The new four-cylinder, oversquare one-litre water-cooled Opel OHV engine weighed only 96 kg and was the first all-new engine developed by Opel since before the war. The side-mounted camshaft was driven not through gear-cogs but using a single chain with a hydraulic tensioner, the valve train employing rocker-arms and light-weight hollow rods. The arrangement was low on weight and on friction, permitting engine speeds above 6,000 rpm. The pistons were formed from an aluminium-alloy and incorporated a bimetal expansion strip to ensure uniform thermal expansion. The three-bearing forged crankshaft was of more traditional construction. The carefully designed aluminium inlet manifold provided for efficient feed of the fuel-mixture to the cylinders and helped provide the engine with good fuel economy characteristics.
The GSi's engine mapping had been carried out by Opel tuning specialists Irmscher. A model with the 82 PS (60 kW) 1.4 L multi point fuel injected engine, which was otherwise mechanically identical to the GSi, also became available as the Nova SRi in the United Kingdom. In January 1988, a turbocharged version of the Isuzu diesel engine was introduced, with power increased to 67 PS (49 kW).