GM 2.5 Iron Duke Specifications

by Alan Edwards
itstillruns article image
Part of old, air-cooled V4 engine at a workshop. image by Thor Jorgen Udvang from

During the mid 1970s the reign of the muscle car was ending. International politics had created a higher cost of gas along with a smaller supply. Gas rationing was in effect. At the same time, market share of Japanese imports with good fuel efficiency was increasing. Despite these factors the federal government was insisting on cleaner cars. Almost magically, the American big cars downsized.

For General Motors, the engine of choice was the Iron Duke, a four-cylinder inline engine with an iron block and head.

Engine Specifications

In 1977 Pontiac began production of the 2.5 L engine. It was GM's acknowledgment of the metric system as it was simply a 2.5-liter displacement engine which is equivalent to 50.79 cubic inches.

Each cylinder has two overhead valves.The engine has a single overhead cam shift (SOHC). Pushrods move the valves. The 2.5 L has a short-stroke crankshaft that runs in five main bearings. The bore is 4 inches and the stroke is 3 inches.

Engine Performance

The original engine puts out 87 horsepower at 4,400 rpm. Torque peak is 128 foot-pounds at 2,400 rpm. The original engine has a Holley two-barrel carburetor and under the EPA's rating system was capable of achieving 37 miles per gallon. Four years after its initial introduction the engine added fuel injection and the engine was officially renamed the Tech IV --- but to everyone it was still the Iron Duke. The Tech IV had slightly more horsepower, it is rated at 90 hp at 4,000 rpm.

Demand in The Mid 1970s

Demand for economical four-cylinder cars was thriving. During the engine's 14-year production run it was used by every GM division except Cadillac. American Motors Company, while developing their own four-cylinder engine, used the Iron Duke in the Concord/Spirit, Eagle and Jeep brands. Even the United Stated Postal Service installed the engine in its Grumman LLV delivery vehicles.

More Articles

article divider