Dear Tom and Ray:

I'm hoping that you two automotive geniuses (I'm not kidding!) can settle a bet that I have with a co-worker. He maintains that the benefit of double overhead cams is that the cams can rotate at half the speed of a single overhead cam. I say he's full of it. I say the rotational speed is the same. What say you? -- Craig

Tom: You're absolutely right, Craig. The speed of a double overhead cam is exactly the same as a single overhead cam. It has to be. All camshafts rotate at half the crankshaft speed.

Ray: The advantage of a double overhead cam is simply that it reduces friction and mass by improving the geometry at the top end of the engine.

Tom: The valves are located at the top of the engine, in the cylinder heads. Those valves need to be opened and closed to let gas and air in and exhaust out of the cylinders.

Ray: In the old days, the camshaft was buried deep down inside the engine. And when the camshaft turned, each lobe operated a lifter, which pushed on a long pushrod, which, in turn, pushed on a rocker arm, which then rotated and opened a valve. That worked, but you can see that there were a lot of pieces that had to move. And lots of pieces means lots of slop, which limits how fast the engine can go, and how efficient it can be.

Tom: So high-performance engines got rid of a lot of that stuff by taking the camshaft out of the middle of the engine and putting it on top of the cylinder head. That's called an overhead cam engine. That eliminated those long pushrods. So the lobes on the camshaft pushed directly against the rockers, which then opened the valves. So that's better. But since a single camshaft in the middle of the cylinder head can't reach all of the valves, you still needed rockers, which sucked up energy and created slop.

Ray: So the next improvement was double overhead camshafts. That has two camshafts on top of the cylinder head-running directly over each set of valves. And on a lot of a double overhead cam engine, the camshaft lobes themselves open the valves. So there's absolutely no energy lost through push rods or rocker arms.

Tom: That reduces friction and valve train mass and allows the engine to rev higher, which improves performance and, often, fuel economy. And this is especially true for the newer, multivalve engines (i.e. four valves per cylinder), which would otherwise need huge rocker assemblies to open and close all of their valves.

Ray: Did that make sense, Craig? No? Well YOU try being an automotive genius without using your hands to explain stuff!

Changing your oil regularly is the cheapest insurance you can buy for your car, but how often should you change it? Find out by ordering Tom and Ray's pamphlet "Ten Ways You May Be Ruining Your Car Without Even Knowing It!" To order, send (check or money order) to Ruin, P.O. Box 536475, Orlando, FL 32853-6475. You can also order online.

© 1998 by Tom and Ray Magliozzi and Doug Berman Distributed by King Features Syndicate, Inc.

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