Slant Six, the unlikely rock star

In the late 1950s, Chrysler engineers, wrapping up work on its last V8 engine families (until 1999), started working on a new six-cylinder. It would become a legend—but it may not have come at all, had things been just a little different.

Slant six development story

Chrysler’s first V8 engines, developed after World War II and integrating wartime aircraft-engine work, were famously efficient; but the new “Hemi” series (a name which was applied later) were expensive, large, and hard to build in big numbers. The company quickly created a version with cheaper heads and valve assemblies, then an even cheaper, easier-to-make version mainly for Plymouth, which had volumes far, far greater than Chrysler, Imperial, DeSoto, and Dodge.

1960 Slant Six

As GM and Ford made ever-larger engines, Chrysler leaders realized they needed a good, cheap, and above all larger V8 engine; so in 1955, they  started to work on a new series, dubbed B engines, with a goal of getting them under the hoods of 1958 model-year cars. Robert S. Rarey led that project, almost magically finishing up in time with 350 and 361 cubic inch engines for that year; the 361 would continue for quite some time, but the 350 quickly vanished. In the end, this would spawn the huge 440 V8 and the unmatched 426 Hemi.

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From there, they went to work on a new engine for the forthcoming Valiant, the first new car in quite some time to be created by a small cross-functional team. It would be a radical car for Chrysler, implementing some old technologies in a new way, and the first to have computer work integrated into the body.

As Willem Weertman tells the story in his book Chrysler Engines, Robert S. Rarey (of B-engine fame) and Weertman gave the Valiant team some guidance possible candidates, such as four-cylinders, in-line sixes, and V6 engines, in both aluminum and iron (the aluminum option may have been influenced by GM’s Vega). They realized they could not use a typical in-line six with the water pump in front, while still keeping to their design goals. But then Fred Rose, the lead designer, and Ray Latham, the engine design supervisor, thought of leaning the engine over by 30 degrees, and moving the water pump to the side of the engine.

1977 Slant Six

In case you wondered, Weertman also told us why they leaned it to the right rather than the left (yes, they thought about it); the main reason was to give the fan just the right spot, near the center of the radiator, given that the engine was offset a little to the right of the car’s centerline (to make more space for the driver’s foot).

It may seem insane today, with engine programs taking years and years, but the engine’s size and type was finalized in April 1958. It was to be a 170 cubic inch in-line six, with a 3.4 inch bore and a 3 ⅛ inch stroke, along with the 30° cant. That same day, Harry Chesebrough, who ran Plymouth, asked for a larger displacement version to replace the existing flat-head six—something not in the slant six’s original job description! Rarey agreed and started working on the problem.

There was no way to bore out the engine, given its chief goal (fitting into Valiants); the head of the Valiant program, Bob Sinclair, refused to even give a quarter of an inch. Willem Weertman and Bob Rarey worked on the problem and, “within five days,” decided they could add one inch more stroke (to 4 ⅛ inches), yielding 225 cubic inches. To do that, they needed a different crankshaft, a new block with more deck height, and longer connecting rods and pushrods. Everything else would be the same. This was still in early April 1958.

1977 Slant Six

On April 16, Paul Ackerman, head of Engineering, looked at three proposals for the Valiant and for replacing the flat-head six. Two of those required the company to create new four-cylinder and new six cylinder engines, and one called for just the one slant six in two sizes; on the same day, Ackerman gave the go-ahead to the slant six, and the die was cast. Two dies were cast, really, because they had decided to make both iron and aluminum versions—one slant six with aluminum heads and blocks, and one with traditional iron blocks and heads.

The engineers wanted to eventually use aluminum for the engines to keep the weight low, reduce machining time, and use fewer machine tools; but they kept the iron version in case they couldn’t make the aluminum version work. Bob Kring, an engineer from Kokomo Casting, worked directly with the engine design people to speed up the creation of the new block and heads; his actions as a go-between were invaluable, since the casting people had never done engines and the engine people had never done aluminum.

Amazingly, the first slant six engine was dyno-tested in the final days of November 1958. Drawing by hand, working with both iron and aluminum versions, the engineers had created a working 170 cubic inch engine; the iron one ran first, and two weeks later was followed by a long-stroke aluminum 225. Kokomo started producing aluminum 170 cubic inch blocks in March 1959.

