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coil question

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Brett E Brett Evans
Columbus, Ohio, USA   USA
1973 Triumph TR6 "Scarlet Harlot"
I have a 1973 TR6 with a Pertronix ignition. My car is supposed to have a ballast resister wire going to a 1.5 ohm coil. I hooked up the Pertronix neg to the coil and the pos to the fuse block per the instructions and the car ran fine for years. In the course of a major project this summer (more on that on a later post), somehow the pos battery cable became loose where it connects to the solonoid and shorted out which burned out the ignition. I'm going to replace the Pertronix as I never had a problem in actual running. My question is this: When I first installed the ignition I wasn't very familiar with how the ballast resistor wire actually hooks up to the coil and didn't realize that a PO bypassed it. If I continue to use the 1.5 ohm coil will it eventually cause problems? Since the ballast wire has been disconnected do I need to use a 3 ohm coil instead?

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Tonyfixit Avatar
Tonyfixit Tony M
Duncan, British Columbia, Canada   CAN
I belive if you continuously run 12-14v to that 1.5 ohm coil it will eventually burn out.
The answer is to either use a 3 ohm 12v coil, or supply reduced current to the 1.5 ohm coil via a ballast resistor.
BUT ensure that you petronix receives it's power BEFORE the resistor.

Also if you opt for using the resistor, you may also want to run a Yellow/White wire from the starter solenoid to the + side of the coil to Boost power to the coil while the engine is being cranked by the starter motor.

Brett E Brett Evans
Columbus, Ohio, USA   USA
1973 Triumph TR6 "Scarlet Harlot"
Thanks Tony--the Yellow/White wire is connected to the pos terminal, it's only the cloth covered resistor wire that's been disconnected. The reason I'm asking the question is I see that the newer Igniter II uses a .6 ohm coil and some companies are marketing it as compatible with the TR6. I've never been accused of being the brightest bulb on the tree, but I don't know how that would work. I would think that running full voltage through it would burn it up in short order but putting a ballast resistor in would defeat the purpose.

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Genoa Bay Eric Williamson
Genoa Bay, BC, Canada   CAN
Brett,
Glad you mentioned the Ignitor II as most forums recommend a 3 ohm coil with a ballast bypass. Whereas Pertronix recommends their 0.6 ohm coil with the Igniter II.
Note that some suppliers are selling the Igniter II but only stock the 1.5 or 3 ohm Pertronix coils.
Eric.

TR3driver Randall Y
Confusion, Los Angeles, USA   USA
The Ignitor II is smarter than the earlier version, and uses variable dwell to limit current through the coil. That makes it much less prone to overheat, as well as work with lower resistance coils.



Randall
56 TR3 TS13571L daily driver
71 Stag LE1473L awaiting engine rebuild
7? Stag awaiting gearbox rebuild

Pumkins John Phillips
Claremore, Oklahoma, USA   USA
The ballast resistor wire reduces voltage to the points to prevent burning them. With Pertronix you have no need for the Ballast Resistor and it should be bypassed. If your coil worked ok before it should still be ok.

Brett E Brett Evans
Columbus, Ohio, USA   USA
1973 Triumph TR6 "Scarlet Harlot"
Part of the reason for my question was because I was wondering if the .6 ohm Flamethrower II can handle a straight 12V, why wouldn't the 1.5 ohm be able to do the same?

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TR3driver Randall Y
Confusion, Los Angeles, USA   USA
It can, provided you use only an Ignitor II to pass power to it. The Ignitor II does it's own current limiting (through variable dwell) and so should work fine with a 1.5 ohm coil and no ballast.

But, the Ignitor I (which is what most people think of when you say "Pertronix"winking smiley does not have adaptive dwell; neither do the points. In both of those cases (or if you think you might ever have to convert back to points on the side of the road); using a 1.5 ohm on 12v requires a ballast.



Randall
56 TR3 TS13571L daily driver
71 Stag LE1473L awaiting engine rebuild
7? Stag awaiting gearbox rebuild

Brett E Brett Evans
Columbus, Ohio, USA   USA
1973 Triumph TR6 "Scarlet Harlot"
I thought the coil passed power to the ignitor, not the other way around. I'm also thinking (now that's a dangerous thing in itself) that both coils convert 12 or 6 volts to 40,000. The 3 ohm coil has more resistance which seems to me that it has to work harder to do the conversion than the 1.5.

Like I said, I'm not the sharpest knife in the drawer!

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TR3driver Randall Y
Confusion, Los Angeles, USA   USA
Well, perhaps I phrased it poorly. The Ignitor is what controls current flow through the coil primary (and voltage across it). The Ignitor II in effect senses the current, and cuts the length of time that voltage is applied (by not connecting to ground during the rest of the period) to limit overall power dissipation in the coil (and in the pass transistor inside the Ignitor module). The Ignitor I (and points) do not do that, they just pass full current (as limited by the ballast and coil primary resistance) for a fixed portion of each cycle (known as the "dwell time"winking smiley.

The coil works by storing energy in a magnetic field, then releasing it suddenly. It's the sudden collapse of the magnetic field that creates the high voltage. The field is stored in a magnetic core; but it has a definite limit in how strong a field (how much energy) it can store. Once the field has reached it's maximum, any additional current just makes more heat.

It does take some time for the magnetic field to build up again after each spark; so the low resistance coils do have an advantage at high rpm where the time between firings is shorter (especially on 8 cylinder engines). But with points (or a 'dumb' Ignitor I), that has to be balanced against excessive power dissipation at lower rpm.

Another point worth repeating, the actual voltage only rises to what it takes to fire the plug. If the plug is firing at 10kV; it makes no difference at all if the coil could generate 40kV. Like being stuck in traffic with a 500hp engine, it just wastes energy and money.



Randall
56 TR3 TS13571L daily driver
71 Stag LE1473L awaiting engine rebuild
7? Stag awaiting gearbox rebuild

Brett E Brett Evans
Columbus, Ohio, USA   USA
1973 Triumph TR6 "Scarlet Harlot"
Thanks for everyone's help--I think the general consensus is to use the 3 ohm coil and that's what I will do (at least until I can afford to upgrade to the Ignitor II)

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