10 Shocking Truths About Mopar’s 225 Slant Six Engine – Chrysler’s “Bulletproof” Engine
Summer of 1972.
On a lonely back road, a battered $500 Dodge Dart shocked the muscle car world.
It outran a big block Camaro, the pride of American horsepower.
Its secret weapon, not a Hemi, not a Magnum, but a 30° slanted inline six.
Engineers hailed it as a masterpiece, while executives dismissed it as an embarrassment.
This was the Mopar 225 slant 6, a humble yet extraordinary engine that could fire up on freezing mornings, survive hundreds of thousands of miles, yet was held back by Chrysler’s obsession with V8 image.
Today, we’re diving into 10 shocking and hidden truths about this engine.
From its unusual tilt design, the forgotten aluminum block experiments to conspiracy theories that it was deliberately sabotaged to protect Chrysler’s HMI legacy.
Number one, the unique 30° slant.
When Chrysler set out to develop the Valiant, their new compact car for 1960, they faced a serious problem.
The all new inline 6 was simply too tall and too long to fit the low, short engine bay.
After countless late night debates, chief engineer William Wittmann floated what sounded like a joke.
What if we just tilt it sideways?
Surprisingly, the idea worked so well it changed history.
Tilting the engine 30° to the right reduced its overall height, allowing it to slip under the hood without compromising cooling or piston stroke.
Even better, the slant brought unexpected benefits.
Lowering the center of gravity, improving balance on the road, and making spark plug access and top-end service far easier.
But here lies the mystery.
Some insiders later claimed Chrysler could have redesigned the block or lowered the hood line instead.
Choosing the 30° tilt may not have been just about packaging.
It could have been a deliberate move to give the engine its own identity, shielding the Slant 6 project from executives obsessed with V8s.
Number two, the pioneering aluminum block that was killed off.
From the very beginning, Chrysler wanted the Slant 6 to be more than just a stop gap solution.
It had to showcase innovation.
So, in 1960, they introduced a special version with an aluminum block, a technology usually reserved for race cars or aircraft at the time.
The lightweight material shaved over 70 lb compared to cast iron, giving the engine a clear advantage in weight distribution and fuel efficiency.
But the dream didn’t last long.
Casting aluminum proved complex.
Production failure rates were high and costs skyrocketed.
Meanwhile, American buyers weren’t impressed by new tech.
They only cared about cheap durability.
As a result, the aluminum block was quietly shelved not long after its debut.
Here’s the controversy.
Was it really just about cost or was something else at play?
Some former engineers believe Chrysler’s executives feared that if the aluminum Slant 6 proved successful, it would overshadow the heavy V8s that defined the brand’s image.
Number three, chromeplated pistons.
Before the Slant 6, Chrysler’s Flathead 6 had a notorious weakness, excessive oil consumption.
It was a nightmare for both customers and mechanics, and it badly tarnished the company’s reputation.
When developing the Slant 6, engineers knew they needed a permanent fix to erase that stigma.
Their answer was a set of tinplated aluminum alloy pistons paired with chromeplated top rings.
The hardened chrome surface resisted wear and sealed the combustion chamber more tightly, dramatically reducing oil seepage into the cylinders.
For a mass market American engine in the 1960s, this was an unusually advanced solution.
What’s curious is Chrysler never bragged about it.
Internal documents revealed that chromeplated rings cost nearly twice as much as standard parts.
Yet, management approved it because they feared another flathead disaster.
Some engineers even claimed Chrysler deliberately kept quiet, wanting the Slant 6 to maintain its cheap engine image rather than admitting they had resorted to high-tech metallurgy to cover up past mistakes.
Number four, the bunch of bananas intake manifold.
One of the most curious details on the Slant 6 is its bunch of bananas intake manifold.
A set of twisted runners resembling a banana cluster.
From the start, it wasn’t a styling gimmick, but a desperate solution.
With both intake and exhaust ports located on the same side due to the 30° slant, under hood space was extremely tight.
Chrysler engineers responded by designing intake runners of varying lengths, snaking around like vines to clear the exhaust and fit under the hood.
This setup also brought an unexpected benefit.
Routing intake air near the hot exhaust allowed the engine to warm up quickly and meet the new US emission standards of the 1960s.
The compromise, however, was obvious.
Unequal airflow distribution robbed the engine of potential power, especially at higher RPMs.
That’s why many Slant 6 owners praised its reliability and economy, but rarely its outright performance.
Number five, the seven main bearing crankshaft.
One of the most important reasons the Slant 6 earned its bulletproof reputation lies in its crankshaft.
While many inline sixes of the era use just four or five main bearings, the 225 Slant 6 was equipped with seven main bearings, giving nearly every cylinder its own support.
This design evenly distributed forces across the crankshaft, minimized flex and wear, and delivered exceptional durability.
As a result, countless Slant 6 engines ran for hundreds of thousands of miles without major rebuilds.
