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Please explain cam lobe overlaps to me.

Hi,

Iím 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 canít make sense of this, hopefully one of you smarter guys can give a good explanation?

Thanks.

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Rutager West

1977 911S Targa Chocolate Brown
Old 04-22-2018, 01:18 PM
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Quote:
Originally Posted by rwest View Post
Hi,

Iím 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 canít 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
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'76 Carrera 3.6RS(nee C3/hotrod), '95 993RS/CS(clone)
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Old 04-22-2018, 03:33 PM
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Both cams turn ccw.
Old 04-22-2018, 04:20 PM
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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 doesnít hold true because the valves arenít right next to each other.

Best,
Rutager
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Rutager West

1977 911S Targa Chocolate Brown
Old 04-22-2018, 04:25 PM
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Quote:
Originally Posted by trials935 View Post
Both cams turn ccw.
Correct, I miswrote that
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Old 04-22-2018, 04:27 PM
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Originally Posted by trials935 View Post
Both cams turn ccw.
Makes sense that the cams are different because they ďseeĒ 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
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1977 911S Targa Chocolate Brown
Old 04-22-2018, 04:43 PM
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