Many of the aluminum engines are still around, treasured by some current slant six owners; but because they could pump out engines more quickly using the iron design, and the slant six was now to go under the hood of just about everything Chrysler made rather than one model with moderate sales expectations, that was more important than some weight savings (especially back in that day). For a while, they made both engines, trying to ramp up the aluminum production speed; but in the end, iron won out and  the aluminum version was dropped. Chrysler would have some cause to regret that, perhaps, a decade later.

Slant six engineering and design

The aluminum blocks had cast-in iron bore liners, a feature that was to be common with Chrysler aluminum engines years later; it was a reliable, durable, cheap method of strengthening the combustion chamber. The pistons were a tin-plated aluminum alloy, with cast-in steel struts to keep everything in check as heat built up. The top ring was chrome-plated, and the second ring was tin-plated. The oil control ring had chrome-plated steel rails.

1960 Slant Six

Pete Hagenbuch told me that early slant sixes had problems with oil leaks; he was called in as an expert. His team looked at the bore finish, ring properties and clearance, and ring designs from all the company’s ring vendors. The 170 seemed to be good enough for the moment, but they switched to Chrysler’s first-ever steel rail oil ring for the 225; this was the chrome-plated Sealed Power SS 50. By the 1961 model year, they were using this chromium plated top ring and a reverse twist second ring; a vendors discovered and implemented a process which abraded the chrome plating to emulate break-in. Moving to the reverse twist ring happened nearly immediately in 1960, since the vendors only had to put their “top” mark on the other side. (The reverse twist moved a chamfer from the top of the ring to the bottom.)

Willem Weertman credited John Hurst (an engine designer at Chrysler, not at Hurst) with the idea for the slant six’s unique and efficient “bunch of bananas” intake manifold, and John Platner and Don Moore for perfecting it. The intake even had a supercharging effect, something Chrysler was to push to extremes in its big V8s; but the main goal was to get the same fuel/air mixture to all cylinders. Along the way, since the intake and exhaust ports were on the same side of the head, exhaust gases warmed the floor of the intake, making warm-up faster.  Other companies’ straight-six manifolds had some cylinders fed by the same tubes, or had sharp bends, or larger differences in the tube lengths.

spark plug tube
Spark plug tube

The engine’s oversquare design helped to build torque, as did the long stroke on the 225, but the small bore size kept the valves relatively small. The heads were otherwise fairly similar to those of the big B V8s, which had just been created; they had overhead valves, wedge chambers, and a tappet chamber which distributed oil without the need to drill holes in the block or heads. Rocker arms were steel stampings. Unlike the V8, they used aluminum tubes (sealed with O-rings) to keep the spark plugs separated from the pushrods; there just wasn’t room for cast walls. Weertman pointed out that this setup cut weight, even if it wasn’t what they wanted (in the 1970s, they finally designed around the tubes). A great deal of ingenuity went into using the same types of inexpensive parts used in the B series V8 engines.

Engine electrical gear was all packed in on one side of the engine; service parts were mostly easy to reach, though the fuel pump and distributor were pretty far down in the engine bay.

One may ask why the engines were kept slanted even in cars and trucks that could take the old in-line sixes. Warren Steele told Allpar that the slant was good in some ways, but hurt access to the spark plugs, starter, and distributor. The reason, not surprisingly, was cost: Chrysler did not want to invest in “straightening” the slant six for other cars. In any case, that would probably have hurt drivability a bit.

Performance

The Slant Six was legendary for endurance, but it was capable of high performance, too, when the company was willing to put some effort in. At launch, even with a one-barrel carb, it edged out Ford and Chevrolet motors—keeping in mind Chrysler had a reputation for being less optimistic in horsepower and torque ratings than Ford. Consumer Reports consistently praised the engine, into the 1970s, for beating GM and Ford’s straight sixes in speed and economy. The intake also helped in cold start drivability.

slant six power table

The dealer-installed 1960 HyperPak consisted of a four-barrel carburetor and numerous other parts which brought gross horsepower to 196, beating many V8s of the day; and some believe that number was conservative. Modern racers have used turbochargers and different carburetion (or fuel injection) and manifolding to produce far more than that now-quaint figure.

