Efficìency, power, detonation and exhaust emissions of S.I. engines largely depend on compression ratio. The adoptiori of a device which allows the compression ratio variation according to engine operating conditions proves to be profitable. Many solutions are known; however none of them is employed in normal production engines, since the following features should be achieved: constructive simplicity, accuracy and ease of regulation, compatibility with an effìcient shape of the combustion chamber. This paper especially deals with a solution that is based on piston length virtual vanation by means of an eccentric surrounding the piston pin. The application of this solution on a series production engine is analysed with the purpose to verify its feasibility.