A cam actuated injection system timed to inject CNG fuel near the engine intake valve in the intake manifold at the end of suction stroke, but just before the closure of intake valve, has been designed, developed and incorporated in the S.I. engine. Details of the injection system hardware and injector drive mechanism developed are presented. A comparative experimental evaluation of the engine fuel economy and exhaust CO and NOx emission levels using premixed CNG-air induction system and Timed Manifold Injection(TMI) system developed is depicted. Engine test results show that with premixed CNG induction system the engine uses a relatively richer gas-air mixture as compared to that with TMI system. Power loss of the engine with premixed charge induction retrofit gets eliminated by using TMI system. Engine exhaust CO and NOx emission levels have been, in general, found to be lower with TMI system developed.