Super Unleaded 98ron - wow!

L John

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Retarding the ignition. The number of plugs has no bearing on the way the timing is controlled.

Nothing to do with ignition timing.
The spark pugs are both energised at exactly the same time in the same cylinder.
Two spark plugs cause two points of ignition in the same cylinder, the flame front will meet at the mid point between the spark of the plugs and cause knock.
My guess is they must be close enough together to minimise the potential knock problem but I'm just guessing as I don't know the answer.
 

LostKiwi

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Nothing to do with ignition timing.
The spark pugs are both energised at exactly the same time in the same cylinder.
Two spark plugs cause two points of ignition in the same cylinder, the flame front will meet at the mid point between the spark of the plugs and cause knock.
My guess is they must be close enough together to minimise the potential knock problem but I'm just guessing as I don't know the answer.
Knock is caused by the sudden spontaneous combustion of the fuel. The two flame fronts are both controlled combustion hence no knock when they meet.
Cylinder hotspots cause knock simply because the hotspot ignites the fuel at the wrong time (i.e too early). The flame front from the spontaneous combustion then expands outwards as the piston is still rising and creates high pressure in the cylinder which then causes the remaining fuel to spontaneously combust and it's this creates the knock. When ignition timing is advanced it's done to ignite fuel at a point in time where the flame front reaches the piston at or slightly after TDC. As RPM increases the number of degrees advance needs to be increased to allow for the rate of burn (which is largely constant for a given engine design). Twin plugs simply help reduce the time required for complete combustion to take place.
 

Botus

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knock sensors pick up a particular type of noise/frequency, not sure if twin plug burn causes them any issue
bang vs whoosh

my 1200GS has twin spark heads and knock sensors
 

L John

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Knock is caused by the sudden spontaneous combustion of the fuel. The two flame fronts are both controlled combustion hence no knock when they meet.
Cylinder hotspots cause knock simply because the hotspot ignites the fuel at the wrong time (i.e too early). The flame front from the spontaneous combustion then expands outwards as the piston is still rising and creates high pressure in the cylinder which then causes the remaining fuel to spontaneously combust and it's this creates the knock. When ignition timing is advanced it's done to ignite fuel at a point in time where the flame front reaches the piston at or slightly after TDC. As RPM increases the number of degrees advance needs to be increased to allow for the rate of burn (which is largely constant for a given engine design). Twin plugs simply help reduce the time required for complete combustion to take place.

Sounds good and logical.

Igniting at exactly the same time and the flame fronts meeting in a controlled manner would reduce the shock waves drastically.
 


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