between the intake and exhaust valves narrower than the 80° used in the majority of the production engines. These heads are called "testa stretta" (narrow head). Some engines also had what was called a "monosleeve" liner, where all four cylinder bore liners are cast side-by-side in a single piece. The 1959 Asardo 1500 AR-S was a one-off non-Alfa prototype that used a custom-built version of the Twin Cam. Starting with a block the bore was increased to by using custom pistons and wet liners from Mahle so that its final displacement was and power was . Forced-induction versions In 1967 the Alfa Romeo GTA SA debuted at the Geneva Motor Show. Built by Autodelta, the Twin Cam engine was boosted by two superchargers driven by an engine-powered oil pump. Power was reported to be . In 1979 the Alfa Romeo GTV Turbodelta was released. The 2 litre engine was augmented by a KKK turbocharger blowing through a pair of Dell'Orto DHLA40H pressurised carburetors and produced in normal tune. This car was only offered until 1981 when Alfa Romeo left Group 4 to concentrate on Formula 1. 400 were built. In 1984 Alfa released the Giulietta Turbodelta. Equipped with a Twin Cam, dual carburetors and an Alfa Avio turbocharger the engine produced . In 1986, a turbocharged version of the single-plug engine developing was used to create the Alfa Romeo 75 1.8 Turbo Quadrifoglio. In Group A touring car racing form this engine made up to . Twin Spark Alfa Romeo's Twin Spark technology used dual ignition to fire two spark plugs in the cylinder head of each piston bore and was first used in the A.L.F.A. Grand Prix car of 1914. Racing versions of the Giulia engine, including the engine used in the GTA, featured a cylinder head with two spark plugs per cylinder. Twin Spark versions of the Twin Cam were also developed for 1750 and 2000 models used for racing. In 1987 the production Alfa 75 2.0 Twin Spark was fitted with a Twin Spark head, which provided improved fuel ignition and allowed an improved combustion chamber shape with narrower angle between intake and exhaust valve. Although developed from the Twin Cam, few if any parts are interchangeable between Twin Spark engines and Twin Cams. For instance, while Twin Cams have six oilways leading to the cylinder head the Twin Sparks only have two, the timing chain cover