911 Cam Durations?

[scarceller]

I've been wondering why the 911 cams have significantly less exhaust duration than intake duration, this is not the case for most other motors. Most other engines have exhaust and intake durations about the same or usually more exhaust duration than intake duration. Just about every air cooled 911 cam exhibits this trend, I'm just wondering why? I'm guessing the exhaust port and valve have something to do with this? One reason I can think of that you may reduce exhaust duration is to increase exhaust port velocity so you still have plenty of it at overlap, but that's just a theory?

Does anyone know why the air cooled 911 responds to less exhaust duration? And care to explain the reason?

I ask because I've been involved in a twin plug 3.4L and testing it with various cams, the exhaust duration discussion has come up but know one has yet to explain the why.

Thanks in advance.

[scarceller]

One thing I forgot, Steve thanks for offering this forum! I'm surprised more folks have not yet found it.
Again thank you.

[Steve]