Angular Inclinations: The Rise And Fall Of The Mopar Slant Six

Angular Inclinations: The Rise And Fall Of The Mopar Slant Six

Small, large, bizarrely shaped, peculiar in function, or outright enormous, oddball engines like the Chrysler Slant Six are something that we take great joy in dissecting over here at EngineLabs.

But while some lean toward the “weirder the better” side of the engineering extreme, we do still value practicality and purpose in very high regard when it comes to production engines. The same can be said for reliability, ease of augmentation and maintenance, and efficiency and performance.

The Chrysler Slant Six offered a tilted take on the inline six-cylinder motor that at the time, only Mercedes Benz had mastered aforehand. For Chrysler, this design provided the perfect solution to a time-sensitive issue, with the Slant Six ending up being produced on a global scale. Was it the most ideal design of all time, or the most powerful six-cylinder powerplant in history? Hell naw. But it sure did have its time in the limelight and did its job with minimal fuss.


All told, Chrysler produced Slant Six car engines in the millions from 1960 all the way up until the 1984 model year, with U.S. markets seeing Dodge trucks rocking the motor all the way up until 1987. Commercial, industrial, marine, agricultural applications… Chrysler popped these reliable straight-six motors into damn near everything.

However, even the best engine designs eventually become dated, and for the Slant Six its demise had just as much to do with emissions restrictions and bizarre electronic controls, as it did with dated designs and carburetors. But oh what a run this powerplant had for those thirty years or so. Thus earning the Chrysler Slant Six a boomer nod of approval, and an EngineLabs certification as a quintessential everyman’s motor.

Manufacturing of the Slant Six for the United States market began in Michigan, but soon swelled to Ontario to provide Canadian customers with engines as well. Production plants in Mexico, Turkey, South Africa, and Australia later provided engines for their respective markets too. Photo Credit: What it’s like/YouTube

Looking for a Fresh Angle

By the late 1950s, Chrysler had come to realize that its straight-six flathead wasn’t cutting the kielbasa anymore.  For nearly three decades, its non-V8 applications had been outfitted with this engine, and by the mid-1950s Chrysler’s execs decided that this engine from 1929 was due for retirement.

But a secondary motive was spurning this executive decision. America’s newfound love affair with smaller, more efficient European (and later Japanese) automobiles had forced Detroit automakers to rethink their heavy-handed investment in lumbering land yachts, performance vehicles, and full-size pickup trucks.

The market’s unprecedented shift toward small and efficient automobiles meant creating an affordable, compact engine, and an all-new automobile to go along with it. And so development of the Valiant chassis began in earnest. But, Chrysler’s engineers soon realized that their all-new inline-six engine they had constructed for the car would not fit within the platform’s smaller engine bay.

Photo Credit: What it’s like/YouTube

Hood clearance concerns were the most apparent, followed by the forward-mounted water pump causing even more concern. The engine was just a fuzz too tall, and a bit long for the bay with those mandatory accessories attached.

After a few days of mulling things over, project leader William Weertman and his team realized that by tilting the engine to the right, hood clearance concerns could be negated, and the cooling fan could remain in place over the radiator. From there, all that was needed was the relocation of the water pump to one side of the engine, a bump in engine mounting so that it was a just a smidge off-center, and the project was back on track.

Photo Credit: What it’s like/YouTube

Sporting a 30-degree off-vertical angle (hence the “Slant Six” nomenclature), this design was just what Chrysler needed to power the compact Plymouth Valiant, as well as the Dodge Dart Lancer, and Chrysler Imperial. It had just enough performance to entice the public.

The initial iteration came in a 170 cubic-inch package, which when pushed, produced 117 horsepower at 4,400 rpm, and 155 peak pound-feet of torque at 2,400 rpm. Constructed almost simultaneously was a larger, 225 cubic inch variant, that made 145 horsepower at 4,000 rpm and 215 lb-ft of torque at 2,800 rpm. However, all of these figures remain contested to this day, with many old-school car guys claiming that like GM and Ford, Chrysler fudged on its figures to bolster buyer appeal.