Some even reached 400,000 m with nothing more than basic maintenance, an almost unheard of feat for a mass market engine of the 1960s.
Number six, Hyper Pack, the suppressed performance package.
Few people realize that the Slant 6 once had a high performance version that shocked the muscle car world.
This was the Hyper Pack, developed by none other than Tom Hoover, the father of the HMI for NASCAR’s compact class in the early 1960s.
The Hyperack featured a Carter, fourbarrel carburetor, longrunner intake, performance cam shaft, domed pistons, stiffer valve springs, and tubular headers.
The results were stunning.
The 170 CI in made 148 horsepower, while the 225 C in produced an impressive 196 horsepower, nearly double the standard output.
At Daytona, the Valiant Hyperack racked up multiple wins, leaving larger cars in the duSt. The shocking part is that Chrysler never fully promoted the Hyper Pack.
Instead of making it a production model, they only offered limited kits through Mopar dealers.
Many believe Chrysler deliberately kept it in the shadows, fearing that a powerful Slant 6 might overshadow their small block V8s and threaten the HMI’s prestige.
Number seven, the Australian Crossflow Hemi, a licensing mystery.
When Chrysler brought the Slant 6 to Australia, local engineers didn’t just assemble it.
They saw untapped potential.
In 1970, the Valiant VG Pacer debuted with a completely new cylinder head, the Crossflow Hemi head.
This design allowed intake and exhaust gases to flow in opposite directions, reducing heat buildup and improving air flow.
Combined with hemispherical combustion chambers, it delivered more efficient combustion and a stock output of 165 horsepower.
With Weber carburetors and aggressive cam shafts, many pacers produced far more than factory figures.
The mystery is why did Chrysler America never adopt this crossflow Hemi?
They held the patents and could have mass-produced it.
Yet US slant 6 engines stuck with the old head design.
Some former engineers suggested Detroit executives feared that a HMI inline 6 would steal the spotlight from the legendary HMI V8, the crown jewel of Chrysler’s image.
Number eight, the chaotic carburetor story.
Starting in 1960, it came with a simple Carter BBS single barrel, easy to maintain and reliable.
But within just a few years, Chrysler began switching rapidly.
In 1962 came the Holly 1920.
Some 1963 models unexpectedly used the Stromberg WA3, and variations even depended on whether the car had a manual or automatic transmission.
This constant turnover left mechanics and owners baffled.
Many joked that Chrysler was throwing darts at the parts bin.
Each model year seemed to have a different carburetor, making diagnosis, repairs, and replacements unnecessarily complicated.
Some observers believe this wasn’t just the result of experimentation, but possibly a deliberate way to keep the Slant 6 humble.
By never giving it a consistent optimized carburetor platform, Chrysler ensured the engine would never reach its full performance potential to threaten the V8 lineup.
Number nine, 60% cheaper to build than a big block.
Later released internal documents revealed a stunning figure.
The Slant 6 cost up to 60% less to produce than Chrysler’s big block V8s.
It sounds counterintuitive since the Slant 6 wasn’t cheap in design.
It had a thick cast iron block, a seven bearing crankshaft, and specially plated pistons.
Yet its simplicity, lower part count, and ease of mass production made it far less expensive to build.
In theory, Chrysler could have leaned on the Slant 6 as a savior during the oil crisis and economic downturns.
A motor that was cheap to manufacture, absurdly durable, and more fuel efficient than a V8 was the perfect formula for survival.
Instead, the company poured its marketing dollars into the HMI, the Magnum, and the 440 big blocks, leaving the Slant 6 dismissed as a grandma engine.
Number 10, the Slant 6 in industry.
The Slant 6 didn’t just serve on the road, it lived a secret life in industry that few people talk about.
By the mid 1960s, Chrysler offered industrial versions of the engine for generators, irrigation pumps, and farm equipment.
In those roles, it had to run at steady RPMS for thousands of hours on end, something most musclebound V8s could never handle.
A Popular Mechanics article in 1963 described a slant 6 powering a farm irrigation pump in the Midwest running 247 for nearly 6 months without being shut off.
Engineers explained that its moderate RPM range, simple cooling system, and the natural balance of an inline 6 configuration made it ideal for such punishing duty.
What’s curious is that Chrysler rarely publicized this.
Perhaps they didn’t want the Slant 6 associated with pumps and generators rather than cars.
Yet, this hidden legacy only reinforced its reputation.
An engine you could drop into almost any job from hauling New York taxis to working Kansas farms and it would keep running seemingly without ever growing tired.
The Mopar 225 slant 6 was never Chrysler’s darling.
Yet it became a symbol of durability and ingenuity.
Built to solve a small problem, it survived millions of miles from city taxis to farm pumps and even spawned hyper pack and cross flow hemi versions that made V8s nervous.
The shocking truth is that Chrysler had an engine cheap to build, fuel efficient, and nearly indestructible, but sidelined it to protect the V8 image.
That’s why today the SL 6 isn’t just an engine, but a living legend for enthusiasts.
