Mopar Slant Six: Unearthing The Powerhouse of Chrysler Engines

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Chrysler looked overseas for inspiration in developing their new Slant Six in 1959. The engine might have had a weird shape, but it proved reliable for years.

Chrysler built the Mopar Slant Six as an inline six-cylinder engine from 1959 to 1984. The engine was developed with a 30-degree offset cylinder bank, which made it perfect for new vehicles that Chrysler was marketing. The engine was used in millions of Chrysler cars and trucks during its run.

By the end of the fifties, American automakers noticed a significant shift in consumer tastes as the demand for smaller import cars grew. Even though Chrysler had used the L-shaped straight six for decades, the motor was too large to be effective in any compact car being considered. Chrysler needed a new, more concise powerplant that could quickly be built, fit into smaller dimensions, and achieve better fuel efficiency. For inspiration, Chrysler turned to a 30-degree offset motor that Mercedes had developed and adapted the design to their needs. Little did they know the slant-six would become one of the most reliable, durable, and versatile engines ever. Chrysler would use the motor in almost a million vehicles and find uses in marine, commercial, and industrial vehicles. Let's look a bit closer at the Slant-Six to unearth the secrets of this Chrysler engine.

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The Beginnings of the Chrysler Slant Six

By the end of the fifties, American consumers were shifting away from the large, cumbersome automobiles that carmakers had been peddling for decades. Even though the nation was reeling from a recession, there were signs of a growing market for smaller, more compact vehicles. The success of the Ford Thunderbird and the Nash Rambler in the late fifties, along with imports like the quirky VW Beetle, was evidence that Chrysler needed to find a way to compete in this new market or risk being left behind.

Chrysler’s attempts to find a solution lay in a compact car design called the Valiant (after Chrysler sold the rights to the original “Falcon” name to Ford for their new compact model). The idea was to build a car with ample interior and cargo space without looking like a modified full-sized model. Chrysler’s design team worked from the inside out, fashioning a car that could compete in quality with European models, but even with its aggressive fresh look styling, it needed an engine.

To develop the new motor, Chrysler tapped a project engineer, William Weertman, and his team to devise a powerplant solution. The design team developed a 30-degree offset six-cylinder motor internally known as the “G” engine, tested numerous prototypes, and finally offered up the Slant Six, which debuted at the British International Motor Show in 1959 as the ‘60 Valiant.

The engine design was similar to the M186 engine Mercedes had used in its 300SL sports car for several years. The unique inclination (30-degree offset) offered several benefits, allowing intake and exhaust manifolds with runners nearly the same size as its current six cylinders. Still, instead of a cross-flow arrangement, the new angle required the runners to be on the same side of the head. With a few other modifications (primarily the location of the water and fuel pump), the result was an engine with improved torque, increased airflow, and higher performance than any flat six-cylinder Chrysler had.

The First Slant Six Engine

Chrysler offered the initial Slant Six to be built in two displacements. The first version was a 170 cubic inch (2.8 L), producing 115 hp and 155 lb-ft of torque. The engine had a short block, earning it the nickname “LG” - (Lower G). With a 3.4-inch bore and 3.25-inch short stoke, the over-stroke design provided respectable power and fuel economy. Chrysler quickly advertised the new Valiant as “nobody’s kid brother” willing to take on all comers, domestic and international. The engine was touted as a gas miser, able to run best on regular gasoline, which saved American families money at the pump.

Shortly after the introduction of the LG engine, Chrysler developed a larger version of the Slant Six motor, displacing 225 cubic inches. This engine used a tall block (which earned the RG ”raised block” moniker). The bore was carried over from the smaller motor, but an increased stroke length (4.25 inches) made the engine an undersquare design, providing excellent low-end torque. The new engine produced 145 hp and 215 lb-ft torque and was paired with a single-barrel carburetor with an 8.4:1 compression.

