Since I didn't know what this meant until I bought my second engine, I'll give you my take on it.
It's using two spark plugs per cylinder to improve detonation in engines that have higher compression ratios. The most visible evidence that it's being used is a distributor with 12 wires coming out of it, or -- in the case of the 964 and 993 -- two distributors working in conjunction. The compression ratio refers to the amount that the engine's pistons compress the air-fuel mixture. Higher compression means reducing the air-fuel mixture's volume to a greater degree, creating greater atmospheric pressure, than an engine with lower compression ratio would. With the right kind of detonation (and detonation timing), you can extract more power out of an engine with higher compression.
At least, that's how my liberal-arts-educated mind imagines it. I hope somebody with more on the ball will correct my errors.
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Jack Olsen
1973 911 T (3.6) sunroof coupe
jackolsen@mediaone.net
[This message has been edited by JackOlsen (edited 06-06-2001).]