Iron Duke engine

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General Motors logo.svg Iron Duke
Iron Duke^ 2.5L four cylinder engine - Flickr - dave 7.jpg
Displacement150.8 cu in (2.5 L)
Cylinder bore4 in (101.6 mm)
Piston stroke3 in (76.2 mm)
Block materialIron
Head materialIron
ValvetrainOHV 2 valves x cyl.
Compression ratio8.2:1 (1977–1983) 9.0:1 (1984–1993)
Fuel systemCarburetor
Throttle-body fuel injection
Fuel typeGasoline
Cooling systemWater-cooled
Power output85–110 hp (63–82 kW)
Specific power34.4 hp (25.7 kW)-44.5 hp (33.2 kW) per liter
Torque output123–135 lb⋅ft (167–183 N⋅m)
PredecessorGM 2300 engine
SuccessorGM 122 engine

The Iron Duke Pontiac engine VIN code A (also called the 2500, 151, Pontiac 2.5, and Tech IV, though the decal on the air filter assemblies actually reads "4 Tech") is a 150.8 cu in (2.5 L) Straight-4 piston engine. All Iron Dukes were built by Pontiac beginning in 1977 and ending in 1993. After this time, the GM 2.2 L OHV 4-cylinder replaced it across the entire lineup of vehicles that offered it. This engine is not to be confused with the Chevrolet 153. Early Iron Dukes had a Chevrolet V8 bellhousing (also shared with its third generation inline six) instead of the Buick-Oldsmobile-Pontiac bellhousing bolt pattern until the early 1980s when the later versions were installed in FWD applications where the 2.8L bellhousing bolt pattern was phased in. Cylinder head design had the intake manifold mounted on the passenger side, and the exhaust manifold on the driver side.

This 151 was also used by American Motors (AMC) starting in 1980, as the base engine option in the RWD Spirit and Concord, and continuing in both cars through 1982. The AWD (4x4) Eagle carried the 151 as standard equipment for 1981, and carried it midway through the 1983 model year. It was also available (as the Hurricane) in economy model Jeep CJs. When coupled to a Chrysler Torqueflite transmission a special version of the TF904 with a Chevrolet V8 bellhousing was manufactured when optioned with AMC/Jeep vehicles; this also holds true with the manual transmission bellhousings. AMC replaced the Iron Duke 2.5L I4 with a 150 cu in (2.5 L) Inline-4 derived from their own six.

Year Power output
1978 85 hp (63 kW) 123 lb⋅ft (167 N⋅m)
1979 90 hp (67 kW) 128 lb⋅ft (174 N⋅m)
1980 86 hp (64 kW) 128 lb⋅ft (174 N⋅m)

Versions of the Iron Duke[edit]

The LS6 was a 151 cu in (2.5 L) Straight-4 engine produced from 1978 to 1979. The LS8 was a 151 cu in (2.5 L) Straight-4 engine produced for 1979. The LX6 was a 151 cu in (2.5 L) Straight-4 engine produced from 1977 to 1978. The LX8 was a 151 cu in (2.5 L) Straight-4 engine produced from 1979 to 1980. This version introduced a cross-flow cylinder head. Cross-flow cylinder heads were added in mid-1979, leading people to refer to this version as the crossflow. Output stood at 90 hp (67 kW). The 151 and 151-S engines were 151 cu in (2.5 L) Straight-4 82–98 hp (61–73 kW) engines produced from 1974 to 1992 for Chevrolet Opala (Brazil). They used only the conventional (not crossflow) cylinder heads from the earlier North American models, like the Chevy Novas'.

Tech IV: (1982-1990)[edit]

Tech 4
2.5l tech 4 engine.jpg
Also calledIron Duke

GM Engine Tech4.png

Iron Dukes were fitted with fuel injection (TBI, via a single injector in the throttle body) in 1982. This version was christened the Tech IV, though Car and Driver later ridiculed it as the low-Tech IV. Power output remained at 90 hp (67 kW).

This was replaced by a swirl-port head with 9.0:1 (instead of 8.2:1) compression ratio in 1984 for a 2 hp (1.5 kW) gain. Other additions for 1985 included roller lifters, improved bearings, and a new crankshaft. Several significant changes were made in 1987, which included: an improved cylinder head, intake manifold and throttle-body fuel injection module, a more-modern serpentine belt with an automatic spring-loaded tensioner for the accessories, and a Distributorless Ignition System (DIS). This revision to the engine increased power to 98 hp (73 kW). In 1988, a balance shaft was added to smooth engine vibrations. Up to this point, the engine incorporated a 'dogbone' upper front engine mount secured to the cowling of the vehicle's hood latch, aiding in controlling the vibration. Further improvements in later years included new pistons, rods, crankshaft, and an in-pan oiling system. The most powerful variant of the Tech IV raised the rev limit to 5500 rpm, and achieved 110 hp (82 kW). The Tech IV uses the same bellhousing pattern as the 2.8 L 60-Degree V6. Over the years, the Tech IV engine has proved to be a reliable workhorse for owners when not pushed to its limits. All 1978-1990 Iron Duke L-4's are outfitted with a micarta camshaft gear that meshes directly with a steel gear on the crankshaft (no timing chain), a design also used by the Chevrolet inline six. 1991-92 VIN R and U engines received a timing chain.

Problems with the Iron Duke[edit]

The timing gear has a tendency to crumble a tooth anytime after 80,000 mi (130,000 km). The cam gear simply shears a tooth at startup and the engine won't start; because of the non-interference design of the engine, no further damage occurs. When the cam gear loses a tooth, the camshaft and distributor stop rotating during engine cranking. Replacing the gear requires heating the new gear in hot oil and quickly installing it for a shrink fit on the cam stub. One upgrade is the use of the aluminum camshaft timing gear from a Chevrolet 250 inline six with the Iron Duke's crankshaft timing gear. Inspection (and replacement, if necessary) of the MAP sensor, and its accompanying vacuum hose, is often a solution to many driveability problems. This sensor largely controls the engine's driveability. Stuck EGR Valves are also very common on the Tech IV.

Racing versions[edit]

Though not a production engine, the Super Duty 4 racing engine was notable and widely publicized at the time. It was the basis for NASCAR's Touring Car series. The Super Duty 4 (usually called the "SD4") is still in use today for ARCA Racing.

A 2.7L 232 hp (173 kW) SD4 engine powered the 1984 Fiero Indy Pace Car to over 138 mph (222 km/h) during the race. The SD4 was never available in production vehicles; however, Pontiac's Performance Parts counter had all the SD4 parts available and one could garner a 2.7L 272 hp (203 kW) version and a 3.2L 330 hp (246 kW) version. All 2000 Indy Fiero replicas came with the 2.5L 92 hp (69 kW) Iron Duke engine. The SD4 was the last in a line of high performance Pontiac "Super Duty" engines. Kansas Racing Products still makes the engines after buying the rights to make them from GM.

Nick Arias Jr. Racing Components offers an aluminum version of the Super Duty engine block, with improvements added for high-output applications.Cosworth also produced a 16-valve head (Cosworth Project DBA, 1987) for the Iron Duke.[1]

See also[edit]


  • 1979 chevy Monza Brochure
  1. ^ "Cosworth Type Reference". Retrieved 2012-06-04.

External links[edit]