If the spark is longer, the burning of the gasoline starts much faster and ends much faster.
It is also not a small effect. If you make the spark twice as long, it does not give you twice the speedup in burning - IT IS ACTUALLY MUCH MORE THAN TWICE !
Engineers call this a 'cubic' effect and it is related to how volume is measured by multiplying length by width by height.
If you make the spark twice as long, the speedup in burning is 2x2x2=8 times faster. Yes, eight times faster.
This faster burning has two good effects:
What we call 'Detonation'
or in slang terms 'Pinging'
is a chemical reaction that takes a certain amount of time to happen. If all the burning takes place super quick, there is no fuel left (called the 'end gas') to detonate.
Second benefit: If the burning is not all that fast, we have to start it earlier. This is 'ignition advance' where the spark is actually fired before the piston rises to top dead center. We need to 'advance the timing' this way to get the maximum pressure on the piston to happen about 10-15 degrees PAST TOP DEAD CENTER, but by starting early we suffer too - because the early burning pushes down on the piston before it reaches the top. In the USA engineers call this backwards pressure on the piston 'negative torque' and in Germany they call it 'Negative Work Index'.
So, big fat long sparks are good
IF THEY ARE USED WITH THE CORRECT IGNITION TIMING.
If big long sparks are used with too much advanced timing - like your stock ignition timing meant for factory spec short sparks - they can hurt and lead to more detonation unless you compensate by using higher octane gasoline.
In a similar way, 2 sparkplugs (like on the 5.7Hemi) are good in the sense that they double spark length - and spread it to two different spots.
3, 4 and even 13 sparkplugs have been tested per cylinder - see 'IC Engines' book by MIT professor R Taylor.
The Bosch+4 plug works by using a big 0.060 spark, but Magnum 5.2/5.9 V8 owners usually don't like it because it makes any pinging present worse.
It is especially hard to get a big spark to jump at full throttle (high pressure) and rpms above 4,000. Supercharging and Nitrous make it even harder still.
Old sparkplugs have bigger sparks naturally as the electrode metal corrodes away, but their dull rounded off edges really make it hard to get the spark to start.
