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jluetjen jluetjen is offline
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Join Date: Oct 2001
Location: Westford, MA USA
Posts: 8,594
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My understanding is that it is a few factors:

1) Size: 4 valve pente-roof combustion chambers are very compact and often can use flat top pistons. So they only take up a band across the center +/-25% of the cylinder diameter. The outer half of the cylinder is the squash zone that promotes turbulance in the cylinder and thus reduces the tendency to knock. Hemi heads on the other hand are a rough hemi-sphere covering the full diameter of the cylinder. So the distance is further from the spark plug to the edge of the combustion chamber at TDC. So where a 911 head may have volume of about 68 or 70 cc's, a pente-roof head from a similar sized cylinder may be only 25 cc's.

2) Shape: Since pente-roof heads have less volume, it is possible to get the desired CR without a dome on the piston which keeps the combustion chamber compact. In the worst case 911 (a 2.0 with S pistons or worse 906 pistons), the combustion chamber's shape is more like an upside down bowl (just the bowl, not the area inside the bowl which is taken up by the piston dome). So at TDC, the flame front has a much longer and contourted distance to travel.

3) Valve size: Hemi heads generally have 1 large intake valve (which is often heavily shrouded by the piston at TDC), which flows a lot of air, but generally doesn't promote much turbulance. 4 valve heads on the other hand are often not shrouded since the piston doesn't have a dome, and the flow through the two intake valves generates turbulance.
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'69 911E

"It's a poor craftsman who blames their tools" -- Unknown
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Last edited by jluetjen; 07-29-2004 at 02:11 PM..
Old 07-29-2004, 02:09 PM
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