It's no exaggeration to say that the VW 1.8L turbo was to European turbocharged four cylinders what the Chevrolet small block was to American V8s. Formally known as the "1.8 R4 20VT" and informally known simply as the "1.8T," this little powerhouse was launched in 1993 and quickly developed a reputation for huge power levels in stock and especially modified form. Outside of Europe, the 1.8T's only real competition in the turbo four market is probably Mitsubishi's 4G63.
The 1.8T has been used in nearly every four-cylinder VW/Audi application from 1993 to present. These applications include (but aren't limited to) the VW Polo Gti, Golf MkIV, Bora, Jetta, New Beetle S, Passat B5 and Sagitar. Since Audi is VW's luxury brand, the engine has also carried over to models in that division, including the A3, A4, A6 and TT Quattro Sport. Other automakers to use the 1.8T have been Spanish manufacturer SEAT (in the Leon Mk1, Cupra R and Toledo) and Skoda for their limited edition Octavia VRS.
The 1.8T used a cast-iron engine block and an aluminium cylinder head with dual overhead camshafts and five valves per cylinder. The engine's actual displacement of 1,781cc comes by way of an 81-mm bore with an 86-mm stroke. Part of this engine's inherent strength comes from its use of a die-forged steel crankshaft, split forged connecting rods and Mahle forged pistons (in some applications).
The 1.8T has come in dozens of different configurations since it was first offered, with power ratings spanning from 150 horsepower at 5,800rpm (in most Polos, Golfs, Beetles and Passats) to 236 horsepower at 5,700rpm for Audi TT Quattro Sports. One common variation was the North America only 177 horsepower version, offered as an upgrade and subsequent replacement for the 150 horsepower version.
Aside from its incredibly free-breathing head, the 1.8T's main secret weapon is its turbo and induction system. All stock 1.8T's use a VW-sourced turbocharger, somewhat equivalent to a Garrett T30. That turbo feeds one of the few true variable-length intake manifolds on the market today. At low rpm, air flows through a set of long, thin intake runner tubes to enhance low rpm torque and driveability. At high rpm, a flap opens to connect the intake manifold's large open space (plenum) almost directly to the cylinder head, bypassing the tubes and enhancing top-end power.
Volkswagen has developed a number of specialised race versions to capitalise on the 1.8T's inherent strength and durability. Engines produced for the Formula Palmer Audi open-wheeled race series use a 300-horsepower version with a Garrett T34 turbo, which has a "scramble boost" option that allows the driver to bump power up to 360 horsepower for short periods. VW also makes a specialised FIA Formula 2 (the next series down from Formula 1) spec engine producing a sustained 425 horsepower with a 55-horsepower scramble boost capacity.