The Iron Duke engine (also called 151, 2500, Pontiac 2.5, and Tech IV) is a
straight-4
A straight-four engine (also referred to as an inline-four engine) is a four-cylinder piston engine where cylinders are arranged in a line along a common crankshaft.
The majority of automotive four-cylinder engines use a straight-four layout ( ...
piston engine
A reciprocating engine, more often known as a piston engine, is a heat engine that uses one or more Reciprocating motion, reciprocating pistons to convert high temperature and high pressure into a Circular motion, rotating motion. This article ...
built by the
Pontiac Motor Division of
General Motors
General Motors Company (GM) is an American Multinational corporation, multinational Automotive industry, automotive manufacturing company headquartered in Detroit, Michigan, United States. The company is most known for owning and manufacturing f ...
from 1977 until 1993. Originally developed as Pontiac's new
economy car
Economy car is a term mostly used in the United States for cars designed for low-cost purchase and operation. Typical economy cars are small (compact car, compact or subcompact car, subcompact), lightweight, and inexpensive to both produce and p ...
engine, it was used in a wide variety of vehicles across GM's lineup in the 1980s as well as supplied to
American Motors Corporation
American Motors Corporation (AMC; commonly referred to as American Motors) was an American automobile manufacturing company formed by the mergers and acquisitions, merger of Nash-Kelvinator Corporation and Hudson Motor Car Company on May 1, 19 ...
(AMC). The engine was engineered for fuel efficiency, smooth operation, and long life, not for performance. Total Duke engine production is estimated to be between 3.8 and 4.2 million units.
Development
At the time of the
1973 oil crisis
In October 1973, the Organization of Arab Petroleum Exporting Countries (OAPEC) announced that it was implementing a total oil embargo against countries that had supported Israel at any point during the 1973 Yom Kippur War, which began after Eg ...
the only engines Pontiac built were , , and versions of
their V8 engine. Recognizing that future products would need to be smaller and more fuel-efficient, Pontiac engineers were tasked with developing a new engine that would be suitable for these future products. The engineers considered developing smaller displacement versions of the existing V8, a
V6 derived from the V8, a
V4 derived from the V8, and an inline-four derived from one of the
cylinder bank
The engine configuration describes the fundamental operating principles by which internal combustion engines are categorized.
Piston engines are often categorized by their cylinder layout, valves and camshafts. Wankel engines are often categoriz ...
s of the V8 (in the same fashion as the
1961 Pontiac Tempest's
"Trophy 4" engine), but ultimately decided to create an entirely new four-cylinder engine.
The development team's design goals were to minimize noise and vibration while maximizing durability,
drivability, fuel economy, and "usable" power at lower engine speeds. They began by analyzing other four-cylinder engines in production at General Motors at the time, and they found that
GM do Brasil's version of the
Chevrolet 153 cu in four-cylinder—with a shorter
stroke
Stroke is a medical condition in which poor cerebral circulation, blood flow to a part of the brain causes cell death. There are two main types of stroke: brain ischemia, ischemic, due to lack of blood flow, and intracranial hemorrhage, hemor ...
and longer
connecting rod
A connecting rod, also called a 'con rod', is the part of a reciprocating engine, piston engine which connects the piston to the crankshaft. Together with the crank (mechanism), crank, the connecting rod converts the reciprocating motion of the p ...
s—had significantly reduced
secondary vibration as compared to the original Chevrolet design and the newer
2.3-liter four-cylinder from the
Chevrolet Vega
The Chevrolet Vega is a Subcompact car, subcompact automobile manufactured and marketed by General Motors, GM's Chevrolet division from 1970 until 1977. Available in two-door hatchback, notchback, station wagon, wagon, and sedan delivery body st ...
. This obviated the need for
counter-rotating balance shafts, which would have increased the weight, complexity, and cost of the engine. Despite sharing the same
bore, stroke, and
cylinder spacing as the Brazilian engine, the majority of parts are not interchangeable.
