Valid reasoning for utilizing dual spark plugs in automotive engines can be easily justified.
Porsche has been doing so since the fifties in its race engines and since 1989 in the two valve per cylinder 911 engine.
Benefits include: (Not in any order of importance).
A) More area under the torque curve. (Less negative torque due to necessary later spark timing). In a typical 911 engine, the spark lead must be reduced by approximately 10 crank shaft degrees with twin plugs compared to a single plug.
B) Reduced octane requirement. This is due to the fact that the characteristic known as detonation, requires time to manifest itself. A faster burn reduces the available time for detonation.
C) Fewer exhaust emissions. Read that; more efficient, more complete burn. Not to mention lower EGT.
D) Reduced specific fuel consumption.
E) Ability to run a higher compression ratio. (Go back to reduced octane requirement above).
There are other attributes to dual spark plugs in the above application but you should get the jist.
Of note is the fact that the Porsche 911 combustion chambers refered to above, are hemispherical and later, semi-hemispherical. In other words, these chambers are inefficient compared to other types of chamber designs and lend themselves to dual spark plugs quite nicely. A better way to word this is that this engine NEEDS dual spark plugs. At least the normally aspirated verion does. The Turbo charged engine has such an active chamber (swirl/tumble/etc.) that it does not respond to the twin plug deal as well.
Hope this is helpful.
