Last year I read a book called How to Get...

Dear Car Talk

Dear Car Talk | Dec 01, 1994

Dear Tom and Ray:

Last year, I read a book called "How to Get More Miles per Gallon." The book mentioned an aftermarket device that deactivates several cylinders while you're cruising. For example, if the car has eight cylinders, when you reach cruising speed, this device shuts down four of the cylinders, so you're running on only four. What do you think of this idea?

RAY: I think my brother must have one of these devices in his '63 Dodge Dart. It always feels like it's running on half its cylinders.

TOM: Actually, Ned, this idea was tried about ten years ago by Cadillac. They called the engine the 8-6-4. It was an eight cylinder engine, but depending on the demand for power, it would switch to six or even four cylinders to increase fuel economy. But it turned out to be a real disaster for Cadillac.

RAY: Not only was the technology complicated, but it just didn't produce enough benefits. And when combined with Cadillac's unreliable engines of that era, the result was many unhappy Cadillac customers.

TOM: And I'm sure this aftermarket device, if anything, is even more crude and ineffective than the Cadillac system.

RAY: Manufacturers eventually gave up on this sort of device because they figured out they could save more fuel by improving their transmissions. That's why most cars now come with four-speed overdrive automatic transmissions and lock up torque converters, which are more effective, more reliable, and cheaper than the "8-6-4" schemes.

TOM: But if you're stuck with a car without overdrive and without a lock up converter, who knows? Maybe this thing will give you a few extra miles per gallon. I suspect it works by cutting off half of your fuel injectors, and that would certainly save gas.

RAY: But on the other hand, you might have to push harder on the gas pedal to keep the car moving. Plus, since an aftermarket device can't control the valves, you'll still be wasting energy drawing air into each of the idle cylinders, and compressing that air. So I don't think you'll save much of anything.

TOM: Well, there's only one way to find out. We'll have to say in our column that this thing is complete garbage. Then we'll wait until the folks who make it get mad, call us up, and tell us to try one ourselves...for free, of course. Then we'll see how much the fuel economy improves on my '63 Dodge Dart when it's running on half of it's cylinders.

RAY: How're you going to run on two and a half cylinders?

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