<https://www.insightcentral.net/threads/new-spark-plug-indexing.18738/>
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"It's really a lot simpler than what you'd think. The reason that we benefit so much from indexed plugs, is because our spark plugs are threaded offset into the combustion chamber. Because of this, if the plug is faced wrong, the spark will be going up into the roof of the combustion chamber, instead instead of down in the air/fuel mixture. Indexing wouldn't be necessary in a dohc head with a plug going straight down into the combustion chamber. \n\nThe Harley's have their plugs indexed toward the intake valve because otherwise would be open toward the wall of the combustion chamber, It wouldn't matter to much if it was open toward the exhaust. In our engine though, since we're a 4 valve sohc, it's important that we have our plugs indexed toward the exhaust valves / firewall so the open side will aim down into the combustion chamber. Here's a quick read about plug indexing:\n\nRe: NITRO can correct me if I'm wrong...\n\nNotice how he says that indexing the plugs on a d16a6 can make anywhere from 4-8hp, (on a 108hp engine). He recommends the d16a6 have the ground electrode aim toward the exhaust (open side face intake valves). This engine spins in the opposite direction as ours,  putting the intake valves on the firewall side. The spark plugs are also on the same side as our cars.\n\nAnd, as I wrote earlier, you will never be able to tell which way they point, \nunless you index them yourself, or watch as someone else does... The\noperation of the car will not change in any perceptible way..  \nRacing, with high-strung engines running nitro is a whole different animal. \n\nAnd, yes, I did index mine - just because.  I used indexing washers."
<https://www.insightcentral.net/threads/new-spark-plug-indexing.18738/page-2>