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JFairman JFairman is offline
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Join Date: Jul 2006
Location: S. Florida
Posts: 7,274
Originally Posted by mark houghton View Post
I spent some quality time last night reading Bruce's and Wayne's books on the care and feeding of the 911, relative to cams, and I kinda walked away with the impression from their quotes that high lift cams aren't compatible with CIS for a couple reasons: domed pistons and possible collision with valves, and valve overlap with both intake and exhaust valves being open briefly at the same time, causing oscillations in the CIS fuel delivery (as in a bouncing metering arm, I guess).

I guess the key word here is "high lift", as he (they) were referencing "S" cams. Apparently 964 or SC cams aren't considered high lift, as evidenced by the fact that many people run them in their 930's. I will eventually be running with an SC330 grind, which I understand has the duration of a 964 with the lift of the SC, and was made to run in place of the SC cam for stock lift applications.

Your first paragraph would apply to normally aspirated CIS engines which had odd shaped CIS domed pistons. 930 engines are lower compression and the pistons are almost flat topped so there is more valve clearance with 930 pistons than domed CIS - SC pistons.

Long duration valve overlap on a normally aspirated motor like the SC would cause the air flow sensor plate to vibrate with the backflow pulses or oscillations at low to medium rpms because the distance between the air flow meter and intake ports is relatively short and unobstructed with spinning devices.

On a 930 the distance from the intake ports to the airflow meter is alot longer and you have the turbo compressor wheel in the intake air path constantly spinning and making some air pressure even at idle and an intercooler to dampen pulsations in the intake air flow.
With all that stuff, especially the compressor wheel spinning away in the intake track between the intake ports and the airflow meter I think the backflow pulses or oscillations would be removed and dampened so they wouldn't cause any vibration or bouncing of the metering plate with a longer duration cam.

The problem with longer duration cams in a street 930 is effective compression ratio becomes even lower with the intake valve closing later in the beginning of the compression stroke with a long duration cam so with the already low compression of a 930 and slow response of CIS the low speed drivability and turbo lag would be unbearable during normal around town and city driving for most people.

The Elgin CIS 330 is a stock early 930 cam with some of the base circle ground away so the nose of the cam becomes higher in relation to it which increases valve lift some and duration also but not as much, and Elgin SC 330 grinds are SC cams which have had the same thing done.

964 cams have higher a little higher lift than SC cams, different lobe centers, and a little longer duration. They move the powerband a little higher in the rpm range compared to SC or SC330 cams and make a little more horsepower while doing it, but you loose some low speed torque at the same time unfortunately.

Too bad variable valve timing hadn't been invented yet..

I wouldn't say 964 cams are high lift, just a little higher lift than SC cams. The specs are in the Webcam website.
Old 11-24-2009, 11:42 AM
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