Formulas for calculating Ignition Advance from Map Values
Ignition Timing, or when the spark plug fires, is controlled by the car's
Fuel Injection computer. For us, it is the BOSCH Motronic series ECU or DME. Sensors use
the starter's ring gear located on the flywheel to measure engine rotation. They count the
teeth as the engine rotates.
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When calculating Ignition advance from Motronic maps, these formulas apply. |
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Porsche 944 ML1.2 |
130 tooth ring gear |
=0.6923ºX (HEX2DEC(hex#)) |
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Porsche 944 ML3.1 |
130 tooth ring gear |
=0.6923º X (HEX2DEC(hex#)-20) |
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hex# - is the hexadecimal number stored in an
ignition map |
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Quarter-Teeth
For Ignition advance, map numbers are in degrees per quarter-tooth. The formula is
(360º / #TEETH) / 4. For a Porsche 944 (ML3.1) with a 130 tooth flywheel gear.
(360º / #TEETH) / 4 = (360º / 130) / 4 = 0.6923º
Each Ignition advance map numbers number is valued at 0.6923º
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Car Type |
944 |
944Turbo |
911 |
most BMW |
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Flywheel Teeth |
130 teeth |
132 teeth |
129 teeth |
116 teeth |
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Map values |
0.692° digit |
0.682° digit |
0.698° digit |
0.776° digit |
The 60-2 timing wheel system
In the late 1980's
Bosch standardized on a dedicated timing wheel with 60 equally spaced teeth. A single
sensor is used. Two teeth are missing at top dead center. Since the teeth are equally
spaced, the leading edge and falling edge are now counted. This gives a total of 120
counts per 360º revolution.
( 360º / #TEETH) / 4 = ( 360º /120) / 4 =0.75º
Each Ignition advance map numbers number is valued at 0.75º
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Porsche 944S/944S2 with M2.1 |
=0.75º X (HEX2DEC(hex#)-30) |
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See www.motronic.ws/zero.htm for explanation of ignition offset numbers (-30 above) |
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