GM Iron Duke engine
|Displacement||150.8 cu in (2.5 L)|
|Cylinder bore||4 in (101.6 mm)|
|Piston stroke||3 in (76.2 mm)|
|Cylinder block alloy||Iron|
|Cylinder head alloy||Iron|
|Compression ratio||8.2:1 (1977-1983) 9.0:1 (1984+)|
|Power output||85-110 hp (63-82 kW)|
|Specific power||0.56 hp/cu in (25.2 kW/L)|
|Torque output||123-135 lb·ft (167-183 N·m)|
The Iron Duke (also called the 2500, 151, Pontiac 2.5, and Tech IV, though the decal on the air filter assemblies actually reads "4 Tech") was a 2.5 L (150.8 cu in) I4 piston engine based on the 301 Pontiac V-8.In fact it was half of a 301 much like the Trophy 4 that was half of a Pontiac 389. 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 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. AMC replaced the Iron Duke 2.5L I4 with a 150cid Inline-4 of their own, derived from their evergreen sixes.
- 1977 Pontiac Astre
- 1977–1980 Pontiac Sunbird
- 1978–1980 Chevrolet Monza, Oldsmobile Starfire
- 1980–1982 AMC Concord
- 1980–1982 AMC Spirit
- 1980–1983 Jeep CJ
- 1980–1984 Oldsmobile Omega, Pontiac Phoenix
- 1980–1985 Buick Skylark, Chevrolet Citation
- 1981–1983 AMC Eagle
- 1982–1986 Chevrolet Camaro, Pontiac Firebird
- 1982–1989 Chevrolet Celebrity
- 1982–1991 Pontiac 6000
- 1982–1992 Buick Century, Oldsmobile Cutlass Ciera
- 1984–1988 Pontiac Fiero
- 1985–1987 Chevrolet S-10 Blazer, GMC S-15 Jimmy
- 1985–1993 Chevrolet S-10, GMC S-15/Sonoma
- 1985–1990 Chevrolet Astro, GMC Safari
- 1985–1987 Buick Somerset
- 1985–1991 Pontiac Grand Am, Oldsmobile Cutlass Calais
- 1986–1991 Buick Skylark
- 1987–1994 Grumman LLV (USPS delivery vehicle)
- 1990–1992 Chevrolet Lumina
|Year||hp (kW)||lb·ft (N·m)|
|1978||85 (63)||123 (167)|
|1979||90 (67)||128 (173)|
|1980||86 (64)||128 (173)|
The LS6 was a 151 cu in (2.5 L) I4 engine produced from 1978 to 1979.
The LS8 was a 151 cu in (2.5 L) I4 engine produced for 1979.
The LX6 was a 151 cu in (2.5 L) I4 engine produced from 1977 to 1978.
The LX8 was a 151 cu in (2.5 L) I4 engine produced from 1979 to 1980. cross-flow head introduced(1)
151 and 151-S 
These engines were 151 cu in (2.5 L) I4 (82-98 hp) engines produced from 1975 to 1992 for Chevrolet Opala (Brazil). They used only the conventional (not crossflow) cilynder heads from the earlier north American models, like the Chevy Novas'.
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).
Tech IV 
|Also called||Iron Duke|
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. These included an improved cylinder head, intake manifold and throttle body 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 5500rpm, and achieved 110 hp (82 kW).
Over the years, the Tech IV engine has proved to be a reliable, if noisy, 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). 1991-92 VIN R and U engines received a timing chain. The timing gear has a tendency to crumble a tooth anytime after 80,000 miles. 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 press fit on the cam stub.
A few Tech IV owners experience minor driveability issues with the engine. There are several cheap/easy repairs that the shadetree mechanic can perform to keep this engine running at its best.
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 
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 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 (250 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.
See also 
- "Cosworth Type Reference". Race-cars.com. Retrieved 2012-06-04.
(1)1979 chevy Monza Brochure
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