Focusing on making power at lower engine speeds was a deliberate consideration in order to meet the rest of the design goals. Careful consideration was made to the design of the
intake manifold
An inlet manifold or intake manifold (in American English) is the part of an internal combustion engine that supplies the fuel/air mixture to the cylinder (engine), cylinders. The word ''manifold (engineering), manifold'' comes from the Old Eng ...
and
exhaust gas recirculation
In internal combustion engines, exhaust gas recirculation (EGR) is a nitrogen oxide () emissions reduction technique used in petrol engine, petrol/gasoline, diesel engines and some hydrogen internal combustion engine vehicle, hydrogen engines. ...
system to ensure power output from each cylinder was equalized. Power consumption of the water and oil pumps were reduced, and the
piston ring
A piston ring is a metallic split ring that is attached to the outer diameter of a piston in an internal combustion engine or steam engine.
The main functions of piston rings in engines are:
# Sealing the combustion chamber so that there is mini ...
s, cylinder bores, and crankshaft journals were designed to minimize friction.
To maximize durability the
engine block
In an internal combustion engine, the engine block is the structure that contains the cylinders and other components. The engine block in an early automotive engine consisted of just the cylinder block, to which a separate crankcase was attach ...
was made of cast iron with five
main bearing
A main bearing is a bearing in a piston engine which holds the crankshaft in place and allows it to rotate within the engine block.
The number of main bearings per engine varies between engines, often in accordance with the forces produced b ...
s, rather than the relatively fragile cast aluminum block used by the 2.3-liter Vega engine. (Even with the cast iron block the Iron Duke only weighed about 20 pounds more.) The 2.3-liter engine's
belt-driven overhead camshaft
An overhead camshaft (OHC) engine is a piston engine in which the camshaft is located in the cylinder head above the combustion chamber. This contrasts with earlier overhead valve engines (OHV), where the camshaft is located below the combustio ...
was eschewed in favor of an
overhead valve design with timing gears. Specially-designed bolts that stretch slightly farther than a conventional bolt were used to secure the intake and exhaust manifolds to the
cylinder head
In a piston engine, the cylinder head sits above the cylinders, forming the roof of the combustion chamber. In sidevalve engines the head is a simple plate of metal containing the spark plugs and possibly heat dissipation fins. In more modern ...
, to allow slight movement while maintaining the seal of the gaskets in order to prevent cracking the manifolds as they
expand with heat.
A two-stage, two-barrel
carburetor
A carburetor (also spelled carburettor or carburetter)
is a device used by a gasoline internal combustion engine to control and mix air and fuel entering the engine. The primary method of adding fuel to the intake air is through the Ventu ...
with electric
choke was used to improve performance in cold starts, while heat shields incorporated underneath the carburetor and between the intake and exhaust manifolds were used to prevent heat soaking the gasoline in the carburetor thereby improving performance in hot weather. Recognizing that cars with four-cylinder engines equipped with
air conditioning
Air conditioning, often abbreviated as A/C (US) or air con (UK), is the process of removing heat from an enclosed space to achieve a more comfortable interior temperature, and in some cases, also controlling the humidity of internal air. Air c ...
tended to experience drivability issues in hot weather, other improvements were made including a cut-off switch that shut the compressor off at
wide open throttle
Wide open throttle or wide-open throttle (WOT), also called full throttle, is the fully opened state of a throttle on an engine (internal combustion engine or steam engine). The term also, by extension, usually refers to the maximum-speed state o ...
and a delay incorporated into the air conditioning's circuitry to prevent the compressor from engaging until twelve seconds after the engine was started.
Early applications
The Iron Duke's first applications were in the 1977
Astre and
Sunbird
Sunbirds and spiderhunters make up the family Nectariniidae of passerine birds. They are small, slender passerines from the Old World, usually with downward-curved bills. Many are brightly coloured, often with iridescent feathers, particularly i ...
subcompact cars, replacing the 2.3-liter Vega engine, and in the compact
Phoenix. As these cars were originally designed for Chevrolet engines, the Iron Duke also used the Chevrolet
bell housing bolt pattern, instead of the Buick-Oldsmobile-Pontiac V8 pattern. The following year use of the engine expanded to the Sunbird's Chevrolet and
Oldsmobile
Oldsmobile (formally the Oldsmobile Division of General Motors) was a brand of American automobiles, produced for most of its existence by General Motors. Originally established as "Olds Motor Vehicle Company" by Ransom E. Olds in 1897, it produc ...
twins, the
Monza
Monza (, ; ; , locally ; ) is a city and ''comune'' (municipality) on the Lambro, River Lambro, a tributary of the Po (river), River Po, in the Lombardy region of Italy, about north-northeast of Milan. It is the capital of the province of Mo ...
and
Starfire.
