low ohms coil ?

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Offline Skunkworks Challenger

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Re: low ohms coil ?
« Reply #15 on: June 26, 2011 - 07:10:18 PM »
You guys are talking way over my head but interesting.  I went to start my 70 Challenger with 440 and Mopar Performance electronic ignition and it would not start.  I would get a spark occasionally
although kind of weak.  The system, including a new coil from NAPA has about two hours run time and had been working well. I had not started the car for about three weeks and it was running
fine the last time I shut it off.  I put on an old coil from the previous engine and it started right up.  I don't know anything about the specifications of either the new or old coil.  Since I have to
buy a new coil what should I get.  I want to stay stock looking but want reliability.  What do I tell my NAPA guy that I want?  PK
« Last Edit: June 26, 2011 - 08:36:14 PM by Skunkworks Challenger »
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Offline Bullitt-

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Re: low ohms coil ?
« Reply #16 on: June 26, 2011 - 08:12:56 PM »
I suspect with some people the size of plug gaps just follows the old American philosophy:

If something is good then more must be better and too much will be just right.

No offence to the yanks - LOL

Works for Bourbon    :drunk:
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Offline UKcuda

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Re: low ohms coil ?
« Reply #17 on: June 27, 2011 - 03:27:04 AM »
Works for Bourbon    :drunk:

 :cheers:

PK,  I don't know what value your ballast resistor is but if your old coil is anywhere round about 1.5 to 1.7 ohm (which it probably is) then you should be OK just staying with that, or get one of around that value.  It probably won't be marked on the coil so if you want to check it you need to get hold of a multi-meter with a very low ohms range (a lot of them have 0-200 as the lowest range and that's too high to be measuring a coil).

Alternatively, if you can get hold  of a voltmeter set it on 0-15v DC (or similar range) and attach it across the coil terminals with the coil connected on the car.  Then ground the -ve side of the coil and switch on the ignition (but don't crank it).   If the meter reads around the 7-9v area then that coil should be fine.  But try not to leave the ignition switched on like that for very long.
« Last Edit: June 27, 2011 - 03:35:33 AM by UKcuda »
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Offline Skunkworks Challenger

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Re: low ohms coil ?
« Reply #18 on: June 27, 2011 - 10:59:28 AM »
UKcuda, thanks, I'll do some testing and see what I have.  You mention not to leave the ignition on too long without running.  Maybe that is what happened to the two hour run time coil.
 I've been doing electrical work to get my radio, turn signals and other items hooked up and probably left the key on when I should not have.  Do they overheat internally if they are not discharging?
PK
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Offline UKcuda

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Re: low ohms coil ?
« Reply #19 on: June 27, 2011 - 07:24:22 PM »
UKcuda, thanks, I'll do some testing and see what I have.  You mention not to leave the ignition on too long without running.  Maybe that is what happened to the two hour run time coil.
 I've been doing electrical work to get my radio, turn signals and other items hooked up and probably left the key on when I should not have.  Do they overheat internally if they are not discharging?
PK

Yes they can do that, the stock ECU defaults to being a closed connection with ground so if you leave it with the ignition on and not running you are basically creating an "all dressed up and nowhere to go" situation which can get hotter than it is designed for. 

Depending on the resistance values and the quality of the components you can end up cooking something that way.  Maybe you cook the coil or maybe the ECU or maybe you get away with it. 

The classic situation is being parked up somewhere with the ignition on so you and your gal can listen to the radio - can you picture the scene, the music, the moonlight etc. etc. ? - everything is going pretty good until you try to drive off = not going so good anymore !
'72 'cuda

Offline Talkwrench

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Re: low ohms coil ?
« Reply #20 on: June 27, 2011 - 08:31:25 PM »
OK UKCuda [or others] I was discussing this with my friend in the US and he said this about the Cuda.. What do you think?

