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-   -   Please explain cam lobe overlaps to me. (http://forums.pelicanparts.com/porsche-911-technical-forum/994374-please-explain-cam-lobe-overlaps-me.html)

rwest 04-22-2018 01:18 PM

Please explain cam lobe overlaps to me.
 
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.

http://forums.pelicanparts.com/uploa...1524431884.jpg

Bill Verburg 04-22-2018 03:33 PM

Quote:

Originally Posted by rwest (Post 10011206)
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.

http://forums.pelicanparts.com/uploa...1524431884.jpg

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
http://forums.pelicanparts.com/uploa...1524438379.jpg

SC
http://forums.pelicanparts.com/uploa...1524439878.jpg


Physically the look different because they need to be asymmetric because one rotates clockwise and the other counter clockwise
http://forums.pelicanparts.com/uploa...1524439748.jpg

trials935 04-22-2018 04:20 PM

Both cams turn ccw.

rwest 04-22-2018 04:25 PM

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 04-22-2018 04:27 PM

Quote:

Originally Posted by trials935 (Post 10011373)
Both cams turn ccw.

Correct, I miswrote that

rwest 04-22-2018 04:43 PM

Quote:

Originally Posted by trials935 (Post 10011373)
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


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