For the 1979 model year, the engine was extensively redesigned. The original
reverse-flow cylinder head was replaced by a
crossflow design, a new two-barrel carburetor called "Vara-Jet" was introduced, the
distributor
A distributor is an electric and mechanical device used in the ignition system of older spark-ignition engines. The distributor's main function is to route electricity from the ignition coil to each spark plug at the correct time.
Design
...
was relocated, and the size of the oil pan was reduced. The only parts carried over from the 1978 engines were the connecting rods.
Peak power increased to 90 hp.
For 1980 the Iron Duke engine was redesigned to be mounted transversely, to suit the new
front-drive General Motors "X-body" cars. The bellhousing bolt pattern was revised to match that of the new
60° V6 engine.

GM also began selling the engine to
American Motors Corporation
American Motors Corporation (AMC; commonly referred to as American Motors) was an American automobile manufacturing company formed by the mergers and acquisitions, merger of Nash-Kelvinator Corporation and Hudson Motor Car Company on May 1, 19 ...
(AMC) starting with the 1980 model year. It was the base engine in
Spirit,
Concord, and
Eagle
Eagle is the common name for the golden eagle, bald eagle, and other birds of prey in the family of the Accipitridae. Eagles belong to several groups of Genus, genera, some of which are closely related. True eagles comprise the genus ''Aquila ( ...
automobiles, as well as in base-model
Jeep CJ
The Jeep CJ models are a series and a range of small, open-bodied off-road vehicles and compact pickup trucks, built and sold by several successive incarnations of the Jeep automobile marque from 1945 through 1986. The 1945 Willys "Universal Jee ...
s. The engines purchased by AMC continued to use the Chevrolet V8 bellhousing pattern. The four-cylinder engine was discontinued from AMC's rear-wheel drive models after 1982. During 1983, the all-wheel drive Eagle base engine switched from the Iron Duke to a new, AMC-developed
four-cylinder
The engine configuration describes the fundamental operating principles by which internal combustion engines are categorized.
Piston engines are often categorized by their cylinder layout, valves and camshafts. Wankel engines are often categoriz ...
. The 1980 through 1983 Jeep CJs were also available with the Iron Duke as the base engine.
* 1981–1982
AMC Concord
* 1981–1983
AMC Eagle
The AMC Eagle is a compact four-wheel drive passenger vehicle manufactured and marketed in a single generation by American Motors Corporation (AMC) for model years 1980 through 1987 and continued by Chrysler, Chrysler Corporation following it ...
* 1980–1982
AMC Spirit
* 1980–1981
Buick Skylark
The Buick Skylark is a passenger car formerly produced by Buick. The model was made in six production runs, during 46 years, over which the car's design varied dramatically due to changing technology, tastes, and new standards implemented over t ...
* 1980–1981
Chevrolet Citation
* 1978–1980
Chevrolet Monza
The Chevrolet Monza is a subcompact automobile produced by Chevrolet for the 1975 through 1980 model years. The Monza is based on the Chevrolet Vega, sharing its wheelbase, width, and standard inline-four engine. The car was designed to accommo ...
* 1980–1983
Jeep CJ
The Jeep CJ models are a series and a range of small, open-bodied off-road vehicles and compact pickup trucks, built and sold by several successive incarnations of the Jeep automobile marque from 1945 through 1986. The 1945 Willys "Universal Jee ...
* 1980–1981
Oldsmobile Omega
* 1978–1980
Oldsmobile Starfire
The Oldsmobile Starfire is an automobile Car model, nameplate used by Oldsmobile, produced in three non-contiguous generations beginning in 1954. The Starfire nameplate made its debut as a convertible concept car in 1953 followed with the 1954– ...
* 1977
Pontiac Astre
The Pontiac Astre is a Subcompact car, subcompact automobile that was marketed by Pontiac (automobile), Pontiac as a badge engineering, rebadged variant of the Chevrolet Vega. Initially marketed in Canada for model years 1973–1974, the Astre deb ...
* 1977–1981
Pontiac Phoenix
The Pontiac Phoenix was a compact car that was sold from 1977 to 1984 by Pontiac (automobile), Pontiac. There were two generations of the Phoenix, both based on popular Chevrolet models, and both using the GM X platform designation. It was named ...
