GM LT engine

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The second-generation Chevrolet Small-Block engine, which uses the LT prefix, was introduced in 1992. The distinguishing feature of this engine family was the use of reverse-flow cooling, which enabled higher compression ratios than previous versions of the small-block Chevy.

A special DOHC engine, the LT5, also used this prefix.

The LT family was replaced with the all-aluminum GM LS engine family in 1997 (in the Chevrolet Corvette), and 1998 (in the Chevrolet Camaro and the Pontiac Firebird Trans Am and Formula models).

Contents

[edit] Generation I

See the GM Small-Block engine page for information on the first generation of the 350 V8.

[edit] LT5

A GM LT5 engine
A GM LT5 engine

Immediately prior to the release of the second generation Small-Block, General Motors released a largely unrelated engine which also used the LT name. The LT5 was a special engine indeed. Head Engineer, Terry D. Stinson, led the project. It was an all-aluminum 349 in³ (5.7 L) small-block V8, but was entirely different from any of the other Chevrolet 350 engines. The bore and stroke were both different at 3.9 by 3.66 in (99 by 93 mm) instead of the usual 4 by 3.48 in (102 by 88 mm) and it featured Lotus-designed 32-valve DOHC heads. It was hand built by specialty engine builder, Mercury Marine. This engine produced 375 hp (280 kW) and 370 ft·lbf (502 Nm) for the 1990-1992 Corvette ZR-1 and jumped to 405 hp (302 kW) and 385 ft·lbf (522 Nm) for 1993 to its final year in 1995, thanks to cam timing changes and improvements to the engine porting. 1993 also added 4-bolt main bearing caps and an exhaust gas recirculation system. The engine was not used in any other vehicle.

[edit] Generation II

The Generation II small block debuted in 1992 on the 1992 Chevrolet Corvette. Few parts from the Generation II engine are interchangeable with the old generation I engine. It uses a new engine block, cylinder head, timing cover, water pump, intake manifold and accessory brackets. On the other hand, the engine mounts and bell housing bolt pattern remain the same, so the new engine is easier to swap into an older car than another model V8 would be. One visible difference is the new "opti-spark" distributor which is located on the front of the engine behind the gear-driven water pump.

A key technical difference between the original 350 and the Generation II small block is the cooling system. The engine employs reverse cooling, meaning that the coolant starts at the heads and then flows down through the block. This allows for a higher compression ratio and more spark advance since the heads are kept at a cooler temperature. A secondary benefit of reverse cooling is that cylinder temperatures are higher and more consistent.

It has already been noted (above) that the LT engine's reverse cooling system enables high compression, and the factory exhaust system and air inlet systems are not well optimized for performance (below). However, the intake manifolds, cylinder head castings, and other base engine components are capable of supporting much higher horsepower than factory-equipped engines produced. Even more readily than the typical Gen 1 small block, these aspects of the LT1 and LT4 enable their horsepower to be significantly increased by the addition of high-performance air intake ducting and exhaust systems. Further, upgrading the cylinder head porting and camshaft design with appropriate supporting modifications can enable these engines to produce in excess of 400 naturally aspirated horsepower at the rear wheels, or about 500 hp (370 kW) at the flywheel. General assumptions for friction loss through the drivetrain are as follows: 10%-12% friction loss for manual 6-speed transmissions, and 15%-20% for automatic transmissions.

[edit] 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:

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 late sixties Chevrolet 302 used in the first-generation Camaro Z28s). 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.

[edit] See also

[edit] External links