GM LT engine



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LT1 GM LT1 from a 1993 Chevrolet Camaro Z28. GM LT1 from a 1993 Chevrolet Camaro Z28.

In 1992, GM created a new-generation small-block engine called the LT1, recalling the 1970 LT-1 moniker. It displaced 5.7 liters and was a 2-valve pushrod design. The LT1 used a reverse-flow cooling system which cooled the cylinder heads first, maintaining lower cylinder temperatures and allowing the engine to run at a higher compression than its immediate predecessors.

This engine was used in:

   *
         o 1992-1996 Chevrolet Corvette C4
         o 1993-1997 Chevrolet Camaro Z28
         o 1993-1997 Pontiac Firebird Formula and Trans Am
         o 1994-1996 Buick Roadmaster
         o 1994-1996 Cadillac Fleetwood
         o 1994-1996 Chevrolet Caprice
         o 1994-1996 Chevrolet Caprice Police Package
         o 1994-1996 Chevrolet Impala SS
         o 1994-1996 Chevrolet Caprice Wagon
         o 1994-1996 Buick Roadmaster Wagon

There were a few different versions of the LT1. All feature a cast iron block, with aluminum heads in the Y and F bodies, and cast iron heads in the B and D bodies. Corvette blocks had four-bolt mains, while most other blocks were two-bolt mains.

The 92-93 LT1s used speed density fuel management, batch-fire fuel injection and a dedicated engine control module (ECM). In 94 the LT1 switched to a mass airflow sensor and sequential port injection. A new, more capable computer controlled the transmission as well as the engine and got a new name: powertrain control module (PCM). Where the ECM held its calibration information in a replaceable "chip", the PCM was reprogrammable through the diagnostic port.

The early Opti-spark distributor had durability problems and a revised version was introduced on the 1994 B-Bodies and in 1995 on the Y and F-Bodies. 1996 saw major revisions for OBD-II - a second catalytic converter on the F-body cars (which was standard since 1995 in California), rear oxygen sensors to monitor catalyst efficiency and a crank position sensor for misfire detection. Some OBD-II features had been added to the Corvette starting in 1994 for testing purposes. The 1997 model year Camaro and Firebird were the last year for this engine in a GM production car.

The 1992 LT1 in the Y-body was factory rated at 300 hp (220 kW) and 330 lb·ft (447 N·m). 96 LT1 Y-bodies were rated at 300 hp (220 kW) and 340 lb·ft (461 N·m). The 93-95 F-bodies were rated at 275 hp (202 kW) and 325 ft·lbf (439 Nm), while the 96-97 cars were rated at 285 hp (210 kW) and 335 ft·lbf (452 Nm). The 96-97 WS6 F-bodies were rated at 305 hp (227 kW). The 94-96 B and D-body version was rated at 260 hp (191 kW) and 330 ft·lbf (446 Nm).

[edit] LT4

The LT4 was a special high-performance version of the new-generation LT1. With the addition of a slightly more aggressive camshaft profile, 1.6:1 full roller bearing Aluminum rocker arms and better-flowing intake manifold (painted red) and cylinder heads, it was specified to produce 330 hp (243 kW) and 340 lb·ft (461 N·m). (459 Nm) of torque (although these numbers were likely underrated in order to "protect" the upcoming LS1's power rating). It was introduced in the 1996 model year, for the last year of the C4 Corvette, and came standard on all manual transmission (ZF 6-speed equipped) C4 Corvettes. The engine was passed down to special versions of the Camaro and Firebird the next model year.

The LT4 was available on the following vehicles:

   * 1996 Chevrolet Corvette when equipped with 6-speed manual transmission (includes all Grand Sports) (Production: 6,359)
   * 1997 Chevrolet Camaro SLP/LT4 SS 6-speed (Production: 100 for the U.S. and 6 for Canada)
   * 1997 Pontiac Firebird SLP/LT4 Firehawk 6-speed (Production: 29)

All 135 production engines for the Firehawks and Camaro SS were completely disassembled, balanced, blueprinted and honed with stress plates. One in 5 engines was tested on a Superflow engine dyno and every car was tested on a chassis dyno in addition to performing a short 6-mile (10 km) road test.

[edit] L99 (4.3L)

A 4.3 L version of the LT1, designated the L99, was introduced in 1994 for the Chevrolet Caprice. It was externally identical to the LT1, but the bore was reduced to 3.736" and the stroke to 3.00" (similar to the original 1955 265ci small block Chevy with a 3.75/3.00 bore/stroke combination). The pistons used in the L99 were the same as the ones used in the Vortec 5000 with the use of 5.94" connecting rods.

Like the LT1, it features sequential fuel injection, reverse cooling, and an optical crank position sensor. Output is 200 hp (150 kW) and 245 lb·ft (332 N·m).

Due to its smaller displacement, it provides better fuel economy over the 5.7L LT1.



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