Please explain cam lobe overlaps to me.

[rwest]

Hi,

I’m guessing that it has something to do with rotation direction or some other odd deal, but looking at my two 77 CIS cams, one side has both of the lobes almost exactly together meaning both intake and exhaust open at the same time? While the other side they are spaced probably about 120 degrees from each other.

My tiny brain can’t make sense of this, hopefully one of you smarter guys can give a good explanation?

Thanks.

[Bill Verburg]

Originally Posted by rwest

Hi,

I’m guessing that it has something to do with rotation direction or some other odd deal, but looking at my two 77 CIS cams, one side has both of the lobes almost exactly together meaning both intake and exhaust open at the same time? While the other side they are spaced probably about 120 degrees from each other.

My tiny brain can’t make sense of this, hopefully one of you smarter guys can give a good explanation?

Thanks.

overlap is exactly as you describe, the time both valves are off their seats

SC cams don't have much overlap but earlier S did
here's what an S looks like, the overlap period is near the end of the exhaust stroke and continues into the beginning of the intake stroke


SC



Physically the look different because they need to be asymmetric because one rotates clockwise and the other counter clockwise

[trials935]

Both cams turn ccw.

[rwest]

Thanks Bill,

Like I said, my simpleton brain sometimes tales a bit to spool up and your explanation and the pictures bumped something loose and I realized my thought that both valves would be open at the same time because the lobes are next to each other doesn’t hold true because the valves aren’t right next to each other.

Best,
Rutager

[Bill Verburg]

Originally Posted by trials935

Both cams turn ccw.

Correct, I miswrote that

[rwest]

Originally Posted by trials935

Both cams turn ccw.

Makes sense that the cams are different because they “see” the location of the valves differently; they are going the same direction, but the cam towers are mirror images to each other, therefore one set of lobes hits the valves from one side the other from the other side.

Thanks guys,
Rutager