* 1977–1980
Pontiac Sunbird
The Pontiac Sunbird (also known as the Pontiac J2000 and Pontiac 2000) is a model line that was manufactured and marketed by Pontiac from the 1976 to the 1994 model years. Loosely deriving its name from the Pontiac Firebird, the Sunbird was intr ...
* 1977
Pontiac Ventura
Tech IV
Iron Dukes were fitted with
fuel injection
Fuel injection is the introduction of fuel in an internal combustion engine, most commonly automotive engines, by the means of a fuel injector. This article focuses on fuel injection in reciprocating piston and Wankel rotary engines.
All c ...
(TBI, via a single injector in the throttle body) in 1982. This version was christened the Tech IV, though ''
Car and Driver
''Car and Driver'' (''CD'' or ''C/D'') is an American automotive enthusiast magazine first published in 1955. In 2006 its total circulation was 1.23 million. It is owned by Hearst Magazines, who purchased it from its prior owner Hachette Fi ...
'' later ridiculed it as the ''low''-Tech IV. Power output increased to .
This was replaced by a swirl-port head with 9.0:1 (instead of 8.25:1) compression ratio in 1984 for a gain. Other additions for 1985 included roller lifters, improved bearings, and a new
crankshaft
A crankshaft is a mechanical component used in a reciprocating engine, piston engine to convert the reciprocating motion into rotational motion. The crankshaft is a rotating Shaft (mechanical engineering), shaft containing one or more crankpins, ...
. 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 . In 1988, a
balance shaft
Balance shafts are used in piston engines to reduce vibration by cancelling out unbalanced dynamic forces. The counter balance shafts have eccentric weights and rotate in the opposite direction to each other, which generates a net vertical force ...
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 . The Tech IV uses the same
bellhousing pattern as the 2.8-liter ''
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 engines used a
micarta
Micarta is a brand name for composite material, composites of various fibers integrated in a thermosetting plastic. Materials such as linen, canvas, paper, fiberglass, carbon fiber, or other fabrics are used to form products for electrical and ...
camshaft
A camshaft is a shaft that contains a row of pointed cams in order to convert rotational motion to reciprocating motion. Camshafts are used in piston engines (to operate the intake and exhaust valves), mechanically controlled ignition syst ...
gear that meshed directly with a steel gear on the crankshaft. 1991-92 VIN R and U engines used a timing chain instead.
The
Grumman LLV
The Grumman Long Life Vehicle (LLV) is an American light transport truck model designed as a mail truck for the United States Postal Service, which has been its primary user since it first entered service in 1986, 39 years ago. It was also used ...
(Long Life Vehicle) built from 1987 until 1994 for the United States Postal Service for use in mail delivery were initially powered by the Iron Duke engine. The Postal Service specifications called for a 24-year service life and those with 2.5-liter engine have surpassed expectations.
File:2.5l tech 4 engine.jpg, "Tech IV" engine
File:1990 GM Iron Duke engine cut-away exhibit.jpg, 1990 GM Iron Duke engine with parts cut away to reveal the inner arrangement
* 1982–1992
Buick Century
* 1982–1991
Buick Skylark
The Buick Skylark is a passenger car formerly produced by Buick. The model was made in six production runs, during 46 years, over which the car's design varied dramatically due to changing technology, tastes, and new standards implemented over t ...
* 1985–1987
Buick Somerset/Somerset Regal
* 1985–1990
Chevrolet Astro
The Chevrolet Astro is a minivan that was manufactured and marketed by the Chevrolet division of American auto manufacturer General Motors from 1985 to 2005. Sold alongside the GMC Safari, the Astro was marketed in multiple configurations, incl ...
Cargo Van
* 1982–1985
Chevrolet Camaro
The Chevrolet Camaro is a mid-size American automobile manufactured by Chevrolet, classified as a pony car. It first went on sale on September 29, 1966, for the 1967 model year and was designed to compete with the Ford Mustang. The Camaro sha ...
* 1982–1990
Chevrolet Celebrity
The Chevrolet Celebrity is a front-drive, Mid-size car, mid-size passenger car line, manufactured and marketed by Chevrolet for model years 1982–1990, over a single generation.
Marking the transition of the Mid-size car, mid-size Chevrolet ...
