The performance of an extended stroke spark ignition engine has been assessed by cycle simulation. The base engine is a modern turbo-charged 4-stroke passenger car spark-ignition engine with 10:1 compression ratio. A complex crank mechanism is used so that the intake stroke remains the same while the expansion-to-intake stroke ratio (SR) is varied from 1 to 2 by changing the crank geometry. When the combustion is not knock limited, an efficiency gain up to 10 percentage-point is obtained. At low load, however, there is an efficiency lost due to over-expansion. Because of the faster expansion rate, the extended stroke renders the engine more resistant to knock. At SR of 2, the engine is free from knock up to 16 bar NIMEP. Under knocking condition, the required knock-limited-spark-advance with the extended stroke is less so that the operating point is closer to MBT timing and the knock limited efficiency improves. With the extended stroke, since less exhaust energy is delivered to the turbine, the engine air throughput and thus the output power is reduced. At low speeds, the increase in efficiency overpowers the decrease in air flow so that the maximum NIMEP at a fixed speed increases with SR. At high speed, however, the reverse is true and the maximum NIMEP decreases with SR. For the engine/ turbocharger combination used in this study, the transition point is at approximately 1500 rpm, independent of the SR.