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The Impossibly Disastrous 4-6-8 Cadillac Engine Of 1981

Before the bankruptcy and the Aztek, and EV-1; General Motors banked on something way ahead of its time. As such, it was a disaster. Not because it wasn’t a good idea, but more because the technology wasn’t refined enough for what it was supposed to do. This is about Cadillac’s revolutionary, impossibly disastrous 4-6-8 cylinder deactivation engine from 1981. It’s also about thinking beyond technological limits without respecting those technological limits.

The idea of an engine that could switch from 4- to 6- and then 8- cylinders was like the Jetsons cartoons

1981 Cadillac 4-6-8 L62 engine | GM
1981 Cadillac 4-6-8 L62 engine | GM

GM always hired the best. Big dreamers that could imagine what would propel us through the atmosphere as we saw in the Jetsons cartoon shows. When it came to engineers, the sky was the limit. But there were reality limits, too.

In the 1970s the American public went through not one but two oil crises. The OPEC oil embargo in 1973 was followed too soon by the 1979 oil crisis. Both resulted in gas lines, jacked-up prices, and the realization that fuel economy was as important if not more so than your new car’s color or whether it had a big enough ashtray.

Eaton Corporation was employed to help develop the 4-6-8 engine

1981 Cadillac Seville | GM
1981 Cadillac Seville | GM

The engineers at GM pulled in the Eaton Corporation, an electronics supplier, to help devise a way to shut down cylinders firing on demand. The idea is almost second nature today but then it was like Star Wars. Engines need all eight cylinders when you stomp on the throttle for myriad situations. But cruising down the highway or idling through a parking lot you don’t need all eight. Maybe you only need four?

That was the idea. To control whether two or four cylinders could be deactivated when a computer sensed there was no load on the engine. Think of the gasoline it would save?

Cadillac started with a standard, off the shelf 368 ci V8, or L61 as it was code-named. It powered many Cadillacs at that time. It was a smaller bore version of its 425 ci V8 that had been around since the mid-1970s. So the foundation was stout and proven in hundreds of thousands of cars.

The 4-6-8 cylinder heads incorporated solenoids to lock down the rocker arms

Manual engine deactivation selection in 1981 Cadillac.

Above the engine block were unique cylinder heads for what was known as the L62. They incorporated solenoids that when triggered locked down the rocker arms. This prevented the intake and exhaust valves from operating. Both were closed in each cylinder when the solenoids were energized. This trapped air inside of the cylinders and acted as a cushion according to GM. 

It might seem at first thought it would have been better to vent some of that air trapped in the cylinders. Something like a pop-off valve used for superchargers. When too much pressure is introduced the pop-off valve will mechanically release some of the pressure. This would lessen the effort pistons encountered having to continually compress air that was going nowhere. Could it have been that the GM engineers couldn’t figure out how to release small amounts of air?

The Computer Command Module was able to process over 300,000 operations a second

1981 Cadillac Eldorado | GM

The 4-6-8 engine debuted in 1981 to a lot of fanfare. Early builds were parcelled out to the automotive media with mostly glowing reports. At least initially, the 4-6-8 worked as promised. The Computer Command Module that monitored and controlled the deactivation was said to be able to process over 300,000 operations per second. What could go wrong with that type of computing power and speed?

But not long after examples were purchased and in the field, the problems started almost immediately. The problem was when the engine went from four cylinders to eight, or six to eight, there was a lag. Engine smoothness was interrupted, and so was the customer’s confidence. 

Here’s a rather humorous example of the Cadillac 4-6-8 manually deactivated and the resulting engine performance, or lack thereof.

When the engine was powered either by four or six cylinders it was also a rough ride. In fact, in the four-cylinder mode, the engine sounded like it was dieseling. At certain RPMs, the engine would “hunt” for the proper amount of activated cylinders. Clumsily switching from the different combinations of four, six, or eight.

What about fuel economy?

And what about the added fuel economy? There wasn’t any. It seems that the economy figures were the same as the previous year. But the emissions standards had gotten tighter. So those who wondered about such things figured that maybe the reason for the cylinder deactivation was to keep the engine economy as good as the previous year under the newer, stricter rules.

It was almost like GM was using future technology to try and limp its cars into the future when technology would be advanced enough to get them better mileage. 

During 1981 Cadillac issued 13 programming updates to try and tame the system. While it’s doubtful GM developed the 4-6-8 system to run for only a single year, that’s all it was offered for-at least for retail purchases. Interestingly, Cadillac left it for its Fleetwood commercial chassis, which were mostly sold for limousines and hearses.

The 4-6-8 was a one-year-only marvel

1981 Cadillac | GM
1981 Cadillac | GM

For 1982 the rest of the Cadillac line got the HT4100 engine, another disaster we’ll leave for another time. Suffice to say it had no power and reliability was bad. This may have been why Cadillac stuck with the 4-6-8 for commercial applications; it was bad but the new HT4100 was worse.

As much of a cluster as things were at the General during those times, there were some daring, ballsy undertakings. They would eventually result in the EV-1, the first electric car in modern times, and of course the Volt. And before that, there were examples of extraordinary engineering like the Corvair and later Corvettes. 

So, while we snicker at things like the 4-6-8 cylinder deactivation, there was a lot of future-think and possibilities being developed at GM back in those glorious days.