"If it were me - I think I’d put the 1.5 OHM  coil in the ‘Cuda - and get rid of the ballast resistor (jumper it with heavy copper), go to full power. This diagram shows how they were configured as designed - they had a full voltage lead off the solenoid that gave the coil full volts when the starter was engaged. There are MOPAR controllers - like the “chrome” base unit - that probably have a chip very similar to the HEI in them."

now his is a Ford guy but it makes sense..?   :dunno:   See attached

Also how far away can you mount the coil from the dissy? would the length in the wire help resistance[add]

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Offline Skunkworks Challenger

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Re: low ohms coil ?
« Reply #21 on: June 27, 2011 - 09:14:51 PM »
I'm sorry if it looks like I'm trying to hijack the thread.  I checked my "failed" coil today and it showed 1.35 ohms.  I checked the old one I had for a spare and it showed 1.8.  The 1.8 starts
the car while the 1.35 does not.  My ballast resistor shows an output of 5.6 volts.  Is the low voltage why the 1.35 ohm coil does not work?  Do I need more voltage at the coil?  PK
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Offline UKcuda

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Re: low ohms coil ?
« Reply #22 on: June 28, 2011 - 01:24:11 AM »
OK UKCuda [or others] I was discussing this with my friend in the US and he said this about the Cuda.. What do you think?

"If it were me - I think I’d put the 1.5 OHM  coil in the ‘Cuda - and get rid of the ballast resistor (jumper it with heavy copper), go to full power. This diagram shows how they were configured as designed - they had a full voltage lead off the solenoid that gave the coil full volts when the starter was engaged. There are MOPAR controllers - like the “chrome” base unit - that probably have a chip very similar to the HEI in them."

now his is a Ford guy but it makes sense..?   :dunno:   See attached

The ballast resistor is bypassed when the starter is engaged because otherwise the battery would struggle to turn the engine and charge the coil at the same time.

Once the engine is running the coil receives power from the battery/alternator system running at close to 14v.  Without a ballast resistor a 1.5 ohm coil would be pulling quite a lot of current through the ECU.  I don't think even the Mopar chrome boxes will put up with that sort of load for too long.  As far as I know none of the Mopar boxes has a HEI style chip in them (unless you put one in there yourself).

I think Pertronix say that you can use their 1.5 flamethrower coils without a ballast resistor for racing, by which they mean for short periods during drag racing passes.


Also how far away can you mount the coil from the dissy? would the length in the wire help resistance[add]

It would not make any difference except maybe cause a misfire, the distributor wires go to the ECU, not to the coil, they only have a small current in them, you want as little resistance as possible in them.

In any event, a long wire would not simulate a ballast resistor, and why bother when you've already got a ballast resistor ??  Fords have a resistance wire (usually pink) instead of a ballast resistor, it's made from special resistive wire.
'72 'cuda

Offline UKcuda

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Re: low ohms coil ?
« Reply #23 on: June 28, 2011 - 01:38:46 AM »
I'm sorry if it looks like I'm trying to hijack the thread.  I checked my "failed" coil today and it showed 1.35 ohms.  I checked the old one I had for a spare and it showed 1.8.  The 1.8 starts
the car while the 1.35 does not.  My ballast resistor shows an output of 5.6 volts.  Is the low voltage why the 1.35 ohm coil does not work?  Do I need more voltage at the coil?  PK

The voltage reading doesn't mean much unless you say which coil was connected at the time.  I am guessing it was the 1.35 ohm coil and the reading of 5.6v across the coil means that the coil is close to being the same resistance as the ballast resistor (because you have approx. 12/2 volts).  If that's right then with the 1.8 ohm coil fitted the reading ought to be more like 10v.

Again, if that's right, then you just need to be using the 1.8 ohm coil or similar (not the 1.35) and you will have a good voltage at the coil.

I don't know why the 1.35 coil doesn't work - it doesn't seem like a voltage problem since it worked before.
'72 'cuda

Offline Talkwrench

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Re: low ohms coil ?
« Reply #24 on: June 29, 2011 - 08:24:58 AM »
Maybe I might be best at just getting the Pertronix 2 with  .06ohms coil for the Cuda? :thinkerg: And later down the track get the second strike to match... :naughty:
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Offline UKcuda

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Re: low ohms coil ?
« Reply #25 on: June 29, 2011 - 03:42:23 PM »
Maybe I might be best at just getting the Pertronix 2 with  .06ohms coil for the Cuda? :thinkerg: And later down the track get the second strike to match... :naughty:

Dunno 'bout the second strike, but if you already have an electronic distributor I think the Pertronix 2 is really just a more expensive way of obtaining what the GM (or Bosch) HEI module can give you for like $25.  And for about another $25 you can get a really good coil to go with it.
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Offline Talkwrench

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Re: low ohms coil ?
« Reply #26 on: June 30, 2011 - 12:55:37 AM »
Yes you are right..  :thumbsup:
"Came too close to dying to stop living now!"