system is designed with a resistor limiting current.  The biggest limitation of running a points ignition is the current limiting necessary to save the points from managing battery voltage in the RUN position.  They will burn up the contacts.  In a Duraspark, there is no voltage going to the distributor.  The magnetic pickup generates the voltage signal to trigger the module to function the coil.  That process is outside of the coil side of the primary ignition.  From what I can tell on the Duraspark module, it has two wires connecting it to the coil; one to coil- and one to coil+.  This is pretty conventional for electronic ignitions.  The coil gets its voltage from the ignition key wiring.  So, battery voltage in the RUN position runs through a resistor, to the coil+ with a wire from the coil+ running to the module.  The Coil- is a tach signal wire and a wire runs to the module.  The module sees current only on the primary side and its only purpose is to function the secondary side of the ignition.  Again, I can't work out why a resistor is necessary in that system.     

[Katz427]

Randy, you read the Davis article too. Yes, the coils used for the duraspark as well as most OEMs were rated for 6.5 volts input on the primary, in order to get acceptable  life from the coil. The Motorsport coil was rated for 14 volts input , it was oil filled but basically the same canister  size, they didn't  last as long, running 12 v, but did make it through a 500 mile race, most times, ( there was always a backup system, at ready).

[427Fastback]

I dont think the dist or the box have a issue with the 12 Volts..Most of the schematics are based on OEM applications using a OEM coil.OEM coils don't like to run on 12 Volts.I have never had a problem with duraspark and all the conversions i have done but i dont run a OEM coil.I use MSD coils.MSD says there stuff is 12 Volt friendly...Just a thought.....Cory