Photo Credit: What it’s like/YouTube

Ingredients, Budgets, and a Whole Bunch of Bananas

While aluminum engine blocks were simultaneously implemented when the Valiant was first launched, iron versions were far more common, due to ease of production. It didn’t take Chrysler long to halt production of the aluminum Slant Six block entirely. This was due in part to a lack of public interest, and also because of the manufacturing headaches.

Internally, pistons were a tin-plated aluminum alloy component, with the top piston ring on the 225 being chrome-plated steel rail units, which was intended to eliminate previously encountered oil consumption issues. More on the bigger 225 here in a bit, as this revised six has its own mini backstory.

Now as for the heads of the Slant Six, these shared many of the same genetics with the Chrysler B V8 engines from that era. Partially done out of budget concerns, Chrysler banked heavily on the fact that these heads worked well and were a familiar service item for dealership service techs.

Photo Credit: Stephen Lafluer/YouTube

Regarding the Slant Six’s bizarre, and efficiency-focused “bunch of bananas” intake manifold, it had everything to do with engine bay constraints, and the the fact that the engine’s intake and exhaust ports were located on the same side of the engine. By utilizing this design, Chrysler was able to meet the U.S. government’s all-new list of environmental regulations, as close proximity to exhaust gases made intake warm-up far more efficient. On the downside, this also caused intake air charges to be anything but cool; and with its unequal runners hampering things further, the design of the manifold proved to be anything but performance-oriented.

The Slant Six used aluminum tubes to separate the spark plugs from the pushrods, which made them prone to leaks. The issue was remedied for 1975 models, when the heads were redesigned. Photo Credit: What it’s like/YouTube

30 Degrees of Success

Like the flathead engines that came before, Chrysler had intentions from day one to slap the Slant Six into damn-near every application imaginable once proven to work reliably in its all-new Plymouth Valiant compact car.

The debut of the Slant Six in 1960 was a resounding success. With the Plymouth Valiant selling like hotcakes, Chrysler quickly set to attaching its space-saving slanted engine into one application after another. Trucks, vans, RV’s, tractors, boats, and even industrial equipment were all tapped to be powered by this offset design.

Intended to address efficiency and engine bay constraints above all else, the 170-cube Slant Six did receive a lot of flack when it was launched, for it was quite the snoozer in the performance department. This was due, in part, to those previously mentioned unequal-length runners that fed its single-barrel carburetor, both of which came precalibrated for maximum fuel efficiency gains.

Photo Credit: What it’s like/YouTube

Naturally, it didn’t take long for Chrysler to get hip to the fact that some of its Valiant buyers wanted a bit more performance. So  426 HEMI mastermind, Tom Hoover, was tasked with developing a solution, and then implementing this design at NASCAR’s first (and last) compact car race in Daytona.

While the race cars received their own Slant Six variants, Mopar dealers were gifted with a detuned “Slant Six Hyper-Pak” to pitch to potential Valiant buyers. With a four-barrel carburetor that had been port-matched to a long-runner intake manifold, a more aggressive cam, stronger springs, domed pistons, a header, and other mods, power figures easily spiked to 148 ponies on the smaller engine. A total of 196 brake horsepower was also observed on the 225, helping the odd six-pot engine outperform most pedestrian V8 engines from that period.

Four main bearings and solid lifters were part of the original Slant Six recipe, as was a forged crank. Updated versions received a cast crank and hydraulic lifters. Photo Credit: What it’s like/YouTube

Dissecting That Second Serving of Slant Six Smarts

Fully aware that Americans wanted more performance and power out of Chrysler’s all-new angled engine, the second engine it released that same year proved to be the long-term winner for the brand. Featuring the same footprint as the smaller model, the more potent Slant Six came loaded with a one-inch longer stroke for more cubic inches and usable power. The changes needed to make this possible included a different crankshaft, a new block with taller deck heights, and longer connecting rods and pushrods to accommodate the internal changes.

