Water injection is a means of internal cooling of the engine. During combustion, excess temperatures generated are absorbed by water as latent heat. Optimum water injection quantities were found to be about 0.015 ml to 0.031 ml of water per cycle on a 592 cc SI engine. The experiments were carried out by tapping the fuel injector signal and designing a circuit to inject water at the instant petrol is injected. Fuel injection duration was tuned by using a Wide Band Lambda sensor. The engine was supercharged as well by means of compressed air supply and regulated by hysteresis control. Water injection was investigated while varying spark advance to find the Maximum Brake Torque (MBT). Maximum obtained torque improvement with water injection was 16 %. This was achieved at a manifold absolute pressure of 120 kPa, with air temperature at ambient. The same load condition, 120 kPa, with air heated to the temperature that would be obtained from isentropic compression, resulted in a torque improvement of 7 %.