Spark plugs gapping

NGK R sparkplug (courtesy, Extrememarine, Uk)

Let’s start with a simple rule of thumb: platinum- or iridium-tipped plugs should never be re-gapped as these come pre-gapped and attempting to alter the gap can/will cause damage to them. If you gap-test and find it's not what you wanted, get another set from your vendor/dealer/supplier.

 Gapping for different engine setups..

Here is a list prepared after some homework on what works best on an array of molded setups 

• NA: 1.1mm (works for most/any CRs)
• stock single/twin turbo (usually 0.3~0.9bar): 1.1mm
• twin turbo upto 1bar: 0.9mm
• twin turbo/hybrids upto 1.3bar: 0.7mm
• single turbo/big twins upto 1.3bar: 0.7mm
• single turbo/big twins upto 1.8bar: 0.6 or 0.7mm depening on specific needs
• single turbo/big twins, 2+bar: 0.6mm (theoretical & mostly untested so take extra precaution)

 Larger or smaller gap, which is better?

This will solely depend on your setup.

 Large(r) gapping

The larger the gap, the bigger the spark but you risk;

(a) not having a sure-fire spark on each ignition cycle... and,

(b) if you run high intake charge pressure levels, you risk spark blow-out -just as its name suggests, the boost pressure in the chamber blows out the spark and thus no bang. no bang = no power :)

 A large spark also needs your ignition system in its entirety to be at 110%, as a large gap essentially makes it harder for the spark to jump the gap and in turn strains your system generally. That means the plugs themselves, HTLs, ignitor/coil-pack/DIS, alternator yiddy yadda. You also MUST have proper engine grounding/earthing as people often only focus on the +ve side of the circuit, and as basic physics states, a circuit can't be complete if you don't have both +ve & -ve so check your grounding!

 Having a larger gap will also make starting in cold conditions and/or having fouled plugs more difficult. This is the reason some NA cars will have trouble starting in cold weather, esp if they also have low(er) CRs.

Small(er) gapping.

Smaller gaps on the other hand will always give you a sure-fire spark in most conditions but with the downside that it may fail to ignite the AF mixture to give a complete burn. this may be especially noticeable if you crank up your fueling (without re-adjusting your plug gapping). It's only beneficial when running higher-than-OEM intake charge pressure. Same reason why the list above indicates a smaller gap for higher boost applications.

 At higher charge pressure, the mixture is already so 'wound up' and ready to explode that is just needs a minor trigger to set it off. back to physics 101, pressure is directly proportional to temperature. So higher pressure = higher temp., and higher temp = higher volatility. also the same reason why NA engines have the largest gaps.

 In summary, NGK and Denso give very good, reliable and dependable plugs. Just remember to pick of the right heat range for your engine and torque them in correctly.