Installation Manual

gap must be adjusted for the vehicle that the spark plug is intended for. Those with modified
engines must remember that a modified engine with higher compression or forced induction
will typically require a smaller gap settings (to ensure ignitability in these denser air/fuel
mixtures). As a rule, the more power you are making, the smaller the gap you will need.
A spark plug's voltage requirement is directly proportionate to the gap size. The larger the gap,
the more voltage is needed to bridge the gap. Most experienced tuners know that opening gaps
up to present a larger spark to the air/fuel mixture maximizes burn efficiency. It is for this
reason that most racers add high power ignition systems. The added power allows them to
open the gap yet still provide a strong spark.
4. Indexing
This is for racers only !!
Indexing refers to a process whereby auxiliary washers of varying thickness are placed under
the spark plug's shoulder so that when the spark plug is tightened, the gap will point in the
desired direction.
However, without running an engine on a dyno, it is impossible to gauge which type of indexing
works best in your engine. While most engines like the spark plug's gap open to the intake
valve, there are still other combinations that make more power with the gap pointed toward
the exhaust valve.
In any case, engines with indexed spark plugs will typically make only a few more horsepower,
typically less than 1% of total engine output. For a 500hp engine, you'd be lucky to get 5hp.
While there are exceptions, the bottom line is that without a dyno, gauging success will be
difficult.
5. Heat Range Selection
Let's make this really simple: when you need your engine to run a little cooler, run a colder
plug. When you need your engine to run a little hotter, run a hotter spark plug. However, NGK
strongly cautions people that going
to a hotter spark plug can sometimes mask a serious symptom of another problem that can
lead to engine damage. Be very careful with heat ranges. Seek professional guidance if you are
unsure.
With modified engines (those engines that have increased their compression) more heat is a
by-product of the added power that normally comes with increased compression. In short,
select one heat range colder for every 75-100 hp you add, or when you significantly raise
compression. Also remember to retard the timing a little and to increase fuel enrichment and
octane. These tips are critical when adding forced induction (turbos, superchargers or nitrous