a purpose-built, turbocharged example; it was solely meant to showcase the capability of the engine's design. After the first few years of positive reviews of the Quad 4, the automotive press began to criticize it for its noise, vibration, and harshness (NVH) levels. At the time, the Quad 4 was generally compared to turbocharged four cylinders, V6s, and occasionally V8s. The first changes of the Quad 4 to address its NVH were two rounds of exhaust port size reductions, followed by the addition of balance shafts in 1995. Further changes were made for the 1996 model year when the engine's bore and stroke were changed, and the engine was subsequently redubbed "Twin Cam". In recent years, the design has gained a minor following in hot rodding circles as a period style engine because it looks similar to a 1930s Offenhauser DOHC design (once you remove the ribbed aluminum cover, exposing the tall "cam towers" and the deep valley between them holding the spark plugs). Versions LD2 The LD2 was the standard version of the Quad 4, introduced in 1987 for the 1988 model year. In base form it put out from 1988 to 1989 and from 1990 to 1992. GM was able to produce the with a 9.5:1 compression ratio, meeting emission standards without using either a turbo or an EGR system. The base Quad 4 saw several slight changes during its production run including different crankshafts, cams, and manifolds, all meant to increase torque and lessen the NVH. In an effort to yield 5 additional HP some highly opted 1992 RPO LD2 SC Oldsmobile Achievas received a vortex valve along the TB induction pipe runner with an injection molded auxiliary induction tube funneling cool air from outside the engine compartment to the airbox via the openings around drivers side headlight (ram/air replete with cold weather block off plate in radiator core support) as until the engine warmed up would cause stumbling issues with the MAF sensor. There was also a transitional version of the LD2 in 1995, with the same displacement as the early Quad 4 but featuring the dual balance shafts of the later quipped 2.4-liter LD9 Twin Cam. Applications: 1988–1991 Buick Skylark 1992–1994 Oldsmobile Achieva 1988–1991 Oldsmobile Cutlass Calais 1990–1991 Oldsmobile Cutlass Supreme 1988–1994 Pontiac Grand Am 1990–1991 Pontiac Grand Prix 1993–1997 Pontiac Trans Sport (Europe only) 1995 For 1995 only, a balance shaft-equipped version of the