An Australian two-barrel version was rated at 160 gross bhp; the late 1970s’ “Super Six” used a two-barrel Carter carburetor, similar to the one used on 318 V8s, to increase responsiveness without loss of economy, but didn’t add much to the official ratings. The 1976 Feather Duster package, which went for economy rather than speed, brought a 30 mpg EPA highway rating.

1977 Super Six (slant)

Pete Hagenbuch told Allpar that the Super Six was developed in 1975, almost immediately before production, by three people—a valve engineer and two road-test mechanics. The trio set up the system, whose main benefit was simply mounting a more appropriate carburetor; even small Japanese engines had two-barrels while Americans stuck with one-barrels, an odd cost-saving measure. The trio didn’t just put on the carb, of course; they tested numerous spark advance schedules, calibrating spark advance with the carburetor, and adding a low-restriction air cleaner. The cheap single-exhaust held output back, but performance was still far more sprightly without hurting economy.

Gene Yetter told Allpar about a hotter Mopar camshaft, part number 3512639 (244° duration, 26° overlap, 0.406” lift on intake, and 0.414” lift on exhaust; 0.010” valve lash on intake; 0.020 on exhaust). This cam was recommended for engines from 1971 to 1977.

The Dutra/Erson RV10-RDP cam is a bit more practical for most, not having access to NOS parts; it can be reground from stock solid lifter cams (grind #2106). They approximate the hot Mopar cam.

The various types of slant six engines

Heavy-duty versions were created for industrial and commercial-vehicle use, including trucks, buses, and possibly some fleet use; these had a double-row roller timing chain, chrome-plated upper piston rings, and, depending on the year and application, valve rotators, Stellite-coated exhaust valves, polyacrylic valve stem seals, and high-volume oil pumps. Allpar’s Bob Rodger noted the possibility of retrofitting the double roller chain system into ordinary engines, using NAPA parts 9168 (chain), S338, and S339 (sprockets) for 1962-75 motors.

The slant six was used for a reasonably long span, 31 years, with relatively few changes. Launched in the 1960 cars, it was used in domestic cars through 1983 and domestic trucks through 1987; it continued in production for marine use all the way into 1991. Originally made in Trenton, Michigan, it moved to Toluca, Mexico when production levels dropped.

The 170 cubic inch was dubbed the RG engine, and can easily be distinguished by the short (1.5-2 inches) hose from the water pump into the lower part of the head. The 198 and 225, both of which were created by giving the 170 a longer stroke, was coded LG and had a 3-inch hose.

Hardened valve seats arrived long before unleaded gasoline became mainstream, as a preparation for a future of catalytic converters and unleaded fuel.

The original slant six was engineered to be 170 cubic inches; the 225 and 198 were add-ons. The head was created for that 170 cubic inch size, and wasn’t ideal for the larger versions. For that reason, Chrysler launched a revised version in 1968; the new heads were better for both economy and power, with the combustion chamber revised to eliminate dead spots, adding quench/squash areas and ways to try to keep the fuel/air mix around the exhaust valve, with the spark plug igniting the center of the mix. The new heads are more open, and eliminate spots caused by surface transitions.

Electronic ignition was added in the 1973 cars; it was a good, reliable system over the long haul, other than ballast resistor failure, which resulted in refusal to start. The ballast resistor continues to sell for under $5, and replacing it is easy, but diagnosis issues gave the rest of the system a bad name.

Another change for the 1973 cars was an alteration to the oil filter standpipe, which until then required long-canister filters, such as the PureOne PL30001; Scott Sigethy told Allpar that the Ford FL-30001 (made by Purolator) worked well because its double seals prevent the engine from being starved for oil. Starting with the 1973 engines, the standpipe was changed, and owners could (and should) use shorter filters; on these engines, the tall filters don’t fill all the way, causing late delivery of oil on startup.

Joseph Newhouse’s chart at Allpar showed power for the various engine; all numbers are for “49-state” versions after California started issuing its own rules. Gross and net horsepower cannot be directly compared.

