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911st 911st is offline
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Join Date: Jan 2004
Location: Sacramento
Posts: 7,269
Not the cam, the intake port!

It looks like it would be much more effective to open up the intake port to 38mm than to change out the cams.

I seems that adding an SC or C2 cam dose not even come close to making the breathing improvement that just opening up the intake ports should.

This is dramatized in my rough drawing that started as a graph of an SC cam. I drew in about what a 930 camís total lift and timing should be.

The cross hatch (xxxx) area represents flow potential of a given port size.

The bubbles (oooo) area represents what is gained by opening up the intake ports from the stock 34mm to 38mm.

Why 38mm?

That should to put the ports in balance with the 49mm intake and 41.5 exhaust valve diameters. How much a valve can flow is a function of lift and the valves diameter (curtain area). However, this assumes the valve balance is near ideal and it may not be.

For comparison a 993TT runs 43mm/38mm ports. This suggests the intake relative to the exhaust can be even a little larger.

It also suggests that our 3.3 port sizes are on the small side relative to the 993TTís 3.6. For a 3.3 to be equal we should be at 41mm intake and 36.4mm exhaust.

With even larger ports, it should be that changing cams would then have a more significant impact.


Old 02-01-2010, 02:01 PM
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