What dictates the width of a spark plug gap is both the amount of voltage and amperage of your ignition system as well as cylinder pressure and air / fuel mixture density. The rule of thumb is that the more power an engine makes, the smaller the gap needs to be. Race engines (contrary to popular belief) use smaller gaps than most "stock" engines due because of higher cylinder pressures and denser air/fuel mixtures. This is because a small, intense spark is better for lighting a highly dense, very wet fuel mixture like you'll find in high powered race or performance engines, kind of like these new lighters you see these days with a small, intense blue flame. They light cigars, fires, etc better in the rain, wind, etc where a weak yellow flame that you get from a match or a typical Bic type lighter isn't as hot, and easily blows out.
A smaller gap will yield a short, intense spark and will usually make the engine real responsive. A wide gap creates a longer spark arc but without sufficient "juice" from your ignition system, (including amps), can be weak. A small intense spark is MUCH better than a big, weak spark, especially when dealing with richer fuel mixtures and more volumes of fuel being burned.
A good starting point and pretty much all around best gap is .035". If you want to experiment to see what works best for your particular engine, ignition, etc, then try going up to .040" and then .045, noting each time how the engine idles, responds, and pulls. Once you've gone bigger on the gaps, now try going smaller to .032", .030" and .025" and again, note how the engine runs. Whatever the gap was when the engine ran it's best overall, is where you should gap your plugs at.
Just an FYI; most very high powered blown gas, blown alcohol, and even blown fuelers run gaps as small as .026" (give or take) and use ignition systems that use 50,000 - 80,000 volts, but more importantly, up to 40+ Amps! Amps are what kill you. Stun guns have up to 1 million volts, but no amps, so they aren't lethal. Hell, static electricity you get when you shock yourself against a door knob or your car door can be upwards of over 100,000 Volts, but does it kill you? Nope. Your house electrical current can kill you too, but it's only 110 Volts, BUT it has 15 - 20 Amps pushing that Voltage behind it. Get hit by your house's 220 Volt system are you will most likely die. Not because of the 220 Volts. Again, it isn't Volts that kill you (again, think of a stun gun), it's Amps. Most 220 Volt outlets are for running heavier equipment such as your dryer, a welder, etc and have between 20 - 50 Amps behind them. That kind of current will light your ass up and fry you, but a 50,000 Volt jolt from a spark plug won't, unless you are screwing around with a serious ignition system like what a Top Alcohol or Fueler engine runs with 40+ Amps behind them. They even have decals on the tops of the dual magnetos saying "Caution! Lethal Current".
Your ignition system may have lots of Voltage, but no Amperage. This is why even on high energy ignition systems, (CONTRARY to what their lame instructions say to gap your plugs at), you CLOSE the gap on engines making more power. Usually .035" is around the best gap, even though many instruction manuals say to open your gap to .045". Go ahead and try that, but when it doesn't run as good, or it drops cylinders because that wider / weaker spark can't light the cylinders consistently, you'll know why. This gets even more true when you are running too rich and have too wet of a mixture. It's harder to light a fire in the rain than it is on a dry summer day. Inside your cylinder isn't much different.
On mild engines, (even you guys who have 400 - 550 HP), that isn't enough power to be concerned with on the gaps, and a gap of .045" will "probably" run just fine because you aren't burning enough fuel (a wet enough mixture) to cause cylinders to drop with a decent ignition system, but start getting over 600, 800, 1,000 horsepower and that .045" or so gap isn't going to work out too well any longer. Either way, your milder engine WILL run better with .035" gaps instead of the .045" gaps that they tell you to run.
Now, while we're on the subject of gaps, a little extra trick you can do to your plugs is to do what's commonly called "side gapping" them. This is when you take a small file or a fine grinder to the ground strap and cut it back so it only covers about half of the center electrode. This exposes the spark to the open cylinder and combustion area and prevents the shrouding the full length ground strap tends to do on regular plugs. This can be worth a few extra horsepower, just as indexing a set of plugs is worth a few extra ponies.
As you can see in the image, the plug on the left is a typical plug with a full length ground strap, where the plug on the right is side gapped. You can clearly see how doing this exposes the spark to the open cylinder more and will promote better flame travel and combustion. It's free horsepower for only a few minutes worth of effort.
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