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Bosch Black Coil vs. Blue Coil

September 23, 2010 | Troubleshooting & Repair
By Joe Leoni

Q1: What's the difference between a Bosch black coil and their blue coil--besides the color?

It took a little time to pull a couple of coils out and check them over. I would like to defer this to the ex-Motorola gentlemen who wrote of the merits of CD ignition, but here it goes:

Black Coil Blue Coil
Size Larger Smaller
Primary Resistance 1.0 ohms 1.3 ohms
Secondary Resistance 10.6K ohms 8.95K ohms
Turns ratio (calculated with 60hz power) 107.5 80.6

The Ignition Transformer is configured as an auto transformer, but the decaying flux in the primary allows a greater induced secondary voltage than the ratio would suggest. The Blue coil has a smaller turns ratio and this would indicate that the current is higher for the same spark voltage. The Blue coil also has a higher resistance primary which would reduce distributor current.

Higher voltage does not necessarily mean a better coil. The spark plug will arc over as a function of the gap/condition of the plug and the pressure in the combustion chamber. Therefore, given a higher voltage coil Vs a lower voltage coil they would both be inclined to fire at the same increasing voltage.

What then is the difference? The difference is current, or watts in the spark and duration of the spark. You have heard of a hotter spark, and you have heard of higher voltage coils. Sure if you have a plug fouling problem a high voltage might help, but to insure a more complete burn the higher current that lasts longer will deliver the best performance.

If just any old spark would do, the spark we get off of the carpet to the light switch would do the job. But the carpet spark is very low power/current and duration yet high voltage.

So much of what we do with these cars is emotion driven, and this is how it should be. If you like a Blue coil buy one. That club badge on the air inlets doesn't help performance. But I/you like it there.

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