And while the previously mentioned “Slant Six Hyper-Pak” for the Valiant saw some success, Chrysler had its sights set on other applications for its brawnier straight-six. The 225-cube versions of the Slant Six were soon powering Dodge trucks and hulking Plymouth land yachts alike, for its smaller size and slanted design allowed it to fit pretty much anywhere.

Interestingly enough, the motor was never straightened, even when plopped inside trucks and cars with generous amounts of engine bay real estate. This would have made maintenance infinitely easier when trying to replace things like fuel pumps or distributors, both of which were stuffed deep within the bowels of the bay.

Photo Credit: What it’s like/YouTube

Puny Performance Packs and Prudish Repackaging

With both versions of the Slant Six engine now in mass production, it was time to sit back and assess customer feedback and long-term reliability, as well as any redesign requirements that might materialize. From its release in 1960, all the way up until 1971, an array of different camshafts and cam timing came and went. These were accompanied by a ton of different carburetors and ignition mapping combos for meeting tightening emissions standards and for toying with things like compression ratios.

It also didn’t take long for the heads to receive a redesign for the 1963 model year, as efficiency, performance, and emissions concerns continued to torment the automobile industry. This resulted in those leaky, washerless spark plug tubes being utilized, which remained in play all the way up until the mid-1970s.

Come 1967, things got a bit more performance-packed, with the 170ci variant receiving a beefier carburetor and the camshaft from the 225. This was attached to an overhauled head that was intended to increase economy and power. Two years later, in 1969, a carburetor de-icer apparatus, using air that had been heated by the exhaust, was applied to the 225ci version of the Slant Six.

But being that the vast majority of Slant Six shoppers were just looking for efficiency and reliability, and weren’t so interested in modest factory upgrade options, the engine continued to be promoted for its practicality, and not for its performance potential.

Does anyone here recall the “Super Six” performance pack? That 10-percent bump in power was Chrysler’s solution to creating more horsepower, all while retaining fuel efficiency figures post-1970s U.S. oil crisis. This was the same engine option that came with hidden idle adjustment screws to prevent DIY-prone dads from tuning their carbs. The same “Super Six” setup used a detuned Carter BBD two-barrel carb from a V8 engine from 1977 to 1983 to cut budgetary corners and remain within those restrictive emissions regulations. Needless to say, this would not be remembered as one of Chrysler’s crowning achievements.

The Chrysler Slant Six wasn’t without its own infuriating issues. Take ignition timing for example, which varied wildly based upon engine size, chassis code, transmission, and emissions requirements for that year. The result? An infinite amount of ignition timing options spread across more than two decades. Photo Credit: What it’s like/YouTube

Good Carbs, Bad Carbs, Empty Carbs… All the Carbs!

When it came to Slant Six carburetors, Chrysler blew through them faster than Homer Simpson in a doughnut shop. While it all started with the base Carter BBS, which remained the entry-grade option all the way up until 1971 on North American cars — and until 1974 trucks — things soon spiraled out of control when it came to multi-barrel flow.

Upgraded Holley 1920 carburetor options were the first to arrive in 1962, and remained a consistent core component on the Slant Six for more than a decade. There were also reports of certain models in 1963 receiving a Stromberg WA-3 carburetor for some odd reason, while manual and automatic vehicles received completely different carburetors across the board.

When the Holley 1920 was finally retired in 1973, the Holley 1945 stepped up to bat. Constructed to meet emissions restrictions above all else, this carb ran far leaner than its predecessor, and was often ditched for a more potent unit by DIY guys or those in search of more torque.

Caption The easiest way to tell a 170 vai variant from larger versions of the Slant Six is by measuring the water pump hose that feeds into the head. Anything lengthier than two inches long is going to either be the 198ci (which later replaced the 170) or the 225 ci engine. Photo Credit: What it’s like/YouTube

Slant Six Evolution or Devolution, You Decide

After about a decade of sales, Chrysler retired the 170 cubic-inch Slant Six for the 1970 model year, and began offering a 198 cubic-inch Slant Six instead. It was a motor that was built around the 225 block, but with a shorter throw on the crankshaft side and even lengthier connecting rods.

