My 17 is still working well with the positive ground and original generator and voltage regulator. But if when it stops working it will be switched over to alt, negative ground.

I think not switching the coil wires makes for a much weaker spark.  Don't ask me how.

AaronSEIA

Never thought about the coil polarity.  I was not aware it made any difference.  I know the old car guys always add a coil resistor when they switch to 12 volt.

The coil polarity is a point of disagreement in many.  With a point-condenser ignition, there were some (earlier) condenser designs that tended not to last long with the battery polarity reversed... and the plating on contact points was chosen and positioned in an attempt to reduce the rate of deposition and erosion as electrons attempted to pass during opening.   Most of this is a moot point now, as neither modern condensers nor points are worth a crap, they don't last long regardless

The next point is actually how it affects (or doesn't) plug electrode temperature.  Just like in DC welding, the flow of current focuses the arc heat either towards one side of the gap, or the other, based on polarity.  The heat, being directed towards the grounding electrode, would cause it to be receiving the arc heat, rather than the center electrode and insulator.  It's the latter which is designed to manage plug temperature, so if the alleged heating-direction theory were true, the engine would not respond much with change of heat range, and it would suffer preignition by virtue of the overheating grounding electrode.

The final point, is how the coil is wound inside, and at which point the coil is grounded.  For a variety of performance reasons, most ignition coil is actually made more like an autotransformer, and the secondary windings carrying spark current are SHARED with the primary, such that when the points open, the primary and secondary are both carrying spark current.  There are three ends to the circuit-  the high voltage tower, the coil + terminal, and the coil point (-) terminal... and then there's the coil case, which is, by virtue of the bracket holding it, also grounded.  The internal windings are at very high voltage in some areas, and those areas are wound to be in farthest proximity to the coil case, so it doesn't internally arc... the primary (contact point current) is usually closest.

The concern for coil polarity (and it's probably well founded in some coils, but not others) is that running polarity reversed would place the HV component within reach of arcing to the case, because the electromagnetic polarity occurring across the polepiece is 'directing' the electrical flow in the opposite direction (towards a grounded surface, rather than the insulated spark tower).  This is simply because an electric arc can be 'directed' with a magnetic field. 

(If you do a bit of welding, and your arc won't go where you want it to go... one of the reasons why this happens, is because there's some strong magnetic field, either in the parent metal, or in the vicinity, that's pulling the arc to one side or the other.  in DC welding, it will 'drift' either towards, or away from the magnetic field, and in AC welding, the arc will splatter right-to-left in line with the field).

But the GENUINE argument, is that in theory, everything matters, but in practice, some of it not-so-much... and that is because in the scheme of things, there are much more significant variables elsewhere, that make coil polarity too insignificant to yield commensurate performance variability. 

I've seen tractors run both ways.  I've seen coils live, and I've seen 'em die... and the worst, are ones that won't die, but they don't live well... which is hell... when you're a half-mile through a mostly-plowed field, and it's starting to rain.

This is the one I used.  I put a diode in where they show the 10 ohm 10 watt resistor.  An idiot light would work as well.

AaronSEIA

That diagram is a pretty simple diagram. The only thing I would loose is the ballast resistor and install a coil with the resistor built inside so Mother Nature doesn't mess with it. Speaking of coils, 4 cylinders on 12V's need 2.7-3.3 ohms of resistance in the ignition circuit, so a 3.0 ohm coil will fit the bill. Also, Alternators and Ammeters don't mix well. A Voltmeter is much safer. With a Voltmeter the wiring would be run a little differently.  Any questions drop me an e-mail. Be glad to help ya out if you decide to change over to 12V. I also offer complete 12V conversion kits to if interested....

Steve@B&B

bb-customcircuits.com

Starters are not 'polarity sensitive'....kinda magical that way...

However hitting a 6 volt starter with 12 volts does make it spin faster..so far after 20 years of abuse 'Troy' is still happy....

OK Great - I was wondering about that as I never see anyone mention the starter in the conversion. 

To be a 100% 12 volt conversion, you 'need' to repalce with a 12 volt starter or have yours converted ( swap field coils, maybe armature ?). Steve @B&B will know for sure. Rumour has it he can do the conversion in his sleep....

Jay

Yup already have the 12v starter.  Already started the re-wire as it came with an Alt also - but the owner had the wiring all copped up.