The Lowdown on Overhead Camshafts
Question: Some automobile manufacturers emphasize the fact that their cars have overhead camshafts. Please tell me as an average motorist what advantage this will have for me over the camshafts in most conventional cars.--P.S.
Answer: Car buyers often hear a lot of technical terms that don’t really mean very much, but are meant to convey the idea that a car has the latest mechanical technology. In fact, a lot of these technical features have been around for a long time and carry disadvantages as well as benefits.
The Ford Pinto, which is now out of production, was notorious for all the problems that its overhead camshafts caused its owners. The cars’ camshafts often had to be replaced at well under 50,000 miles. Nonetheless, many cars have overhead camshafts that operate well.
An overhead camshaft has fewer moving parts and theoretically is more efficient in doing its job, which is to open and close the valves through which fuel flows into the cylinders and exhaust flows out. The overhead camshaft should allow the engine to rev up to a higher speed than regular camshafts, but this apparent advantage is of minimal benefit in normal driving.
At the same time, the overhead camshaft is usually powered by a rubber belt, which is less reliable than the metal gears or chains that operate a regular camshaft. It is not unusual for the belt on an overhead camshaft to wear out or fall out of adjustment, requiring a costly repair.
The overhead camshaft gets its name from its position at the top of the engine. A cam or hump on the shaft, which converts rotating motion to reciprocating motion, presses down on the valve stems to open the valve. A powerful spring shuts the valve as the camshaft hump rotates away.
A regular camshaft is positioned deep inside the engine and is connected to the valves by push rods and rocker arms. The additional parts result in a loss of efficiency, because every time motion has to change directions, power is lost.
Auto manufacturers push overhead camshafts because they are lighter in weight and more efficient, which improves gas mileage. They are also cheaper to produce and don’t generally last as long as the older design.
As an average motorist buying a car, you need to evaluate the overall car. You certainly would not want to buy one car because it has an overhead camshaft. The same goes for most of the technical design features of a car, despite the standard sales pitch to the contrary. Instead, try to examine overall performance, comfort, styling and the car’s reputation for reliability.
Q: Here’s yet another complaint about a GM automatic transmission. I have a 1985 Pontiac 6000 with a four-speed transmission and 13,000 miles on the odometer. It has a rough engagement problem. When I accelerate at the same time that the transmission downshifts, there is a really loud clunk. Can the problem be eliminated?--C.F.
A: You could eliminate the problem by buying a new car. Unfortunately, the condition you describe is inherent in your transmission, which is kindly described by GM as a “firm shifting transmission.”
In 1987 models, the transmission was modified to smooth out the shifting. Modified springs and a redesigned valve body were included.
I would not attempt to drastically alter any of the adjustments possible in the transmission, such as the detent cable from the engine. You run the risk of ruining the transmission.
