Cam rotaion relative to crank rotation?

[1GENZED]

Hi

When I was in college studying Tool and Die and CAD-CAM , a freind and I designed and built a complete 1:2.5 scale V-twin based on various Harley pushrod engines for our final project. This thing was a beauty, we cut all the gears and cam lobes on a wire EDM and the heads where first turned on a 3 axis lathe and then machined on a four axis Mill, can you say tricky.

We had a "constructive" argument about the cam ratation relative to the crank rotation. He claimed that the cam always had to turn in the same direction as the crank, but didn't know why. My argument was that the cam didn't have to turn in the same direction as the crank, as long as the engine was designed for the reverse rotation. I still can't figure out why the cam would have to turn in the same direction as the crank. Can someone explain why this would be. The only reason I can think of is possible vibration problems, but then again it's a "Harley".

Thanks

Ahmed.

[MikeW]

You're right. I see no reason the cam would have to turn in the same direction as the crank. However, since most cams are chain driven it's only natural for them to turn in the same direction. All the cam is doing is opening the valves. For it to turn in the opposite direction it would have to be ground in the mirror image to keep the valve timing the same. Here's a timing gear kit that replaces the timing chain on a Mini Cooper. Note that it has three gears to keep the rotation direction the same as stock:

http://www.slarkraceengineering.co.uk/camshaft_kit.htm

[Phred]

1GENZED,

I have built USAC midget engines with both clockwise and counterclockwise cams. The only difference being the gear drive arraignment. And a reverse ground cam. All other engine parts being the same. There seems to be no advantage one way or the other, except that there are less parts associated with a reverse, two gear setup. But these gears, and block machining must be very accurately built, as there is no adjustment available to set gear clearance. With the third idler gear setup, gear clearance can be set by adjusting the idler gear. Which allows the block to be re-align bored, with no concern about gear clr. So, there seem to be more clockwise cam engines in race cars. Friendlier for the engine machinist, and more aftermarket parts availability. BTW, these engines can also be built to run backwards at the crank, with a reversed ring and pinion rear end. This is done to utilize the torque of the engine to help plant the car on acceleration out of the turns.

Phred