there is a "timing trimming socket" located near the ignition system ECU in the engine compartment. It is a socket with a number of fixed value resistors in it. The socket can be rotated to different positions to alter ignition timing. It is usally preset at the factory for the octane rating available in area where the car was originally sold. The socket I believe has 7 differnt positions.

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1990 190E 2.6
1996 Grand Voyager 3.3
1985 Mustang GT 5.0 5 SPD
1982 Suzuki GS 750T