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Old 04-22-2004, 02:02 PM
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blndweasel blndweasel is offline
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perhaps I can try to explain it in yet another way...

Try this little experiment. take a big book. stand it up on one end on a table somewhere, and let it fall over to one side. You should notice a big wash of air come out from under the book as it approaches the surface of the table. It's elementary physics. As the surface of the book comes closer to the surface of the table, the air has to go somewhere, and it wooshes out wherever it possibly can.

Take a look at the combustion chamber on your cylinder heads. There's an open part next to the spark plug and around the valve seats, and if you were to scribe a circle around the circumference of where the piston bore meets up with it, you should notice that a little less than half of that area is completely flat and flush with the surface of the mating surface between the head and the block. So you understand that there is a flat part on the head, right? The flat portion of that head is like the table.

Now take a look at the face of your piston. If it's flat all across the top, the analogy is simple. The face of the piston is like the book. As the piston approaches top dead center, the air in between the piston face and the flat portion of the combustion chamber on the head is forced out... into the cavity portion of the combustion chamber on the cylinder head. Air is being forced from the flat portion to the open portion, creating movement of air (i.e. turbulence) It's the turbulent movement of air and gas that encourages an even burn throughout the combustion space. In fact, the piston face gets so close to the face of the cylinder head at its flattest part, that (correct me if I'm wrong) very little combustion occurs in this .035" space between the surfaces.

As soon as you start to add volume between the flat surface of the cylinder head and the piston, namely as you increase that .035" gap to, say, .060 or .100 by adding a full face piston dish, or setting the piston lower than the surface of the deck, you introduce an area where essentially "stagnant" air can reside.

If you take a bowl and perform the same experiment as with the book, and let the open face of the bowl fall over onto the table, you're going to notice much less air evacuating from under the bowl, because it's getting trapped inside. Understand why the bowl is producing less air movement / air turbulence / quench than the book? Same difference between full dish pistons or pistons set low in the block, and zeroed in flat tops that have a proper quench height.

D-shaped dishes work differently, because the piston still maintains a flat face at the area where quench occurs, at the flat portion of the cylinder head within the diameter of the piston bore. So essentially if you only look at the quench area of the piston, D-shaped dishes look identical to flat top pistons in this area. That's why both flat tops and D-dishes produce quench.

Does this help at all, or am I just rambling?

the blonde weasel
san diego, CA
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