A year later, Chrysler switched these engines over to the twin-barrel carbs it had been using on its marine Slant Sixes with revised solid lifters for all of the Michigan and Ontario engines. This was followed a couple of years later with the introduction of an electronic ignition and a conversion to universal Chrysler V8 oil filters.

By 1975, the 198 had been retired due to power-robbing emissions restraints, leaving all future Slant Six engines to have 225 cubic inches of displacement. To celebrate, the engines received an all-new cylinder head for that model year, which eliminated the leaky spark plug tubes and brought back the crush washers from the early production years. Unfortunately, this “upgrade” necessitated the removal of the head to replace or adjust the lifters.

It was around this same time that emissions regulations forced Chrysler to add catalytic converters to its lineup. Along with a few other mandated emissions changes, the 225ci Slant Six was hobbled to the point where it was now only producing 95 horsepower and just 185 lb-ft of torque.

The same emissions regulation changes also forced the automaker to replace the mechanical lifters with hydraulic units. In return, this required the regrinding of the rear portion of the camshaft to allow more oil to access the rocker arms and pushrods before reaching the lifters.

Starting in the 1976 model year, Chrysler cut corners even further, with the introduction of a cast-iron crankshaft replacing the forged steel unit. Three years later, in 1979, Chrysler implemented its “electron beam welding” technologies on an entire line of inexpensive, lightweight aluminum intake manifolds. Coincidentally, this ended up causing more problems than solutions, as these intakes have not been known for their reliability.

The Slanted Ship Slowly Sinks

By the time 1979 materialized, and with it those crappy aluminum intakes, some seriously intriguing advancements were being put to the test via a massive North American debut. And no, we’re not talking about the Sony Walkman or the McDonald’s Happy Meal.

Unlike portable cassette tapes, headphones, and fast food kid’s meals with toys inside, Chrysler’s decision to attach electronic feedback carburetors on its Slant Six engines proved to be a massive failure. Theoretically speaking,  this design wasn’t all that bad, with its oxygen sensor adjusting air-fuel ratios during warm-up or when under heavy load. But once paired with a problematic Holley 6145 carb setup, disaster was literally in the air.

Finnicky and unnecessarily complicated, this standard-issue carbureted “upgrade” for the 1981 model year may not have been the first nail in the Slant Six’s coffin, but it definitely was a big one. Coupled with dual catalysts that came with their own maintenance requirements and trigger-happy warning lamps, this was the beginning of the end for the venerable Chrysler Slant Six.

Attempts at turbocharging, fuel-injecting, and converting Slant Sixes over to diesel either failed or were sidelined for newer engineering advancements or less expensive endeavors. And with a 1980s-grade ECU controller slapped atop the air cleaner, vacuum leaks and electrical problems soon ran rampant, along with consumer disinterest in the Slant Six.

By 1984, only trucks intended for sale in North America were being equipped with the tilted Chrysler motor. Even more confounding was the fact that these trucks were only made available with a single-barrel carburetor. And while the Slant Six soon disappeared from the U.S. market in favor of diesel, V6, and V8 engines, production continued down in Mexico for quite some time afterward.

From what we’ve been able to glean, the last Slant Six motors were marketed for international marine applications sometime in the early 1990s, which, needless to say, saw lackluster sales figures. Making for a rather sad send-off to one of the most widely produced and reliable carbureted motors ever to bear six pistons and a compact footprint.

Photo Credit: What it’s like/YouTube

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About the author

Micah Wright

Raised on LEGOs by grandfathers who insisted on fixing everything themselves, Micah has been a petrolhead in training since age four. His favorite past times include craft beer, strong cigars, fast cars, and culinary creativity in all of its forms.
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