Spark Plug Gap | Spark Plug Gap Chart

Spark Plug Gap setting is important for proper engine operation. To set the spark plug gap correctly, you need to use a  spark plug gap tool. For reference, see our spark plug gap chart below, this chart converts metric measurements to standard.

How to Change Spark Plugs - Spark plug Gap setting and Gap cross reference chart

Spark Plug Gap cross reference chart - metric to standard (mm to inch)

Spark Plug Gap - understanding the basics 

Spark plug gap is where spark plug spark discharge is designed to take place.  On a conventional spark plug, it is the area between the center and the ground electrode.  As the spark always follow the path of least resistance, spark gap is generally the closest point between the spark plug center electrode and the spark plug ground electrode which is sometimes formed by the spark plug shell itself.  The only case when the spark will travel longer path to the ground, is when the longer path is more conductive (provides less resistance).  This can be caused for example by loss of insulating property of ceramic due to conductive carbon build up from the combustion process (spark plug fouling).  

Large spark plug gap setting demands higher voltage (electrical pressure) for spark to jump the large distance gap.  That means that there is higher voltage build up in the ignition system (ignition coil, distributor, ignition wires) prior to the spark discharge.  This is generally desired in applications with late model high output ignition coils and lower compression naturally aspirated engine applications where cylinder pressures are relatively low and easy for spark discharge to occur.  

However running a large gap in high cylinder pressure application (which makes it very hard for spark to occur), will demand such a high voltage (electrical pressure) for spark to jump the spark plug gap, that the spark will find an easier way to the ground, possibly where the igniton wire is close to the ground (engine block etc...) or inside of distributor cap, inside of the ignition coil, on the outside of the spark plug (between the plug and the boot) etc...  In either way the spark will not occur between the spark plug electrodes inside the combustion chamber, and engine misfire will occur.  

Generally the applications with low cylinder pressures (low compression ratio) running lean A/F (Air/Fuel) mixtures have large recommended gap (around .050"), and applications with high cylinder pressures (high compression, forced induction - turbo, supercharged, nitrous applications,...) require spark plug gap smaller than .032".  The dense A/F mixture is much harder for spark to penetrate, and require much higher voltage (pressure).  Higher fuel concentration has the same effect (lower A/F ratio).  That reason is why high power vehicles (1,000 horspower and more) often run a spark plug gap as small as .016".  Some applications such as ProMod turbo cars using M1 fuel frequently run a spark plug gap as small as .012", even though they are equipped with some of the highest output ignition systems providing in excess of 60,000 Volts high current and high mJ power output.  

Sometimes customers wonder, why there is so many different spark plug gap firing configurations.  The reason is, that in order to get the best possible performance, the spark plugs have to be carefully matched with the appropriate application and intended use.  The like to be "experts" often say that there is no difference in spark plugs, and that the mixture either gets ignited or not.  They can not be further from the truth!  Even standard vehicle Modern OBD-II (On Board Diagnostic) engines can determine from a simple crankshaft position sensor the angular speed of the crankshaft.  Mounted on the crankshaft is a trigger wheel that has many teeth, as the crankshaft spins, this wheel inducts a pulse when each tooth passes the sensor.  If every cylinder generates exactly the same amount of power, than the time between each pulse is in the same pattern.  However if one cylinder produces slightly less power, the time between the corresponding pulse is slightly increased.  This is calculated as a % of misfire.  Modern engines often turn the check engine light on with a misfire code (generally P03xx) with corresponding cylinder number when only as little as 10% misfire occurs, which is far before anyone will even feel that something is wrong or even think that a misfire is occuring.  

There can be a lot of power gained by using an optimal spark plug for given application, as well as installing a fresh set of spark plugs.  It is not uncommon to gain as much as 20 Hp just by replacing used spark plugs.  Used spark plugs are hard to fire as the gaps are worn out, and insulation properties of the ceramic is compromised by solid combustion deposits and old fuel saturation.  This robs available voltage at the spark plug gap,  as it "leaks" through the low resistance carbon deposits to the ground, resulting in a weak spark. Think of a worn out spark plug gap like an old leaky water hose that requires more water pressure, and is like a restricting the flow at the end with your finger.  More pressure is required form the leaky hose, because more water leaks out at other places than the hose end.

There are many different spark plug gap styles available, some with low gap capacitance, some with high gap capacitance, some with low ignition voltage requirement, some with high ignition voltage requirement, some with projected tip, some with retracted tip, some with fixed gap, some with adjustable spark plug gap, some with several parallel gaps, some with several gaps in series.

For the best engine and spark plug performance,  the size, style and shape of the spark plug gap should correspond with many factors.  Some of them are:

1) Potential of the ignition system - Spark Voltage (sometimes explained as Voltage potential or electrical pressure) corresponds with the distance that spark can cross at given air pressure and enviroment (concentration of the A/F (Air/Fuel) mixture etc... between the spark plug gap (center electrode and ground electrode).

2) Pressure in the combustion chamber at time of spark discharge - Depends on compression and aspiration of the engine.  High compression engines and engines with forced induction have higher combustion chamber pressures at time of spark discharge.  Higher cylinder pressure requires higher voltage for the spark to occur.

3) Type and concentration of the fuel - for example fuels with high alcohol content requires lower A/F ratio and smaller spark plug gaps.

4) Engine ignition timing advance - the closer to the engine TDC (Top Dead Center) the spark occurs, the higher is the A/F mixture pressure it has to fire, and the harder for spark it is.

5) Engine load and intended use - when engine is under load, the pressures in the combustion chamber is higher as there is more air aspirated, making it harder for spark to occur.  High RPM applications also shorten the ignition coil saturation (on time or DWEL) resulting in less available energy.

6) Type of Ignition system - magneto, CD (Capacitor Discharge), or Inductive.  CD ignitions have very fast Voltage rise (short rise time) and therefore they also work very well with spark plugs that have high capacitance, such a spark plugs with multiple gaps and/or surface discharge spark plugs.  On the other hand Inductive ignition systems generally provides longer spark duration...

 

Spark Plug Heat range 

Vehicles produced in series (which are not additionally modified for engine power output enhancement), whose engines are properly adjusted and in good technical condition, can be equipped according to the current application tables.

Any comparison charts of spark plugs are always for informative purposes only and they do not substitute the current application tables in full.

In the case of engine power output enhancement by means of additional modifications it is suitable to contact a producer's representative. There always applies a principle that in the case of medium modifications of the power output it is suitable to use the spark plugs "colder" by 2 degrees than those forming its original equipment (e.g. a change from 15 to 12). After driving several miles and subsequent assessment of the insulator tip appearance it is possible to decide about the most suitable equipment. This operation requires enough experience.

Proper length of the Spark Plug threaded shell

When changing the spark plugs, always ensure that the spark plug threaded portion of the shell (from the spark plug seat) is same length (or very close to) the O.E. spark plug.

If the threaded portion is slightly longer, spark plug indexing washers can be used to obtain the desired spark plug reach.