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SDI may refer to:

SDI (arcade game)

SDI - Strategic Defense Initiative, or simply titled SDI and sometimes known as Global Defense, is a side-scrolling shoot 'em up arcade game originally produced and released internationally by Sega. It was later ported to home computers and game consoles, some by other companies. Players control a satellite and must destroy enemies by moving a crosshair over them and firing the satellite's weapons.

SDI (engine)

The SDI engine is a design of naturally aspirated (NA) direct injection diesel engine developed and produced by Volkswagen Group for use in cars and vans, along with marine engine ( Volkswagen Marine) and Volkswagen Industrial Motor applications.

The SDI brand name (derived from "Suction Diesel Injection" or "Suction Diesel Direct Injection", the latter a translation of the ) was adopted in order to differentiate between earlier and less efficient indirect injection engines, called SD or "Suction Diesel", which were also produced by Volkswagen Group.

SDI engines are only produced in inline or straight engine configurations; and as they originate from a German manufacture, are designated as either R4 or R5, taken from the . They are available in various displacements (from 1.7 to 2.5  litres), in inline-four (R4 or I4) and inline-five (R5 or I5), in various states of tune, depending on intended application.

The SDI engine is generally utilised in applications where reliability and fuel economy are of primary concern. These engines lack any type of forced induction, hence the use of 'suction' in the title, and as such, their power output is lower when compared with a similar displacement turbocharged engine. For example, the 2.0 SDI engine fitted to the Volkswagen Golf Mk5 has a peak power output of ; whereas the same engine in Turbocharged Direct Injection (TDI) form is rated at or , depending on specifications. This clearly shows that turbochargers provide more power and torque.