| Iron Duke | |
|---|---|
 | |
| Overview | |
| Manufacturer | General Motors |
| Also called | Tech IV (1982-1990) |
| Production | 1977–1993 |
| Layout | |
| Configuration | Straight-four |
| Displacement | 150.8 cu in (2.5 L; 2,471 cc) |
| Cylinder bore | 4 in (101.6 mm) |
| Piston stroke | 3 in (76.2 mm) |
| Cylinder block material | Cast iron |
| Cylinder head material | Cast iron |
| Valvetrain | OHV 2 valves x cyl. |
| Compression ratio | 8.25:1 (1977–1983) 9.0:1 (1984–1993) |
| RPM range | |
| Max. engine speed | 5000 rpm |
| Combustion | |
| Fuel system | Carburetor Throttle-body fuel injection |
| Fuel type | Gasoline |
| Oil system | Wet sump |
| Cooling system | Water-cooled |
| Output | |
| Power output | 85–110 hp (63–82 kW) |
| Specific power | 34.4 hp (25.7 kW)-44.5 hp (33.2 kW) per liter |
| Torque output | 123–135 lb⋅ft (167–183 N⋅m) |
| Dimensions | |
| Dry weight | 375 lb (170 kg)[1]: 3 |
| Chronology | |
| Predecessor | GM 2300 engine |
| Successor | GM 122 engine |
TheIron Duke engine (also called151,2500,Pontiac 2.5, andTech IV) is a 151 cu in (2.5 L)straight-4piston engine built by thePontiac Motor Division ofGeneral Motors from 1977 until 1993. Originally developed as Pontiac's neweconomy car engine, it was used in a wide variety of vehicles across GM's lineup in the 1980s as well as supplied toAmerican Motors Corporation (AMC). The engine was engineered for fuel efficiency, smooth operation, and long life, not for performance.[2] Total Duke engine production is estimated to be between 3.8 and 4.2 million units.[3]
At the time of the1973 oil crisis the only engines Pontiac built were 350 cu in (5.7 L), 400 cu in (6.6 L), and 455 cu in (7.5 L) versions oftheir 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.[4] The engineers considered developing smaller displacement versions of the existing V8, aV6 derived from the V8, aV4 derived from the V8, and an inline-four derived from one of thecylinder banks of the V8 (in the same fashion as the1961 Pontiac Tempest's"Trophy 4" engine), but ultimately decided to create an entirely new four-cylinder engine.[1]: 1 The 'all-new' claim was partially marketing terminology as many of the external parts, such as the distributor, water pump, manifolds, and carburetor were taken directly from the earlier Chevrolet 4-cylinder engine some with small dimensional changes making them non–interchangeable. Further confusion is due to the bare blocks looking visually nearly identical even though almost none of the internal parts are interchangeable between the two engine series. Borrowing parts like this greatly reduced development, tooling, and production time and costs. The early few years of the Iron Duke are more closely related to the earlier Chevrolet 4-cylinder than the later versions with numerous design changes such as fuel injection, flat belt accessory drive, cross flow heads, and balance shafts. Interestingly, the water pump from the late production years will bolt onto a first year production 1961 Chevrolet 151 4-cylinder engine, although designed for reverse rotation.
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.[1]: 1 They began by analyzing other four-cylinder engines in production at General Motors at the time, and they found thatGM do Brasil's 151 cu in (2.5 L) version of theChevrolet 153 cu in four-cylinder—with a shorter 3-inch (76 mm)stroke and longer 6-inch (150 mm)connecting rods—had significantly reducedsecondary vibration as compared to the original Chevrolet design and the newer2.3-liter four-cylinder from theChevrolet Vega.[1]: 1–2 This obviated the need forcounter-rotating balance shafts, which would have increased the weight, complexity, and cost of the engine.[1]: 3 Despite sharing the samebore, stroke, andcylinder spacing as the Brazilian engine, the majority of parts are not interchangeable.[1]: 2
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 theintake manifold andexhaust gas recirculation system to ensure power output from each cylinder was equalized. Power consumption of the water and oil pumps were reduced, and thepiston rings, cylinder bores, and crankshaft journals were designed to minimize friction.[1]: 3–5
To maximize durability theengine block was made of cast iron with fivemain bearings, 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'sbelt-drivenoverhead camshaft was eschewed in favor of anoverhead 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 thecylinder head, to allow slight movement while maintaining the seal of the gaskets in order to prevent cracking the manifolds as theyexpand with heat.[1]: 6–7
A two-stage, two-barrelcarburetor with electricchoke 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 withair conditioning tended to experience drivability issues in hot weather, other improvements were made including a cut-off switch that shut the compressor off atwide open throttle and a delay incorporated into the air conditioning's circuitry to prevent the compressor from engaging until twelve seconds after the engine was started.[1]: 7–10
The Iron Duke's first applications were in the 1977Astre andSunbird subcompact cars, replacing the 2.3-liter Vega engine, and in the compactPhoenix. As these cars were originally designed for Chevrolet engines, the Iron Duke also used the Chevroletbell housing bolt pattern, instead of the Buick-Oldsmobile-Pontiac V8 pattern. The following year use of the engine expanded to the Sunbird's Chevrolet andOldsmobile twins, theMonza andStarfire.