* 1990–1992
Chevrolet Lumina
* 1985–1993
Chevrolet S-10
* 1985–1987
Chevrolet S-10 Blazer
* 1985–1987
GMC S-15 Jimmy
GMC may refer to:
Government India
* Gandhinagar Municipal Corporation, in Gujarat
* Gobichettipalayam Municipal Corporation, in Tamil Nadu
* Guntur Municipal Corporation, in Andhra Pradesh
* Guwahati Municipal Corporation, in Assam
* Gwalior ...
* 1985–1993
GMC S-15/GMC Sonoma
* 1985–1990
GMC Safari Cargo Van
* 1987–1994
Grumman LLV
The Grumman Long Life Vehicle (LLV) is an American light transport truck model designed as a mail truck for the United States Postal Service, which has been its primary user since it first entered service in 1986, 39 years ago. It was also used ...
(
USPS
The United States Postal Service (USPS), also known as the Post Office, U.S. Mail, or simply the Postal Service, is an independent agency of the executive branch of the United States federal government responsible for providing postal serv ...
delivery vehicle)
* 1985–1991
Oldsmobile Calais/Cutlass Calais
* 1982–1992
Oldsmobile Cutlass Ciera
* 1982–1991
Pontiac 6000
The Pontiac 6000 is a mid-size automobile manufactured and marketed by Pontiac from the 1982 to 1991 model years. As Pontiac transitioned to a numeric model nomenclature in the early 1980s, the 6000 replaced the LeMans as the mid-size Pontiac, s ...
* 1984–1988
Pontiac Fiero
The Pontiac Fiero is a Rear mid-engine, rear-wheel-drive layout, rear mid-engine, light sports car manufactured and marketed by Pontiac (automobile), Pontiac for model years 1984 – 1988. Intended as an economical commuter car with modest perf ...
* 1982–1985
Pontiac Firebird
The Pontiac Firebird is an American automobile built and produced by Pontiac (automobile), Pontiac from the 1967 to 2002 model years. Designed as a pony car to compete with the Ford Mustang, it was introduced on February 23, 1967, five months a ...
* 1985–1991
Pontiac Grand Am
The Pontiac Grand Am is a car model that Pontiac (automobile), Pontiac Division of General Motors produced in various years between 1973 and 2005. The first and second generations were Rear-wheel drive, RWD mid-size cars built on the Pontiac Leman ...
Super Duty
The Iron Duke block formed the basis of Pontiac's ''Super Duty'' four-cylinder racing engines of the 1980s, the last in a line of
high-performance Pontiac Super Duty engines. The engines were featured in
NASCAR
The National Association for Stock Car Auto Racing, LLC (NASCAR) is an American auto racing sanctioning and operating company that is best known for stock car racing. It is considered to be one of the top ranked motorsports organizations in ...
's
Charlotte/Daytona Dash Series, the
IMSA GT Championship
IMSA GT was a sports car racing series organized by International Motor Sports Association. Races took place primarily in the United States and occasionally in Canada.
History
The series was founded in 1969 by John and Peggy Bishop, and Bill ...
(in GTP and GTU class cars), and even in
American Power Boat Association racing boats. Super Duty engines continued to be used in
ARCA racing until well into the 2000s.
In addition to parts matching the Iron Duke's stock 2.5-liter displacement other crankshafts and their corresponding connecting rods were offered by Pontiac Motorsports, resulting in displacements ranging from 2.1-liters to 3.2-liters. A 2.7-liter, Super Duty engine powered the 1984
Fiero Indy Pace Car to over during the race, but Super Duty engines were never available in factory-built GM vehicles. However, GM sold the Super-Duty-specific parts at authorized dealers and all of the parts required to convert a stock Iron Duke engine to a Super Duty version were available.
Kansas Racing Products continued to make the engines in the early 21st century after buying rights to make them from GM.
Cosworth
Cosworth is a British automotive engineering company founded in London in 1958, specialising in high-performance internal combustion engines, powertrain, and electronics for auto racing, automobile racing (motorsport) and mainstream Automotiv ...
also produced a
16-valve, double-overhead cam head for the 3.0-liter version of the racing engine (
Cosworth
Cosworth is a British automotive engineering company founded in London in 1958, specialising in high-performance internal combustion engines, powertrain, and electronics for auto racing, automobile racing (motorsport) and mainstream Automotiv ...
Project DBA, 1987).
References
* 1979 Chevy Monza Brochure
External links
Jeep 151 information
{{GM late engine timeline
Gasoline engines by model
Iron Duke
Straight-four engines