CIDYearsC/RHorsepower
* = Net
Torque
1701959-19638.2 to 1101 @ 4400155 @ 2400
1701964-19668.5 to 1101 @ 4400155 @ 2400
1701967-19698.5 to 1115 @ 4400155 @ 2400
1981970-19718.4 to 1125 @ 4400180 @ 2000
19819728.4 to 1100 @ 4400*160 @ 2400
19819738.4 to 195 @ 4000*150 @ 1600
19819748.4 to 195 @ 4000*145 @ 2000
2251960-19628.5 to 1145 @ 4000215 @ 2800
22519638.2 to 1145 @ 4000215 @ 2400
2251964-19718.4 to 1145 @ 4000215 @ 2400
22519728.4 to 1110 @ 4000*185 @ 2000
22519738.4 to 1105 @ 4000*185 @ 1600
22519748.4 to 1105 @ 3600*180 @ 1600
22519758.4 to 195 @ 3600*170 @ 1600
2251976-788.4 to 1100 @ 3600*170 @ 1600
225-2V1977-788.4 to 1110*180

In the middle of the 1976 model year, the slant six swapped to a nodular iron crankshaft, replacing the prior forged steel crank. The two are both apparently more than strong enough for even hefty power gains. Steel-backed aluminum-alloy main bearings replaced the original ones in 1978, a bit after catalytic converters.

Chrysler Canada started developing hydraulic lifters for the slant six at the Windsor Engine Plant, and produced some for testing in fleet service; the idea was to have some serious miles racked up on these engines before they went into full production. They started working on them around 1976 and ran the test engines in 1978. Finally, the hydraulic lifters made it to mainstream slant sixes in the 1981 model-year. Unlike the 1960s design, they did not need to add an oil galley; instead, they grooved the rear cam bearing to feed oil up to the rocker shafts. Then it went through the rocker arm bodies and through hollow pushrods, finally reaching top-feed lifters. Thus, the block did not have to be changed, saving a good deal of corporate cash.

Numerous carburetors were used over the years, as one can imagine. They started out with a Carter BBS, which lasted all the way into 1971. From 1962 to 1973, a Holley 1920 was also used. In 1963, and only 1963, Chrysler put on a Stromberg WA-3 carburetor, for some cars; Stromberg and Carter, incidentally, were among Chrylser’s earliest suppliers.

With the emissions era in force, the Carter single-barrel was dropped after the 1971 cars and trucks; the Holley 1920 gave way to a Holley 1945 starting with the 1974 cars. Much lambasted by those who missed the simpler 1920, the 1945 is actually quite similar and works well over the years. The 1945 was used until 1981, when a feedback carb, the Holley 6145, replaced it until the final car/truck slant sixes in 1987. (This list doesn’t include marine/industrial or Australian applications.) The final Toluca slant sixes used a rejetted Carter YPF two-barrel carburetor and recurved distributor to reduce emissions by 30%. This setup was rated at 98 horsepower—a net figure, almost identical to the 1972 single-barrel output.

From 1976 to 1981, as noted earlier, a Super Six setup was optional; that used a special Carter BBD two-barrel, which was similar to the 318 carb but smaller.

Oddball slants

The first Bricklin concept car used a slant six engine; the first production cars used AMC V8s pushing out 220 horsepower (net).

Executives refused many alternatives, from a big 1967 246 cubic inch version (boasting hydraulic lifters), presumably for trucks and needing an extra oil gallery in the block, to turbocharged and fuel-injected versions. Given that the engine already had an aluminum variant, albeit one dropped in 1963, one can see engineers trying to make it more fuel efficient for future use, perhaps to supplement the 318 V8, perhaps to avoid using Mitsubishi V6 or turbocharged four-cylinder engines. Daniel Stern told Allpar about some of these.

Early on, in 1962, engineers tried out an overhead cam version of the 225; this was tested from 1962 to 1966.

Slightly larger versions, at 180 and 246 cubic inches, were tested in 1964-65; the larger one may have helped for truck use. The hydraulic-tappet version tested in 1967-68 was based on the bigger of these.

A four-cylinder version of the slant six was tested in 1969, presumably for export markets, where Valiants were just about the only popular car Chrysler Corporation had. It seems this was either too expensive to create or wasn’t economical enough to justify the cost, because it didn’t go anywhere–yet. In 1980 to 1983, four-cylinders were tested again, this time based on a 1975-80 diesel slant six (which was never actually produced); one of these had a turbocharger, the other did not. From 1981 to 1983, the company tested a diesel conversion, this time using a turbocharged version. It may be just as well the diesels never saw the light of day.

