Why the Cadillac V8-6-4 Failed

There was no other automaker in the U.S. that was more adversely affected by the government mandated Corporate Average Fuel Economy (CAFE) standards during the late-1970s and 1980s than Cadillac. When the CAFE standards were first enacted back in 1975, Cadillac’s big iron block V8 had an engine displacement of 500 cubic-inches which was 8.2 liters and was the perfect motor to power its very large full-size cars. Seven years later Cadillac’s V8 was a new much smaller 249 cubic-inch (4.1 liter) V8 which had an aluminum engine block and was exactly half the engine displacement size of the old 8.2 liter V8.

Before the the 4.1 liter V8 and after the 500 V8 was Cadillac’s 425 cubic-inch (7.0 liter) V8 which was replaced by Cadillac’s 368 cubic-inch (6.0 liter) V8 for the 1980 model year. The 368 V8 shared the same engine block with Cadillac’s 425, 472, and 500 V8s. The 368 had the same engine stroke as the 425 and 472 V8s and had an engine bore smaller than the 425, 472, and 500 V8s. For 1981, the 368 returned but was rebranded as the V8-6-4 engine since it had a computer controlled cylinder deactivation system in order to improve fuel economy. It was rated at 140 horsepower and 265 lb-ft of torque. For all intents in purpose it had appeared that Cadillac with the V8-6-4 had found the perfect solution to comply with the CAFE standards and still provide a smooth operating V8 with enough horsepower and torque to keep its customers happy. This engine was a big undertaking and a massive gamble back in 1981 for both Cadillac and GM. Today not even a second thought is given to this technology since GM, Stellantis, and other automakers have reliably used computer controlled cylinder deactivation systems in some of their engines since the mid-2000s. Worth noting Cadillac also offered in its cars during this time to boost fuel economy numbers the Buick 4.1 liter V6 as an engine delete option and the optional Oldsmobile 5.7 liter Diesel V8.

Compared to other automakers, Cadillac had been at the forefront of computer technology since the 1970s, implementing state of the art electronics and computer technology with its cars which also included an array of computer controlled digital dash readouts. One such readout called the MPG Sentinel which gave occupants the exact mpg of the engine at any precise moment. Cadillac’s first implementation of a computer controlled port-fuel-injected V8 in the mid-1970s was a big success. Unfortunately Cadillac’s cylinder deactivation equipped V8-6-4 engine was nothing short of a disaster. Cadillac called this new cylinder deactivation system “Modulated Displacement”. The concept was innovative and technologically sound with the V8-6-4 using eight, six, or four cylinders depending on the what the power requirements of the engine were at any given time.

The problem wasn’t mechanical, for the concept was simple, solenoids mounted on the rocker arm studs (above each cylinder that could be deactivated) would close the valves of the cylinders not needed at any particular time. When cylinders were deactivated they still moved in conjunction with the cylinders in use. The closed valves kept the deactivated cylinders from participating in the combustion process. Since the deactivated cylinders remained in motion, when cylinders were reactivated, the valves of these cylinders would re-open thus causing them to be a part of the combustion process again. Additionally when cylinders were deactivated the computer would adjust the fuel and air flow from the engine’s throttle body fuel injection (TBI) system to compensate.

Unfortunately the problem was flawed computer programing which in some cases couldn’t react in enough time, causing delayed cylinder deactivation or reactivation which created engine drive-ability issues. The V8-6-4 became a real headache for owners and Cadillac mechanics. It was also a big public relations disaster for Cadillac which before this fiasco had a stellar reputation. Today’s modern cylinder deactivation systems have much more sophisticated central computers that can detect and make changes in milliseconds making cylinder deactivation and reactivation seem seamless.

Cadillac to its credit tried very hard to remedy the situation by offering several prom upgrades to the central computer, the latter prom versions greatly improved drive-ability. Unfortunately with the multitude of angry customers many Cadillac dealers didn’t have the luxury of waiting for these future prom upgrades. Instead they just deactivated the Modulated Displacement system which in effect converted the V8-6-4 into a 368 V8 that ran on all eight cylinders all the time. With this corrective fix, fuel efficiency may have dropped but customers didn’t care they were just happy having a smooth and predictable engine under the hood. The V8-6-4 was replaced with the 4.1 liter V8 for the 1982 model year, however the V8-6-4 would live on through the 1984 model year powering Cadillac’s rear-wheel drive commercial limousines. In summary Cadillac was right with its development of the V8-6-4 engine, however it was wrong in rushing it into production for the 1981 model year. Had Cadillac spent a few more years testing and perfecting its Modulated Displacement system, the V8-6-4 could have been a big success. The sad truth was after the public relations disaster the V8-6-4 with the latter prom upgrades ended up being a fuel efficient, smooth operating, and reliable engine.


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