Is the rotational speed of double overhead cams the same as a single?

Dear Car Talk

Dear Car Talk | Jul 01, 1998

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!

* * *

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