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Registered
Join Date: Jun 2009
Posts: 1,241
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quench, squish area in head
A 3.6 track engine. the piston has a flat step approx 2mm at its perimeter to match the step on the cylinder head at its perimeter before the dome begins. I have .065'' clearance at the step and .095 at the dome area. Was considering shortening the cylinders to close up the gap as the piston to head dimension seems to be a topic of conversation as in closer is better. By closing up that gap, other than raising the compression, does that enhance performance. I have heard that on a piston that is more flat than domed, the squish aspect is something to consider, but on a domed chamber, not so much. .20'' is probably the most I want to cut which still leaves a good bit of excess clearance. Would appreciate info on this subject. Thanks, Bob
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04-15-2024, 05:22 AM
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Registered
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Your valve clearance is important and may determine what you can get away with?
Alan
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83 SC, 82 930 (track) - Stock except for RarlyL8 race headers, RarlyL8 Zork, K27-7006, 22/28 T bars, 007 Fuel head, short 3&4 gears, NGK AFR, Greddy EBC (on the slippery slope), Wevo engine mounts, ERP rear camber adjust and mono balls, Tarret front monoball camber adjust, Elgin cams, 38mm ported heads, 964 IC. 380rwhp @ 0.8bar Apart from above, bone stock:-) |
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04-15-2024, 09:00 PM
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PCA Member since 1988
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Squish increases combustion chamber turbulence, which improves mixture, flame spread, and efficiency. How much a difference that makes in domed Porsche engines I'm not sure, as I have not done back-to-back dyno testing. On my build 2 years ago, I aimed for .030"/.75mm clearance between the stepped part and the piston dome to head, and got within a few thousandths of that goal. In my engine, the clearance at the stepped part of the piston, and at the dome were about the same. IN yours, they are significantly different.
On engines with flat top pistons and a large flat squish areas, such as American 2-valve V-8's, the rule of thumb is to stay within .030' and .040". As that clearance opens up, squish is decreased. If the squish clearance increases to more than .060" there is little squish effect. With your clearances, I would think you would have to take off too much to get the squish into the zone where it provides significant benefit (if there even is a benefit on the rounded dome Porsche design).
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04-15-2024, 10:24 PM
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