Most drivers know that you should change your spark plugs routinely to keep your car's engine running smoothly. However, when it comes to gapping spark plugs, many people may find themselves confused. Though most modern spark plugs come pre-gapped, it's still wise to check the gap before installation, and some folks go one step farther by side gapping their plugs.
Side gapping a spark plug is one of those old-school tricks that sounds like it was invented on a garage floor next to a half-empty case of Stroh's. In essence, it's the fine and exacting art of taking a Dremel or file to a perfectly good spark plug, all in an attempt to squeeze out a few more drops of horsepower. The whole point of a spark plug is to create a tiny bolt of lightning in your engine's cylinder, igniting the air-fuel mixture to make the glorious magic happen. A standard plug has a ground electrode that hooks over the top of the central electrode, looking a bit like a metal claw.
The theory behind side gapping is that the standard ground electrode gets in the way, shrouding the initial spark and the tiny flame kernel it creates. By cutting the ground electrode back so it's flush with the edge of the central one, you supposedly unshroud the spark. This, in theory, gives the spark a clearer, more direct path to the rest of the air-fuel mixture. The goal is a faster, more complete burn, which should lead to more power and better efficiency -- the holy grail for anyone who's ever turned a wrench.
So, it's just garage sorcery, right?
This feels like one of those tales you feed the apprentice just to see if they'll play along -- there's no way this is real, right? Well, there's actual science to back up the basic premise. The idea of exposing the flame kernel to as much of the combustible mixture as possible is a legitimate engineering goal. In fact, major manufacturers like DENSO design high-performance spark plugs with moderately shortened ground electrodes for the exact same reason.
Better yet, someone actually put it on a dyno -- the end-all and be-all of internet horsepower debates. In a 2019 study published in the International Journal of Recent Technology and Engineering, testers bolted a stock 1.6-liter engine to a dyno to see what would happen. The result? The side-gapped plugs delivered a measurable, if modest, increase in performance. The best configuration saw a power bump and a consistent increase in torque across the rev range. The biggest change, however, was in fuel efficiency. The side-gapped plugs demonstrated significantly lower fuel consumption, along with a 20% to 30% reduction in unburned hydrocarbons -- a clear sign of a more complete burn. So, maybe the garage gurus were onto something.
The catch, because there's always a catch
Before you run out to buy a Dremel and a fresh set of plugs, there are some other factors to consider. The standard ground electrode has a broad surface for a reason -- it distributes the electrical erosion of the spark over a wide area. When you side gap a plug, you force that spark to jump between a newly created sharp edge and the center electrode. This concentrates the wear in one tiny spot, reducing the lifespan of the plug. You're not just messing with the plugs -- you're also messing with the maintenance schedule. Then again, if you're out here side gapping plugs, that probably isn't high on your list of concerns.
Ultimately, the reason manufacturers don't side gap spark plugs from the factory is because the trade-off isn't worth it for 99.9% of cars. Modern spark plugs with fine-wire iridium tips can accomplish the same unshrouding effect through engineering, all while lasting for normal service intervals. So, while side gapping is a cool piece of hotrod history, it's a modification best left in the past.
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