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911nut 911nut is offline
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Join Date: May 2001
Location: Michigan
Posts: 2,511
Originally Posted by 911st View Post

Compare port volume to area under valve seat:
2.78-3.21 sq in....area at full lift for 930 to SC cam verses,
1.23-1.95 sq in....area for 32-40mm intake ports.

Put an SC cam with a stock 930 port volume and there is way more lift than is need with 3.21 sq in. area volume under the intake valve seat at full lift for a port that can only flow 1.23 sq in of volume.

This would make the area under the valve 260% larger than the port.

Kind of cool info and not what I expected. I thought lift equaled more air flow.

It dose some but because of the increased lobe ramp angle, not because of the higher gross lift.

Keith, you've calculated what's known as "curtain area".

You are correct in saying that absolute lift by itself doesn't allow more mass flow in the stock head. Time is the important parameter here. In order to control valve velocity and keep it to an acceptable level, a valve must open earlier and close later if it is to open higher. Mass flow is increased, therefore power is increased.

I work in engine manufacturing. We grind our own cams. Ramp velocity is a big deal and must constantly be measured.

IMO, if one is upgrading cams for increased power, the 964 cam is the way to go, especially on the cars with 5 speed transaxles. The Group B and SC cams don't go far enough and all of these choices will degrade low-rpm performance to some extent.
Paul B.
'91 964 Turbo
Port matched, SC cams, K27/K29 turbo, Roush Performance custom headers w/Tial MV-S dual wastegates, Rarlyl8 muffler, LWFW, GT2 clutch & PP, BL wur, factory RS shifter, RS mounts, FVD timing mod, Big Reds, - 210 lb
Old 01-30-2010, 06:03 AM
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