For the 1979 model year, the engine was extensively redesigned. The originalreverse-flow cylinder head was replaced by acrossflow design, a new two-barrel carburetor called "Vara-Jet" was introduced, thedistributor was relocated, and the size of the oil pan was reduced. The only parts carried over from the 1978 engines were the connecting rods.[5] Peak power increased to 90 hp.
For 1980 the Iron Duke engine was redesigned to be mounted transversely, to suit the newfront-driveGeneral Motors "X-body" cars. The bellhousing bolt pattern was revised to match that of the new60° V6 engine.
GM also began selling the engine toAmerican Motors Corporation (AMC) starting with the 1980 model year. It was the base engine inSpirit,Concord, andEagle automobiles, as well as in base-modelJeep CJs.[6] 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 150 cu in (2.5 L)four-cylinder. The 1980 through 1983 Jeep CJs were also available with the Iron Duke as the base engine.[7]
| Year | Power | Torque |
|---|---|---|
| 1978 | 85 hp (63 kW) at 4400 rpm[8]: 40 | 123 lb⋅ft (167 N⋅m) at 2800 rpm[8]: 40 |
| 1979 | 90 hp (67 kW) | 128 lb⋅ft (174 N⋅m) |
| 1980 | 90 hp (67 kW) at 4000 rpm[9]: 47 | 134 lb⋅ft (182 N⋅m) at 2400 rpm[9]: 47 |
| 1981 | 84 hp (63 kW) at 4000 rpm[10] | 125 lb⋅ft (169 N⋅m) at 2400 rpm[10] |
Iron Dukes were fitted withfuel injection (TBI, via a single injector in the throttle body) in 1982. This version was christened theTech IV, thoughCar and Driver later ridiculed it as thelow-Tech IV.[citation needed] Power output increased to 90 hp (67 kW).[11]
This was replaced by a swirl-port head with 9.0:1 (instead of 8.25:1) compression ratio in 1984 for a 2 hp (1.5 kW) gain. Other additions for 1985 included roller lifters, improved bearings, and a newcrankshaft. Several significant changes were made in 1987, which included: an improved cylinder head, intake manifold, andthrottle-body fuel injection module; a more-modern serpentine belt with an automatic spring-loaded tensioner for the accessories; and aDistributorless Ignition System (DIS). This revision to the engine increased power to 98 hp (73 kW). In 1988, abalance 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 samebellhousing pattern as the 2.8-liter60-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 amicartacamshaft gear that meshed directly with a steel gear on the crankshaft. 1991-92 VIN R and U engines used a timing chain instead.
TheGrumman LLV (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.[12]
| Displacement | Stroke |
|---|---|
| 2.1 L (131 cu in) | 2+3⁄5 in (66.0 mm) |
| 2.5 L (151 cu in) | 3 in (76.2 mm) |
| 2.7 L (163 cu in) | 3+1⁄4 in (82.6 mm) |
| 3.0 L (182 cu in) | 3+5⁄8 in (92.1 mm) |
| 3.2 L (198 cu in) | 3+15⁄16 in (100.0 mm) |
The Iron Duke block formed the basis of Pontiac'sSuper Duty four-cylinder racing engines of the 1980s, the last in a line ofhigh-performance Pontiac Super Duty engines. The engines were featured inNASCAR'sCharlotte/Daytona Dash Series, theIMSA GT Championship (in GTP and GTU class cars), and even inAmerican Power Boat Association racing boats. Super Duty engines continued to be used inARCA 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, 232 hp (173 kW) Super Duty engine powered the 1984FieroIndy Pace Car to over 138 mph (222 km/h) 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 also produced a16-valve, double-overhead cam head for the 3.0-liter version of the racing engine (Cosworth Project DBA, 1987).[13]
