GM Iron Duke engine

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Iron Duke
Bore4 in (101.6 mm)
Stroke3 in (76.2 mm)
Displacement151 cu in (2.5 L)
Block alloyIron
Head alloyIron
Cooling systemWatercooled
Power output85 hp (63 kW)
Specific power0.56 hp/cu in (25.2 kW/L)
Torque output123 ft·lbf (167 N·m)

The Iron Duke (also called the 2500, 151, Pontiac 2.5, Cross Flow, and Tech IV, though the decal on the air filter assemblies actually reads "4 Tech") was a 2.5 L (151 cu in) I4 piston engine. All Iron Dukes were built by Pontiac beginning in 1977 and ending in 1993.

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.

The Iron Duke is often confused with Chevrolet's Stovebolt-derived 153 from the 1960s Chevy II, but the engines are entirely different - the Iron Duke's intake manifold is on the passenger side, as opposed to the driver side.


Year hp (kW) ft·lbf (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 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

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.

A more-modern serpentine belt and crank-triggered ignition was added in 1987, increasing horsepower to 98. The engine was updated one final time one year later with balance shafts, new pistons, rods, crankshaft, and an in-pan oiling system. This 1988 Tech IV produced 110 hp (73 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, 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. 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. When replacing the EGR, a mechanic should only use a new, Delphi-sourced or AC Delco part, as aftermarket EGR valves have diaphram springs that are too weak for this engine. This causes hesitation, sag, stumbling, and sometimes, hard starting. Oxygen sensors should also be replaced every 30,000 miles (48,000 km) on this engine.


Racing versions

Though not a production engine, the Super Duty 4[1] racing engine was notable and widely publicized at the time. It was the basis for NASCAR's Touring Car series.

Cosworth also produced a 16-valve head (Cosworth Project DBA, 1987) for the Iron Duke.

See also

  • GM H platform
  • GM engines

External links