The engine family known today by most as the Quad 4 was debuted to the public in the spring of 1987. The Quad 4 was a 2,260.5 cc (2.3 L; 137.9 cu in) DOHC inline 4-cylinder produced by General Motors' Oldsmobile division.
The Quad 4 name is derived from the engine's four valve, four cylinder layout. The engine was a modern design for it's time, using a cast iron block and an aluminum head, and it produced higher power levels than it's foreign and domestic competition. It is interesting to note that the Quad 4 was the first wholly domestic regular production DOHC four cylinder engine designed and built by GM; the only prior DOHC four cylinder engine offered by GM was the Cosworth Vega, which featured a DOHC head designed by Cosworth in England.
First released to the public as regular production option for the 1988 Oldsmobile Cutlass Calais and Pontiac Grand Am, the engine's availability expanded to Buick in late 1988 and Chevrolet in 1990, after which it became a mainstay in GM's lineup till it's cancellation after the 2002 model year.
Simultaneous to the engine's release in 1988, Oldsmobile attempted to further the Quad 4 name through highly publicized events featuring such cars as the Oldsmobile Aerotech. Another such event was the 1988 Indianapolis 500, Oldsmobile was chosen to pace the Indy 500 that year with a convertible version of their Cutlass Supreme, and a pre-production turbocharged Quad 4 was the engine of choice for their pace car. Although, it was publicized in 1988 that the Quad 4 was forthcoming in a 180 hp "HO", as well as a 250 hp turbocharged version (the same engine that was in the pace car that year), GM never released the turbocharged version as an option. Also, the engine in Oldsmobile Aerotech was a purpose-built, turbocharged example, it was solely meant to showcase the capability of the engine's design.
After the first couple years of rave reviews of the Quad 4, the automotive press began to criticize it for it's NVH - even though at the time, there were no four cylinder engines that were comparable to the power levels that the Quad 4 put out. At the time, the Quad 4 was generally compared to, or against turbocharged four cylinders, V6's, and occasionally V8's. The first changes which the Quad 4 saw to address NVH was 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 where the engine's bore & stroke were changed, and the engine was subsequently redubbed Twin Cam. During the course of production the engine family was noted to have spurious problems with connecting rods, head gaskets, and timing chains however no information exists that prove these issues occurred more frequently than is typical for an automotive engine of this type.
In addition to the 2,260.5 cc (2.3 L; 137.9 cu in) DOHC versions, there were also both a short lived SOHC variant, Quad OHC, available from 1992 through 1994, and the 2,392.3 cc (2.4 L; 146.0 cu in) Twin Cam for 1996 through 2002. The Twin Cam was replaced by the Ecotec for the 2003 model year.
All engines in the Quad 4 family were produced at the Lansing Engine Plant (plant 5) in Delta Township near Lansing, Michigan. The Twin Cam was the last engine that was engineered and produced by Oldsmobile; the Aurora V8 and Intrigue V6 sharing architecture with the Cadillac Northstar.
The LD2 was the standard version of the Quad 4 and the first incarnation having been introduced in 1987 for the 1988 model year. Even in base form the Quad 4 was impressive putting out 150 hp (112 kW) from 1988 to 1989 and 160 hp (119 kW) from 1990 to 1992. For the time, the Quad 4 made very competitive horsepower figures; the Chrysler 2.2 and the Ford 2.3 both needed a turbocharger and an intercooler to match horsepower. GM was able to produce the 160 hp with no turbo, only 9.5:1 compression ratio, and did not need an EGR system to pass emission standards. The base Quad 4 saw several slight changes during it's production run including different crankshafts, cams, and manifolds, all meant to increase torque and quiet NVH. There was also a transitional version of the LD2 in 1995, which displaced the same 2.3 liters as the early Quad 4 but featured the dual balance shafts of the later 2.4 L LD9 Twin Cam.
The High Output 2.3 L LG0 was rated at 180 hp (134 kW) from 1989 to 1992, 175 hp (130 kW) in 1993 and 170 hp (127 kW) in 1994; both the 1993 & 1994 power reductions were a direct result of the first two rounds of the Quad4's NVH reductions; both being exhaust port size reductions. Changes that differentiated the LG0 from the LD2 engine were more aggressive camshafts and an extra half point of compression; 9.5:1 to 10.0:1. These engines were recommended to run exclusively on premium fuel of 91 octane rating or higher due to the compression ratio.
The High Output engine was introduced with both a special production run of 200 Cutlass Calais International Series coupes, as well as 200 Pontiac Grand Am SE coupes - all featuring Bright Red paint, and grey interiors. The LG0 engine was only available with the heavy duty (for it's time) Getrag designed HM-282/NVG-T550 5-speed transmissions. 
For twenty years, 1989-2009, the Quad 4 held the title of being General Motors most powerful naturally-aspirated regular production 4 cylinder engine. Only recently was the LG0's 180 hp rating ever so slightly eclipsed when the 2010 model year 2.4 L Ecotec LAF launched in the Buick Lacrosse and Chevrolet Equinox. The LAF has a rated output of 182 hp (136 kW) but does so with 11.4:1 compression ratio, gasoline direct injection and variable valve timing - neither of the latter two being available during the Quad 4's time production run.
There was a limited production version of the LG0 engine offered in select Oldsmobiles which used the designation W41 (listed below).
The W41 was the highest output Quad 4. The 1991 and 1992 W41 were rated at 190 hp (142 kW), while the 1993 W41 lost five hp, and was rated at 185 hp (138 kW) as a result of the first round of NVH reductions; exhaust port size reduction. The additional 10 hp (7 kW) came from longer duration cams and a different PROM. A part of the W41 drivetrain was a specific version of the HM-282/NVG-T550 with a gear set otherwise unavailable. All W41 5 speed transmissions had a FDR (final drive ratio) of 3.94:1 whereas the 1988-1992.5 LD2 & LG0 transmissions all used a 3.61:1 FDR; the 3.94:1 FDR was used on all 1992.5-1994 LG0 transmissions, but retained the standard HM-282/NVG-T550 gear set.
A SOHC variant of the Quad 4 was intended to replace the OHV Tech IV. Debuting in 1992, this Quad OHC was an 8-valve engine and produced 120 hp (89 kW), 40 hp (30 kW) less than the Quad 4 of the same era. Torque was 140 lb·ft (190 N·m). Power dropped to 115 hp (86 kW) in 1993 with an attempt by GM to reduce the NVH of the engine.
For 1995 only, a balance shaft-equipped version of the 2.3 L version was produced. A clever arrangement ensured a constant load on the shafts: the crank drove one shaft, which drives the second, which then drives the oil pump. The shafts spun at twice the engine RPM, forcing the redline to be reduced from 6800 to 6500 RPM. Output was 150 hp (112 kW) and 150 lb·ft (203 N·m). This was the only Quad 4 produced in 1995.
This was known as a transitional year for the engine family.
The LD9 Twin Cam was a 2.4 L (146 cu in) Quad 4 variant with balance shafts, and a redesigned cylinder head which debuted in 1996. Bore was decreased from 92 mm (3.6 in) to 90 mm (3.5 in) and stroke increased from 85 mm (3.3 in) to 94 mm (3.7 in) for better torque. Power came in at 150 horsepower (110 kW). This engine received a minor update halfway through the 1999 model year that eliminated the EGR, increased the compression ratio from 9.5:1 to 9.7:1, and switched from low impedance fuel injectors to high impedance.