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Hi Jeremy, Let me explain it another way. The primary & secondary resistances have really nothing to do with whether a spark generated by that coil is weak or strong. Yes coils can break down, but I'm not talking about used ignition coils; I'm referring to new coils. The coil with a primary resistance of 3.0 ohms can deliver the exact same energy as a coil with 1.5 ohms. It just takes longer for the coil to charge up. Yes, if you have a V8 doing 10,000 rpm, you have very little time between firings, to charge up the coil for the next firing, so may need a low primary ohm coil to make it charge quicker. We are talking here about a 4K engine with half as many cylinders, and half that sort of revs. It has plenty of time to charge up a coil with a 3.0 ohm primary. GT40, no ballast resistor… 14V / 3.4 ohms = 4.1 ampsGT40R 1.4 ohm with 1.6 ohm ballast resistor… 14V / 3 ohms = 4.6 amps As far as the secondary resistance goes, the voltage developed by the coil, is only as much as needed to jump across the spark plug gap. It you close the gap right up, a spark can jump it with probably 2-5K volts. In a working engine, under pressure more volts are needed to jump the gap, but any coil capable of around 20K volts will suffice for our humble Rollas. It's only extreme performance cars that may need potential voltages like 30K - 40K volts to fire a spark. I have a little spark gap here on the bench, you can buy on ebay for less than $ 5.00, that allows you to increase the gap until it won't spark. The rough scale on the side tells you what the voltage is to jump that gap. At 30K volts the gap is 20mm wide ! The indications of a good strong coil, is one that produces a spark that is bright blue/purple & white in colour. A weak coil, is one that produces a very yellowy spark colour. Yes, there are weak coils. One of the big dangers with coils is operating them without a spark gap, which can lead to breakdown inside on the secondary winding, which has a lot of turns of fairly fine wire. Don't forget there other things in the secondary of the ignition coil that can create weak sparks. The main culprit is bad HV dizzy leads. P.S. As a rule of thumb, a good coil should produce a clean sharp bright purple spark with an average dwell time (charging time) of about 3 milliseconds. If you are getting a weak spark, I'd be looking at other things in your system, than just the coils specs. Cheers Banjo

Hi Jeremy, I'd be more concerned with the primary resistance/impedance, than the secondary resistance. The secondary resistance basically only controls the maximum Kilo Volts you get out of the coil. The very low primary resistance of 0.5 ohms, is known as a High Energy or HEI type coil. These low impedance coils are not suitable for use with a 4K electronic dizzy with an internal ignitor. It might work initially, but will eventually burn the ignitor out. The Toyota Yellow Bible gives specs. for the coil primary for use with a 4K dizzy with internal ignitor as minimum 1.3 to 1.6 ohms. A coil with 0.5 ohm will eventually kill it. A fellow RollaClub member had this problem, just 2 weeks ago, which I had a look at. Brand new 4K electronic dizzy from ebay & a HEI coil. Ran for several months then, "no start". Diagnosed ignitor stuffed. Saved dizzy, by removing internal ignitor, & connected the reluctor pickup to the original ignitor wires out to the coil. Fitted a genuine external Bosch BIM-024 ignotor, and back in business. However, to be on the safe side, replaced coil with a GT40 which has a higher primary resistance than the HEI coil that killed the ignitor. I have intended to write a piece for the forum about the importance of matching coil specs to ignitor specs. There are a lot of people out there, that don't fully understand how important this is, is you want outright reliability. Basically the coil primary resistance determines the maximum current that can flow into the coil, whilst it is charging. So Ohms Law says I = E/R If the Volts are 13.5 & the resistance of the coil is 0.5 ohm, then I = 13.5/0.5 = 27 amperes. Ignitors come in various specs, one of which is the maximum DC amperes it can switch. I've seen various ignitor specs ranging from, 3A, 6A, 10A, 16A, & 19A max. Switching those sorts of current generates heat in the ignitor, so cooling them and having good airflow over them, is important for long term reliability. Heatsinks are important. I don't know specifically what the max. amperes the 4K dizzy with internal ignitor can handle. Its not in the manual, & doesn't appear on any website I have come across. They just provide a resistance range for the associated coil primary. So if we assume 1.3 ohms as spec'd, & assume a nominal voltage of 12 volts, then R = 12/1.3 = 9.0 amperes. Be very careful with this one, as it is so easy to lose an ignitor. There are "smart" ignitors, that prevent the ignitor from charging the coil, once the coil has reached saturation. I'm playing with some "smart" Toyota COPs at present, which do just that, but I can assure you the ignitor in the olde 4K electronic dizzy, has no such feature, like that. Cheers Banjo