Aluminum Block ?

Feature Article from Hemmings Classic Car
April, 2006 - Steve Magnate


If you're like us, the words factory prototype conjure thick veils of secrecy, industrial espionage, spy rings, Area 51 zero-point anti-gravity machines and all manner of things mere mortals just aren't supposed to see. So who would have expected an engine as mundane and workaday as Chrysler's Slant Six to have its own talisman of rare, exotic factory prototype parts and stories? With more than 12.5 million Slant Sixes built between 1959 and 1991, it's common knowledge that they were all made of cast iron, right? Not so fast. The one on display here breaks that rule--it's all aluminum!
Remember, for every automotive development that makes it to the showroom, dozens of dead ends are explored. Some of these brainstorms even make it from the drafting table to the proving grounds before they're unceremoniously junked. There's a dumpster in automotive heaven brimming with rejected ideas, and this aluminum Slant Six offers a glimpse. It features a trio of exotic tidbits that include a modern Clifford Performance reproduction of the once rare 1960-'62 A785 Hyper-Pak aluminum four-barrel intake manifold, a one-of-46,767 made die-cast block from Chrysler's ill-fated 1961-'62 A826 aluminum-block program, and the crowning touch, an honest-to-gosh 1960-vintage factory prototype aluminum cylinder head. Best of all, the engine runs and lives today in a 1962 Plymouth Valiant that gets driven, and drag raced, regularly in Los Angeles.

While the reproduction Clifford Hyper-Pak intake manifold was recently purchased new and the block was rescued from the recycling bin by a thoughtful L.A. area machine shop, the aluminum head is particularly unusual. It is one of two that were spirited away from Chrysler's Kokomo, Indiana, Forge and Foundry division some time in the 1960s by a former employee, now deceased. These heads resurfaced at his estate auction and were purchased by a fellow Kokomo employee and Mopar enthusiast who wishes to remain anonymous. This fellow sat on the parts for several years before selling them to me.

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Iron or Aluminum?
Few people realize that when Chrysler established the final design parameters for the 170-cu.in. low-deck and 225-cu.in. raised-deck Slant Six engines in May 1958, the blocks, cylinder heads and intake manifolds were intended to be made of aluminum. After all, this was to be an all-new engine for the corporation's first post-war compact car, the 1960 Plymouth Valiant.

According to detailed accounts given by Willem Weertman, the managing engineer of Chrysler's Engine Design group between 1955 and 1962: "From the beginning, the Valiant engine program had the optimistic view that, with concerted design, development and manufacturing effort, we could have an aluminum engine at little cost over a cast-iron engine. As insurance for the aluminum program, cast-iron versions of the block and head were also designed and procured for the 225 as well as the 170 engine... It was understood that if the development of the aluminum engines did not encounter major problems, the development of the cast-iron versions would be terminated at a future date when the success of the aluminum engines was assured."
By November 26, 1958, the world's first Slant Six was assembled and running in a Chrysler test cell. It was an iron 170-cu.in. version. Within two weeks an aluminum 170 was running and, by February and March 1959, prototype iron and aluminum 225-cu.in. engines were also running. But it was not all sweetness and light. Numerous setbacks with the aluminum engine block die-casting equipment forced Chrysler to play it safe and the Slant Six made its 1960 debut with a traditional cast-iron block. The aluminum-cylinder-head program was also running into trouble.

"All foundry attempts to improve the casting quality of the aluminum heads had been unsuccessful and this was just too big a risk to chance in the first year," Weertman said. "This meant iron heads also appeared on the inaugural-year Slant Six."
One respite from the gloom was the success of the sand-cast aluminum, single-barrel intake manifold. Every 1960 Slant Six engine was so equipped but cost cutting got the best of it. For the 1961 model year, a virtually identical cast-iron version replaced it and remained in use until an updated aluminum part, with EGR provisions, was released for a 1976-'77 engine weight-reduction campaign.
Despite the slow start, Chrysler was persistent in continued aluminum-block development and, in 1961, some 10,767 of the engines were built and installed as optional equipment in regular-production Plymouth Valiant and Dodge Lancer compact cars. Approximately 36,000 more were installed in 1962 Valiants and Lancers. According to the 1962 Blue Book, the optional aluminum-block 225 for Valiants and Lancers carried option code 502 and cost $47.35 ($38.60 wholesale) over the base 170 cast-iron Slant Six. By comparison, the optional Music Master AM radio cost $58.50 ($45.45 wholesale). Interestingly, the Blue Book does not list the optional aluminum 225 for any other Chrysler Corporation automobile or truck, dispelling many myths and supporting the probability that it was only offered in compacts.
Despite selling nearly 50,000 units, the aluminum option was soon cancelled.
"While the lower weight was still a desirable feature, this held little perceptible advantage to the consumer, either in terms of fuel economy or performance," Weertman said. "Taking everything together, it simply wasn't worth the effort."

Moreover, the slow-selling aluminum engine was sapping production time and space from its wildly successful cast-iron counterpart--which further sealed the lightweight engine's fate.
"To continue further development of the aluminum Slant Six manufacturing process at the Trenton Engine Plant would have reduced plant efficiency at a time when the demand for Slant Six engines was escalating," Weertman said. "By Engineering and Manufacturing consensus, the aluminum-engine program was terminated at the end of the 1962 model year and only iron cylinder blocks were produced after that."

The sun was also setting on aluminum head development. While engineers continued to make do by repairing flawed prototype castings and numerous engines and vehicles sporting alloy heads racked up dyno and test track time, the end was inevitable.