Jim Grundy, a Chrysler engineer in 1967-71, told Allpar that he had designed an overhead cam slant six at Chrysler, installing it into a Dart, but couldn’t find any interest in the project; he sold the Dart and bought it back in 1992.

More conventional updates for greater efficiency or power included a third-valve pre-chamber, similar to the Mitsubishi MCA-Jet system, tested in 1973-74; Chrysler owned a large share of Mitsubishi and could likely have tapped their patents. A more conventional solution for greater power or economy, an aluminum fast-burn cylinder head, was tried out from 1976 to 1979. Finally, the company tried turbocharging in 1977-78 and using multiple-port electronic fuel injection in 1979-80. Either of those last systems would likely have made the slant six competitive with small V8s of the period; the fuel injection, while not as sexy as turbocharging, would have made the cars far more drivable and desirable, especially when compared with primitive spark control computers.

The turbo six was used in at least one Dodge Monaco, a full-sized car.

Despite the aluminum engines having been dropped in early 1963, because they could not achieve needed production, a lightweight 225 was eventually produced from 1976 to 1980, but they were only able to shave 12 pounds off.

Tips for owners

Put two or three drops of light machine oil (e.g. engine oil) onto the felt pad under the rotor in the distributor to prevent problems. Clean or replace the crankcase inlet air filter periodically, or live with the blue smoke. Stalling in wet weather is usually the result of a low-quality distributor cap and rotor or wires. A distributor cap gasket can help.

Timing chains to stretch over time, and can even jump a sprocket or two.

Valve cover gaskets seem to work better as rubber-and-cork designs without sealers.

Rodger claimed it was easier to get a muffler with an inside diameter of 2.25 inches, and a two-inch inside-diameter outlet, recommending a long body. He wrote, “On the original engine, they used a smaller than needed diameter exhaust pipe to the muffler; this saved money. The high performance variations are different.”

The Lean Burn system, (the world’s first such computer-controlled system, except the electronic fuel injection used on some 1958 Chryslers), was basically a renaming of the old Electronic Spark Control ignition system. The ignition computer was, oddly, mounted on the air cleaner. The early version had two pickups in the distributor; a later one was able to use just one pickup.

To change spark advance on a 1971-76 engine, according to David Wordinger, the first step is to set the idle speed and idle timing according to specifications, usually shown on a sticker in the engine bay. A bolt at the distributor base can be loosened to allow the distributor to rotate. The vacuum advance hose must be taken out of the distributor and plugged before setting the timing or speed; the speed must be set first, then the timing, because the mechanical advance will still be in place after the vacuum advance is stopped. This is a good time to lubricate the felt pad under the rotor.

Cold running issues are usually due to incorrect choke and fast-idle settings, but may be due to bad accelerator pumps within the carburetor, or a slipping vibration damper outer ring, which moves the timing mark. The Carter BBS has a better reputation than the Holley carburetors. Timing chain stretch is another issue, as noted before.

Cracking manifolds remain an issue, one which can be temporarily, at least, addressed by POR15 manifold repair kits (for hairline fractures).

The two-barrel intakes made in 1979-82 were somewhat troublesome, using a new technology (electron beam welding) to put together two pieces; the single-piece two-barrel intake from Mopar Performance has a better reputation. Porosity issues with manifolds can be addressed with powder coating or a thorough cleaning followed by several coats of epoxy paint. The two-barrel manifolds may also warp at the head surface and junction, an issue addressed in some cases by thick gaskets.

Success

It took less than two years to fully develop the slant six from nothing to production. Yet, it ended up as a legend of durability, more efficient than GM and Ford sixes for two decades, with a national club devoted to it (the Slant 6 Club of America), a national racing series, and a newsletter for slant-six racers.

Today, slant sixes are beloved by many people, particularly in Mexico and South America, where they were in more upscale cars back in the day. People around the world are modifying slant sixes to produce two, three, … five, or six times as much power as they originally did. There are adaptations using used parts (e.g. Jeep 4.0 inline six fuel injection and computers), custom parts, turbos, advanced fuel injection systems, even custom designed, newly machined high-performance heads and manifolds. It’s done well for an engine designed and put into production within just two years.

Some links…

Slant Six Club • Slant Six Forums

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