Concept: Performance of SI engine [ alt="F1 S.C Madhu 07.04.20 D4" src="//storage.googleapis.com/tb-img/production/20/04/F1_S.C_Madhu_07.04.20_D4.png" style="width: 374px; height: 351px;"> To achieve maximum power and quick speeding/overtaking, the engine needs a rich mixture. (12:1 to 13:1) To achieve maximum fuel economy the engine needs a leaner air-fuel ratio of 16: 1 to 17: 1 Effect of Supercharging The power output of a supercharged engine is higher than its naturally aspirated counterpart. The mechanical efficiencies of supercharged engines are slightly better than the naturally aspirated engines. In spite of better mixing and combustion due to reduced delay, a mechanically supercharged Otto engine almost always have specific fuel consumption higher than a naturally aspirated engine. The tendency of knocking in CI engine Due to an increase in load, the knocking tendency in the CI engine increases. Low ignition temperature, speed reduces knocking tendency.