<VV> Spark Plug Gap size question

Jim Simpson simpsonj at verizon.net
Sat Jun 21 13:34:04 EDT 2014


The short answer is that "it depends".  The slightly longer answer is that
the stock gap is "about right".  The long answer is that there are a number
of trade offs.

If you are getting good ignition throughout your engine's operating range,
then there's no benefit to changing the plug gap.  Once you have good
ignition, changing the plug gap won't improve economy, torque, hp, etc.

A wider spark gap requires a higher initial ignition voltage to start the
spark.  That puts additional strain on the ignition system in the sense
that everything needs to be essentially perfect.  That includes the spark
plug wire insulation, distributor cap and anything else that might fail due
to the higher voltages involved.

And high compression 0r turbocharged engines require even higher voltages
to begin with to initiate the spark.

Higher rpms give less time for the stock ignition coils to build up the
magnetic field that generates the spark.  Hence at high rpm, there is less
ignition energy -- which translates into max voltage -- available for
ignition.

And finally keep in mind that as a spark plug is used, the required
ignition voltage rises.  The gap wears wider, the electrode edges round
off, and debris accumulates on the electrode that bleeds off energy.

So that all suggests staying with the standard spark plug gap.

Once the spark is initiated, the voltage drops down to a sustaining voltage
that is relatively the same regardless of the spark plug gap.  (An ionized
path has been established that has relatively low resistance.)  At that
point, gap is not a significant factor.

Wider gaps do help somewhat in establishing ignition on some engines,
particularly under lean conditions.  (More of the fuel/air mix is exposed
to the spark.)  This usually shows up in an improved idle on some modern
lean burn engines.  But once ignition is achieved, there's essentially no
effect on power/economy.

Racing engines are all over the map regarding plug gap.  Many (most?) are
near "standard" gaps.  But some builders swear by wider gaps.  But keep in
mind that racing engines typically use pretty high end ignition system --
very fat (heavily insulated) spark plug wires, very high voltage
(multi-strike, capacitive discharge) ignitions, and so forth that can
provide extreme voltages and large amounts of power to the plugs reliably.

Now if you want to open a can of worms, ask about extended nose and
multi-electrode spark plugs!

Jim Simpson
Group Corvair


More information about the VirtualVairs mailing list