[KTL]

From what I remember reading, twin plugs improve the combustion efficiency of the 911 motor due to spark plug location. Due to the orientation of the spark plug on the upper valve cover side, the single plug is somewhat shrouded within the combustion chamber and cannot light off the air-fuel mixture very well at higher than a 10:1 compression ratio. Single plug setups risk detonation at higher than 10:1 compression ratio. The addition of the second plug on the lower valve cover helps to light off the air-fuel mixture more effectively and reduce the chance of detonation occurring.

You'd like to have the plug located centrally within the combustion chamber, surrounded by the intake and exhaust valves. This setup is commonly seen on your twin cam inline fours and sixes. It's more effective in igniting the air-fuel mix and doesn't necessarily need twin plugs. Look at an Integra Type R (11:1 I think) or a Corvette ZR1 (11.25:1 I believe). Both are twin cammers and use only one plug.

Jack,

Detonation is BAD. Combustion is good. Detonation (also known as preignition) occurs when the air-fuel mixture ignites (combusts) when you don't want it to ignite. That's why the knock sensor is used on your 964 motor- to detect detonation and retard the ignition timing accordingly.

Compression ratio:

Volume of the combustion chamber & cylinder bore with piston at BDC divided by the volume with piston at TDC. Pretty much the same as saying how much the air-fuel mixture is compressed.



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Kevin
87 Carrera coupe