How does an 8-6-4 engine work?

Dear Car Talk

Dear Car Talk | Jul 01, 1990

Dear Tom and Ray:

A few years ago, one of the Big Three offered, on a limited basis, a car with a V8 engine which had a switch on the dashboard that allowed the operator to "turn off" two or four of the cylinders while cruising. I'm interested in knowing how they worked it, since it seems like the "turned off" cylinders would be such a drag on the working ones that performance and mileage would both suffer. What do you know about this car?

RAY: The car was first offered by Cadillac during the 1981 model year, and the engine was known as the 8-6-4. In theory, it was a great idea. You would start off with eight cylinders for good acceleration. Once you reached cruising speed, two, then four cylinders would shut down. If you came to a hill or needed extra power to pass, two or four cylinders would kick back in. It was not controlled by a switch on the dash, it was controlled automatically by a computer.

TOM: The inactive cylinders were not a drag on the active ones. The valves simply closed, and the inactive cylinders just kept compressing and decompressing the same stuff. No new gas came in, and the small amount of wasted energy was more than offset by the gain in mileage.

RAY: If you want more technical details on how this engine worked, Gerald, I'm sure you can go to any Cadillac dealer and find one of the 8-6-4 service manuals in the dumpster. As I said, it was a great idea in theory. In practice, it was a disaster for Cadillac. The execution of the idea was poor. The engines had lots of problems, and customers were particularly unforgiving since they had spent a lot of money to buy a Cadillac. After just one year, General Motors did what any intelligent car company would have done; they discontinued it.

TOM: It's an experiment that Cadillac would like to forget. In fact, when the folks over at Cadillac see this column today, they're going to ask "won't they ever forget about the 8-6-4?" Well guys, we were trying, but Gerald reminded us.

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