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For the last time, the Iron Duke was not the same engine as the Chevy II four-cylinder

Published in blog.hemmings.com

GM Media images.

Naming an engine beyond its actual displacement can do wonders for an automaker’s marketing efforts. The capital-H Hemi has inestimable brand value, and Ford for many years relied on the Thunderbird script on V-8 rocker covers to impart the image of performance. Naming an engine can also lead to plenty of confusion years down the road, as we see with GM’s Iron Duke four-cylinder.

The Iron Duke itself is a fairly straightforward engine. Its pushrod overhead-valve design was typical of its time, as was its iron-head and iron-block construction. It had no balance shafts, multiple valves, cam phasing or any other advanced technology. On its introduction, it had a carburetor, a non-crossflow head, and absolutely nothing that would have made a buff book-reading car nerd of the time geek out. Pontiac engineers needed a durable powerplant, and they seemed to agree that durability derived from simplicity.

Conversely, the Iron Duke’s history – or, rather, the legends about its history – can be rather convoluted to some people. The most common mistakenly held belief regarding the Iron Duke’s history maintains that it’s little more than an evolution of the Chevy II’s 153-cu.in. four-cylinder – derived from the 230-cu.in. six-cylinder and used from 1962 through 1970 in the United States – and that the Iron Duke name actually applies to those engines. The second-most commonly held belief posits that the Iron Duke is somehow based on the Pontiac 194-cu.in. slant-four used in the Tempest. And finally, some people argue the Iron Duke was actually based on the Pontiac 301, based on the same introduction date for both engines (1977), the same bores (4.00 inches), and the same valve sizes (1.72 inches intake, 1.50 inches exhaust).

All are, as Pontiac engineer John Sawruk noted in his SAE paper on the Iron Duke, incorrect. However, as with all legends, there’s a kernel of truth at the core of the above theories.

The story of the Iron Duke starts, according to Sawruk, in 1973 with the OPEC oil embargo. Pontiac’s engineering staff realized the need for smaller engines and came up with five main goals for any new, smaller engines: minimal noise and vibration, maximum power, durability, driveability, and fuel economy. “Durability of the new engine was of prime concern,” Sawruk wrote. Indeed, Sawruk and his colleagues likely had the Vega’s four-cylinder engine – and its reliability problems – foremost in their minds in 1973 and wanted to do everything they could to avoid repeating those missteps.

They started by considering a number of different configurations: smaller V-8s; V-6s and V-4s derived from cut-down V-8s; an S-4 using the outer two cylinders of one bank of a V-8 and the inner two cylinders of the opposite bank; an all-new straight-six; and an all-new L-4. They even thought about resurrecting the Tempest slant-four though, as Sawruk pointed out, it wouldn’t have worked. “While it was likely that the least tooling expenditures would have resulted from creating an updated version of the 1961 Tempest L-4, it was felt that this approach would not yield the optimum design in light of the goals for the new engine,” he wrote. “It was also felt that lower mass and smaller external size would result from designing a new engine rather than copying the 1961 Tempest engine.”

Technically, the Pontiac engineers went with an all-new inline-four, and they did derive inspiration and some elements from the Chevy II four-cylinder, but just not the one we’re familiar with here in the States.

As part of the decision to build the new L-4 engine, a review was made of recent General Motors engine designs. This review revealed a 2.5 Litre (151-cu.in.) L-4 engine was being produced by General Motors do Brasil. This particular engine had started life as a version of the early 1960s Chevrolet II 153-cu.in. L-4 which is currently produced by General Motors of Mexico. General Motors do Brasil had reduced the stroke from 3.25″ to 3.00″ and increased the rod length from 5.70″ to 6.00″ to reduce the vertical shaking forces inherent in an L-4 without counter-shafts. At the same time, they had increased the bore from 3.88″ to 4.00″ to maintain the displacement. Pontiac’s analysis indicated that this engine had significantly reduced secondary vertical shake versus either the 153-cu.in. engine of the aluminum 2.3 Litre (140-cu.in.) engine then being used in small General Motors vehicles. In addition, since the 2.3 Litre was currently being produced by Chevrolet, the 2.5 Litre, being slightly larger, fit well into Pontiac’s marketing plans.

So it really was based on the Chevy II 153, right? Except, nope.

While some features of the General Motors do Brasil engine were retained, such as displacement and bore centers, the majority of the pieces are not interchangeable with the new Pontiac L-4. For example, such basic parts as the intake manifold, the cylinder head, the exhaust manifold, the rocker cover, the oil pan, the connecting rod, and the piston cannot be mixed between the engines. Different bolt patterns and all new tooling are used to build the Pontiac L-4.

The 151-cu.in. Pontiac L-4 debuted in 1977 as an optional upgrade to the 140-cu.in. overhead-camshaft four-cylinder in the Astre and Sunbird and as a credit option (with the four-speed manual transmission only) in the Ventura and Phoenix. A year later, it made its way into the Chevrolet Monza, and by 1980 every GM division except Cadillac was using it, as was AMC.

As for the Iron Duke name, it was no mere nickname bestowed by Pontiac fans. Instead, Pontiac itself used the name in its advertising announcing the new engine, as seen in the above image showing an Astre, Sunbird, and Ventura. The name came, in part, from the aforementioned desire to distance the new engine from the aluminum Vega four-cylinder.

“Pontiac makes no apologies for the lack of overhead cams or exotic carbs,” according to a 1977 Pontiac brochure. “This cast-iron four is a remarkably rugged new engine.”

(The Duke part? Any one of a number of people, including the first Duke of Wellington, could have inspired it, as could any number of films, trains, ships, or even British pubs.)

The engine went on to gain a cross-flow head, throttle-body fuel injection, and a new name – the Tech IV – over its 16-year lifespan. Pontiac engineers even developed a Super Duty version that, though never released to the public, made for a formidable racing engine. A 3.0-liter version of the Super Duty that appeared in the 1989 Pontiac Stinger concept used 16 valves to make 170 horsepower and shouldn’t be confused with the 3.0-liter Vortec version of the Chevy II 153-cu.in. four-cylinder that GM apparently still sells as a marine and industrial engine.

GM eventually replaced the Iron Duke with the longer-lived 122-cu.in. four-cylinder, an engine that seems to have suffered from far less confusion over its origins, if only because nobody seems to have ever given that engine a second thought. Better to be misunderstood than forgotten, perhaps?