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The dashed lines, determined with a fuel of sufficiently high octane rating to avoid knock, show MBT timing as a function of speed, along with the spark-advance limits for con-
Relation between spark advance, speed, and torque loss, for spark-ignition engine at wide-open throttle, showing knock limit for specific gasoline and typical spark-advance schedule that avoids knock problems.28
The lower solid curve represents a typical spark-advance schedule at WOT.
It is that cylinder which limits the spark advance, f A third reason for the discrepancy between actual spark-advance calibration and the knock limit for a given engine and fuel is the octane requirement increase associated with the buildup of deposits on the combustion chamber walls over extended mileage (see Sec. 9.6.3).
Whether low-, medium-, or high-speed knock is the limiting factor in a particular engine depends on the sensitivity of the fuel, on engine design features, and especially upon the engine's spark-advance requirements for MBT.
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