Bigger gap is better, but the spark has to be able to jump the gap. The reason bigger is better is because a bigger spark has more surface area to create ignition.

You do need to be able to supply voltage to jump that spark and that starts at the coil. In general, it is dependent upon the entire secondary electrical system, including the wires to ensure that the voltage is properly distributed to teh plug.

Someone mentioned misfire. That is correct, that you do not want misfire. To reduce misfire, use an MSD or similar Multiple Spark Discharge (more then one spark each time). This helps a lot with wider gaps.

For the op, err on the side of a smaller gap. You'll get a better performing engine for everyday use. Then, play around and gradually make it a little wider if desired.

 

Dead on accurate on this. Mags put out more voltage and amperage as RPM increases, unlike conventional battery powered ignitions. They put out very little voltage at cranking speed. The output voltage is DC during each secondary spark event, but in most mags it reverses polarity each time it fires because of the reverse in the magnetic field between each event, primary and secondary current both reverse polarity.

There are a lot of things that can effect the plug gap. The higher the cylinder pressure, the more voltage it takes to jump the gap. The higher the rpm, the more voltage drop most ignitions will have. In both of these cases you will have to close the gap down to get the job done unless you can increase the voltage output. High RPM, high boost turbo applications can need as small as a .012" gap, even with high output race ignitions. Increasing the gap increases the voltage it takes to jump the gap. Voltage is what jumps the gap. On a low RPM, low compression street engine with a high output ignition, the gap can be pretty large. It will increase the voltage output from the coil and provide a larger spark surface to fire the air fuel charge. As someone has stated, this helps with lean AFR applications and will help to a lesser degree with richer AFR settings.

Amperage is what creates the heat. The more amperage the hotter the spark. Spark duration is also important. Look up specs on racing ignitions and they will have the milliseconds of spark duration. The longer the spark duration, the more chance of getting a more complete ignition of the air fuel charge. In a perfect world you increase the voltage, amperage, and spark duration all at the same time, however, it's not that easy. Generally working within the realms of a street ignition, increasing voltage usually is accompanied by a loss of amperage. It's a balancing act.

One down side to running as wide of a gap as you can get away with and not have misfire is that you are asking the coil to produce closer to it's maximum output for more of its operating time. You will stress the ignition and a shorter life is possible.

 

Since this discussion is generally about the overall optimum plug gap for general conditions for a more or less low compression motor and this is something that within a range effects a motor a small % but one that helps to get right along with timing, good valves and all the rest of the systems including clean air, etc., does having ethanol in the gas have any effect on how you set a spark plug gap?

P.S. I'm not a fan of ethanol for a number of reasons.

LP

 

No.

One more myth down, a thousand to go.