911 Cam Durations?

[scarceller]

Been doing a lot of studying on camshafts and how they are designed and speced. Something that I've noticed is that in the 911 air cooled motors the cams do NOT follow the basic rules for camshafts in other motors. I'm wondering why the 911 engine has different cam requirements from other motors I'm studying?

Here's the difference in question:
Most cams have exhaust durations about equal to the intake duration or if anything the exhaust duration tends to be longer than the intake. But the 911 air cooled cams tend to always have significantly shorter exhaust duration than the intake. Why?

I'm sure something about the exhaust valve and the exhaust port calls for this but I'm not sure what it is? A bad flowing valve/port tends to require longer exhaust duration to get the cyl empty of exhaust on the power stroke and I've read that the 911 head does not exactly have great flow, so why would you shorten the duration on a head that doesn't flow that well? Just does not make sense?

Other theory is that by shorting exhaust duration you could achieve more exh velocity at overlap but at the cost of possibly still having significant pressure within the cyl on the exh up stroke? That's not a good trade off as it eats HP. Generally you want to sacrifice a tad of the remaining cyl pressure near the end of the power stroke to help clear the exhaust before you reach BDC on power stroke. It's very common to see exhaust valves lift off the seat at 50-70deg BBDC on the power stroke to achieve this.

Any theories or concrete test results explaining why our 911 air cooled motors would favor shorter exhaust durations when compared to the intake duration?

Thanks.

[prschmn]

Hey Sal Short version- typically 80+ % of exhaust happens on blowdown. Early closing of ex valves favors intake vacuum-which CIS/trap door systems need-as you know.

[scarceller]

Originally Posted by prschmn

Hey Sal Short version- typically 80+ % of exhaust happens on blowdown. Early closing of ex valves favors intake vacuum-which CIS/trap door systems need-as you know.

But even early cams (pre CIS) have short exh durations, why?
I get that they want to reduce overlap on the CIS.

[prschmn]

longer exhaust duration.... why and when?? • Speed Talk