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Camflan is close, but not totally accurate...
"Using a higher octane in a car that doesn't require it will reduce efficiency of the engine."
This is incorrect. Using higher octane rated fuel than required only drains you pocket quicker...the octane rating is a measurement of the "flash point" of the fuel...which has nothing to do with the actual ignition of the fuel...the spark from the plug is much hotter than the flash point rating, therefore you can use a higher octane fuel without any loss of performance or economy. The knocking isn't something that is caused by the spark plug, but the residual heat in the cylinder that "pre-ignites" the fuel before it is supposed to...this can be due to the tuning of the engine (in the OP's case) or due to a high mileage engine that has some carbon deposits on the under side of the valves or on the top of the piston that don't cool as fast as the other engine parts and cause the fuel to ignite prematurely...that's why some older cars start to develope a "knock"...go up a fuel grade in these old cars and the knock should go away...
BDC was much closer with his info on the knock sensor in engines...
"...today the OEMs use knock detection feedback to allow people who don't read their owner's manual to drive on regular or midgrade with just degraded performance..."
That feedback causes the engine controls to "retard" the timing (fire the plug a little later) to minimize the impact of the pre-ignition situation...so it basically "detunes" the engine to allow it to run on a lower grade of fuel without any real consequences.
But Camflan nailed this one...
"Use the octane recommended to you by the car manufacturer."
Later.
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woodysweats 
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