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 by changing the crank geometry. The study is limited to the thermodynamic aspect of the extended stroke; the changes in friction, combustion characteristic, and other factors are not included. When the combustion is not knock limited, an efficiency gain of more than 10 percent is obtained for SR = 1.5. At low load, however, there is an efficiency lost due to over-expansion. At the same NIMEP, the extended stroke renders the engine more resistant to knock. At SR of 1.8, the engine is free from knock up to 14 bar NIMEP at 2000 rpm. Under knocking condition, the required spark retard to prevent knocking is less with the extended stroke. Then the operating point is closer to that of most efficient timing and the efficiency penalty due to knock constraint is reduced. 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.