"On February 17, 1960, the Forge and Foundry Division requested that the aluminum head be cancelled from production consideration due to an inability to obtain castings free of porosity and that only the cast-iron heads be produced," Weertman said. "As such, the aluminum head never reached dealer showrooms, and it is especially ironic that the relatively few 1961 and '62 aluminum-block production engines were equipped with comparatively mundane iron heads."

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Aluminum Fever
Chrysler wasn't the only company afflicted with aluminum fever during the post-war era. All of Detroit saw aluminum as a means of saving weight, facilitating annual brightwork updates, improving vehicle dynamics and reducing manufacturing cost. You could say aluminum had the same marketing and manufacturing buzz factor that carbon fiber does today, probably more. A study conducted by Alcoa in 1956 revealed that one of 1955's top users of aluminum was the Chrysler Imperial, carrying 65 pounds per car. For 1956, the top honor went to the Cadillac Eldorado, with nearly three times as much--191.5 pounds. The study showed that the average 1955 model contained 29.6 pounds, with the number growing to 35.2 pounds for 1956, and predicted averages of 50 pounds by 1960 and 81 pounds by 1965.

Parts manufactured from aluminum typically weigh one-third to one-half as much as similar parts manufactured from cast iron. So using 300 pounds of aluminum could cut car weight from 500 to nearly 1,000 pounds versus iron. Automatic transmissions were among the first components liberated from excess girth by aluminum. Packard chopped an amazing 90 pounds from its Ultramatic by switching to an alloy case and select internal parts. The famed GM Hydra-Matic and Dynaflow also evolved with steady annual influxes of aluminum. Soon aluminum clutch covers, brake drums and countless bits of interior and body trim were commonplace.

But in the realm of engines, factory-sourced aluminum was limited mostly to pistons, high-compression Nash cylinder heads and certain Corvette and Thunderbird valve covers. This would change by the late 1950s as Detroit mounted its response to the growing foreign car invasion. Chrysler's aluminum Slant Six wasn't alone in preparing to drive the VWs, Renaults and Simcas back into the ocean. GM was busy readying its mostly aluminum 300-pound Corvair six-cylinder and had a reported $50 million invested in developing the 350-pound all-aluminum Buick/Olds 215-cu.in. V-8. American Motors was also in the game with an aluminum block version of its 195.6 OHV straight-six. Detroit's strategy was to lure foreign compact buyers with larger offerings that delivered similar fuel efficiency and cost less. Aluminum engines were a key factor in achieving an acceptable size-to-weight balance, and Detroit hoped these engines would cost less to make.

On the surface, aluminum engines don't seem cheaper due to the fact that aluminum alloys cost five to seven times as much per pound as iron and steel. The secret lies in casting and machining costs. Then as now, costs were Detroit's toughest problem, and any promising way to reduce them--whether through savings in material, labor, or overhead--was sure to get a lot of engineering attention.

Aluminum, with its relatively low melting point of around 1,200 degrees Fahrenheit could be cast in permanent steel molds or dies. American Motors and Chrysler took full advantage and invested heavily in gigantic water-cooled die-casting machines. It worked like this: You clamp the die machine shut with 2,000 tons of force, squirt in molten aluminum at 8,000 p.s.i., let it cool and set for a few seconds, open the machine, pop out the finished casting and you're ready to cast another one. Contrast this with the time-consuming sand-casting process, where you make up the sand mold around a pattern, pour the molten iron, let cool for 30 minutes or so, then shake out the casting from the sand. This process is necessary with iron because of its higher melting point.

General Motors' aluminum casting technique was different. They chose the less capital-intensive, and slower, method of semi-permanent mold casting utilizing steel external molds with internal passages formed by traditional sand cores. The molten aluminum was fed into the mold by gravity or under a very low pressure of just a few pounds. Not a bad system but one which forfeited some of aluminum's benefits. The upside was that heartier alloys could be used than the rather brittle SAE No. 303 alloy (3 to 4.5 percent copper, 10.5 to 12 percent silicone) required for high-pressure die casting.

Still, high-pressure die-casting promised a greater level of savings versus sand molds because the high injection pressure allowed much closer dimensions. In many cases the casting tolerances were so close that AMC and Chrysler could eliminate whole machining operations--like milling a surface, grinding flash or reaming a hole. This is where they saved more money. Get rid of a machining operation and you get rid of machines as well as the labor needed to operate them. The savings can pile up fast.

Another advantage of casting in steel molds versus sand is the ability to cast thinner sections. Because of inaccuracies (core shift) in sand molds and the flow characteristics of metal in a gravity-fed mold, designers had to leave a 3/16-inch wall, for example, where a 1/16-inch wall would do the job. With a die they could cast right to the needed size and net-shape parts resulted. This saved metal as well as dead weight in the finished product. Finally, aluminum had an inherent advantage on machining costs simply because it's soft; it cuts five times faster with a given tool than iron or steel. By cutting the machining time in half, one machine can do the job of two--meaning less investment in tools, less labor and less factory floor space.

These are the basic reasons why all of post-war Detroit was gung-ho on programs to replace cast-iron engines with aluminum. Then reality set in. While battling ongoing quality control problems, automakers quickly discovered the buying public wasn't all that impressed by these lightweight engine designs. By 1965, the market had spoken and, excepting Corvair, Detroit engine designers lapsed back into a cast-iron mentality. Sure, aluminum alternators, water pumps, carburetors, distributors and timing covers continued the alloy march, but the expense of aluminum heads and blocks would only be justified on exotic factory performance engines like the 426 Race Hemi, Ford SOHC 427 and ZL1 Chevrolet big-block. All of which makes the existence of the aluminum Slant Six all the more incredible.

This article originally appeared in the April, 2006 issue of Hemmings Classic Car.