Initially, Chrysler used an aluminum block due to the weight reduction that the alloy provided (the lighter engine also improved fuel economy). Over the first three years of production, 52k aluminum blocks were cast and placed in Chrysler, Dodge, and Plymouth vehicles. Chrysler soon discovered that the aluminum engine block increased production costs, so they reverted to cast iron blocks after 1963, used four-bolt mains, and many of the same internals that had been a part of larger V8 engines.

The Valiant proved to be an excellent seller in its first year, and based on its early success, Chrysler began implementing the Slant Six in several different models, including the Dodge Dart, Lancer, Plymouth Fury, and Belvedere. In addition, Dodge offered the motor in its D-Series lineup of pickup trucks.

The Hyper-Pack

Chrysler understood that Slant Six might be underwhelming with an 8.5:1 compression to a generation of buyers who loved getting the best performance out of their cars. Toward that end, the company offered a detuned version of their NASCAR Hyper Pack through their dealers. Customers could purchase the modifications, which included a specially designed air cleaner, a four-barrel carburetor attached to a longer intake manifold, dual cast iron exhaust headers, and stronger pistons with enhanced rods and valve springs, among other components. The Hyper-pack increased the compression to a muscle car worthy 10.5:1, raising the power output to 196 hp (225 engine). The increased performance made the compact Valiant almost unbeatable, giving the car a broad power band with excellent acceleration. The downside of the racing modification is that it killed the car's fuel economy, made the engine ride rough, and required constant maintenance. Chrysler quietly dropped the Hyper-Pack in 1962, as the new raised block wedge motor and Hemi began to be developed for the racing program.

The RG Builds A Record Of Reliability

For the next decade, the Slant Six engine proved to be a capable performer. As its use widened into marine and commercial applications, the lower compression enabled the engine to meet many new emissions regulations that other car companies struggled to meet. The excellent fuel economy helped the engine exceed EPA mileage requirements. Its unique configuration (slant) made maintenance easy for mechanics who could easily reach spark plugs and other components.

Chrysler played with different carburetion techniques during the decade, but in 1970, the smaller 170 ci engine was replaced with a 198 ci Slant Six as the base engine. Chrysler saved considerable production costs by building the new motor on the 225 cast iron block, and even though it continued to try and tweak the engine during the early seventies, the engine remained a staple for much of the Chrysler family of vehicles.

The Times Begin To Change

Toward the mid-seventies, consumer reaction to higher gasoline prices, an oil embargo, and ever-tightening emissions regulations hampered the Slant Sixes performance. The 198 engine was scrapped due to its inability to meet the new standards, and the introduction of catalytic converters robbed the 225 of much of its power. By 1975, the RG engine was only producing a wimpy 95 hp and 185 lb-ft of torque. Dodge tried to minimize this loss of power by continuing to push the economy of the Slant-Six, as was evidenced in their sales brochure, which conveniently omits any power ratings.

Faced with rising production costs and dwindling sales, Chrysler revised the motor to hydraulic lifters and introduced a cast iron crankshaft instead of the forged steel unit it had been using. A new exhaust system with a dual catalytic converter meant even less power and worse gas mileage. Shortly afterward, when Chrysler instituted electric welding on the engine’s inexpensive aluminum intake manifold and exhaust manifold, the new procedure turned out to be a disaster. Customer complaints poured in as the engine began to have reliability and performance issues. Chrysler found customers abandoning the brand for imports made with fewer-cylinder engines and better exhaust systems.

The End Of An Era

Chrysler struggled to keep the engine in play by offering new carburetion and electronic fuel injection in the early eighties. Still, with the new electronic ignition, the motor did not adapt well to the new technologies. Sensing that the Slant Sixes had run their course, they discontinued the engine in everything but pickup trucks after 1984. The engine would continue in Dodge Ram pickups until 1987 and in various industrial and marine applications until 2000.

Despite its lack of power, the Slant Six was a reliable and durable performer. As Americans began to shift away from larger vehicles, losing their muscle car longings, the 225 Slant Six helped Mopar to stay competitive in an ever-difficult automotive landscape.