The Vortec 4300 is a 90° V6 truck engine, replacing the Chevrolet 250 in light trucks and 200 in³ (3.3 L) and 229 in³ (3.8 L) 90-degree V6s in passenger cars. The 4300 is based on the 350 in³ (5.7 L) Chevrolet small-block V8, and first appeared in 1985 with the throttle-body injected LB4 in passenger cars; light trucks and vans had carburetors until 1987. In 1991, the limited-edition GMC Syclone featured a 280 hp (210 kW) and 360 lb·ft (488 N·m) turbocharged and intercooled LB4 with the first use of multi-port fuel injection on a Vortec V6. The central-port injected L35 (Vin 'W') debuted in 1992; the cylinder block was slightly changed, a balance shaft was added to remove minor vibrations, and better breathing yielded 200 horsepower (150 kW). Another CPI engine, the LF6, joined in 1996 with the introduction of Vortec cylinder heads, while the LB4 was retired after 1998. In 2002, GM introduced a new multi-port injected LU3 engine, and a LG3 variant appeared soon after. This engine's origins date back to 1955, when the original Chevy small-block V-8 was introduced.
All Vortec 4300s use a cast iron block and heads and 4 in (101.60 mm) bore and 3.48 in (88.39 mm) stroke, both of which are the same as a 350, which gives them a displacement of 262.39 in³ (4.3 L). Connecting rods still measure 5.7 in (144.78 mm) although the rod journal diameter is 2.25 in (57.15 mm). 1992 and later cylinder blocks used a different timing cover since these engines used a balance shaft (some 1992 production cylinder blocks for the LB4 with TBI induction used the 'traditional' front timing chain cover from the small block Chevrolet). They are OHV engines with two valves per cylinder and are produced in Tonawanda, New York and Romulus, Michigan. Power output of the new LU3/LG3 engines is 180 hp (130 kW) to 200 hp (150 kW) and 245 lb·ft (332 N·m) to 260 lb·ft (353 N·m).