 |  | | | Indexing spark plugs sounds high-tech. But what is it
and what good does it do?
Before we dig into spark plug indexing, we should
point out that every engine responds differently to indexed spark
plugs. Why? The shape of the combustion chamber, the shape of the
piston dome, the type of ignition system and many other variables
influence the "location" of the spark plug in the cylinder head. In
engines that feature a healthy piston dome coupled with a tight
combustion chamber, indexed spark plugs are almost mandatory
because the close internal clearances "tighten" the spark plug gap
every time the piston comes to top-dead-center (TDC). When it
comes to performance, some engine combinations simply respond
better to indexed spark plugs than others.
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What Is Indexing?
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Generally speaking, the idea of indexing is to position
the spark plug so that its gap is facing the center of the cylinder,
angled slightly toward the exhaust valve—the most common
arrangement. (Some engines work better with other gap locations.)
This is important because, as the piston approaches TDC, the
air/fuel charge is being compressed. The charge or "mixture" is
being forced toward the area of the spark plug—and normally,
the exhaust valve. The true speed of this force inside the
combustion chamber is extremely fast. Some experts speculate that
it surpasses supersonic speeds.
Because of this, the spark generated from the plug
should be in a "position" to create the best possible flame front.
Looking at a typical side-gap spark plug, you'll note that the
electrode can actually block the flame process. On the other hand, if
the electrode gap faces the on-rushing air/fuel charge, it stands a
much better chance of igniting a flame front.
How To Index
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How is a spark plug indexed? Simple: Mark the spark
plug insulator body with a felt marker (never use a pencil, which
acts the same as a carbon track inside your distributor cap) on the
side where the ground electrode attaches to the spark plug body.
Instead of rummaging through boxes of spark plugs to find the
elusive combination of perfect plug threads that match the cylinder
head threads, use indexing washers. These soft copper washers are
available from the aftermarket and will fit both flat- and tapered-
seat spark plugs. They're available in three thicknesses: 0.060-inch,
0.080-inch and 0.100-inch.
Copper's inherent softness along with the various
washer thicknesses make it simple to thread the spark plug into the
cylinder head and tighten it to the point that the index mark is
situated in the correct position relative to the combustion chamber.
(Never double up washers; they aren't intended to be used in
tandem.)
Power Gains
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Real-world power gains vary. Some engines or
combinations respond differently than others. In fact, all engines
will pick up power, but some gains will be more dramatic than
others.
In the end, it can't hurt to index the spark plugs. This
can contribute to superior engine efficiency and improved
economy.
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