Most every explaination of the 968 vario cam says that the vario control advances the intake timing with respect to the exhaust cam (reducing overlap), for idle to 1500 rpm, the it retards the intake cam (increasing overlap) for 1500 to 5500 rpm, the it advances back to the idle position again for 5500 to redline. This was always strange to me. Studying cam theory for 15 years, in most cases, reducing overlap at high rpm's reduces hp!! Why would Porsche do that?

I found this description of the 968 vario cam function. It says that it goes into a third adjustment at 5500 rpms...not to reduce overlap...but to increase it once again. This makes sense to me.

Take a look. http://autozine.kyul.net/classic/924.htm

graham.

 

The details on that website are wrong, there aren't three settings but only two - one setting is used once [torque setting] the second setting is used twice [power, low rpm setting].

Advancing the intake cam increases overlap duration, it does not reduce it.

idle > 1500 [or 3000 without engine load]: intake retard
1500 > 5500: intake advanced [earlier intake valve closing makes torque]
5500 > redline: intake retard again just like the idle > 1500 setting [later intake valve closing makes horsepower]

The best explanation is here:
http://www.968.net/pressroom/images/...208_43_600.jpg
http://www.968.net/pressroom/images/...208_45_600.jpg

You don't need a lot of overlap at 6500 rpm's. This peak engine speed is relatively low...the cams still have overlap but not very much. If the 968 engine did 8000 rpm's then you'd need more overlap.

 

Woops..I swapped the advancing and retarding .... now who is retarded Trev...thanks for setting me straight. Now I know why the 968 doesn't require as much overlap above 5500...it relies on the dynamic "supercharging" effect of the intake. COOL!