The Stovebolt Site Forums The Shop Area The Electrical Bay Confused about Coil Ohms

I have a 12V Pertonix ignitor that has been in use for years now with no trouble. I purchased it for a GMC 228 12V conversion. I don't want to get into a discussion about the merit of these - but rather, I would like to purchase one of their Flame Thrower 40,000 volt coils. The old NAPA one I have is leaking oil so its time.

I see that there are a number of 12V Flame Thrower coils but I am stumped about the Ohm variations . They range from .6 to the 3.5 ohms and not being an electronic wizard - I quickly became stumped on which one would best work for my application. Finding information about this online has been challenging.

Can someone help me decide which coil would best fit?

Thank you. tom

A quick copy and paste from the Internet. Choosing The Correct Ignition Coil.

For specific recommendations we contacted our pal, Don Lindfors, at PerTronix to get the straight story on coils. As he explains, PerTronix offers three oil-filled, canister-style coils and they are identified by the ignition system they are suited for:

The Flame Thrower 40,000V coils have internal resistance rated at 1.5 or 3.0 ohms. The 3.0-ohm coil should be used on PerTronix Ignitors installed on four- and six-cylinder engines while 1.5-ohm coils should be used for eight-cylinder applications. These coils can be used on virtually any inductive (non-capacitive discharge) ignition system.

Flame Thrower II coils have lower resistance, 0.6 ohms for use with Ignitor II ignition systems as well as many other high-energy ignitions. The low resistance helps to produce up to 45,000 V. This higher voltage allows larger spark plug gaps for added power and better fuel economy.

Flame-Thrower III coils were developed for use with the new Ignitor III electronics. The extremely low resistance of 0.32 ohms results in 45,000 V and a coil that charges to peak, current typically 30-70 percent faster the other coils. It should be noted that these coils are compatible with Ignitor III electronics only.

Thank you. this is a big help. tom

I believe the original 6V ignition with points and condenser will have a coil that can produce maybe a maximum 30,000 (30KV) volts but it only takes around 8-12KV to fire a stock spark plug under normal load. Being that you already have the Pertonix Ignitor, I would go with the company's recommendation.

Any aftermarket coil manufacturer who makes claims about being able to provide extreme voltages at the spark plug is counting on having customers who don't understand what's going on in an ignition system. There is a huge difference in the theoretical maximum open circuit voltage that's available with a hotrod coil and one that flows less primary current and develops less magnetism around the secondary windings, but in the real world, firing voltage is determined by the condition of the spark plugs and the width of the gap. Once the gap is ionized and the spark begins to flow, it is IMPOSSIBLE for the coil to create any more voltage. If that happens at a clean, properly gapped plug with squared-off electrodes, it happens at less than 10K volts. Whether the coil is capable of 25K or 100K volts is a moot point. The higher secondary voltage coils do create a lot more load, and an increased probability of failure of the primary current control system, whether it's a set of points, or some exotic electronic pulse system dreamed up by P.T. Barnum and sold to gullible people who should know better. In the early 1950's, my father was modifying Lincoln Zepjyr 12 cylinder distributors to run on flathead V8 race cars, and ending up with a system that would jump a 3/4" gap with a hot blue spark- - - -and doing it with points, on 6 volts! He did so by manipulating the dwell time and allowing the coil to develop a stronger magnetic field before the points opened. A set of points only lasted a few races, but the hot spark was there when it was needed to ignite the alcohol/peroxide fuel mixture.

A standard 12 volt coil off the shelf at the local parts store will fire the plugs just as effectively as the same coil with a fancy paint job and a slick advertising campaign by Pertronix, no matter how it gets turned on and off. Does anybody really believe they make their own coils?
Jerry

I was waiting for your input on this, Jerry. I understand your points. There was a reason why I used a standard NAPA coi with this set up a few years back. You just sparked my memory. Tom