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Old 03-26-2006, 05:53 PM   #16
Section 9
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Join Date: Nov 2003
Location: Horizons Cave

Originally Posted by Les, slight limp
As i see it (just a theory you understand) the walls of the combustion chamber act as a barrier to the flame front, tight squish improves things by leaving less unburnt fuel in the squish area and so reduces the chances of detonation, the grooves actually encourage the flame into the squish area leading to a more complete burn and that's why the squish clearance needs to be around 70 thou so there is enough fuel in the squish area to provide a consistant burn, otherwise a clearence of 70 thou would actually aggravate the engines tendency to detonate, alternate theories welcome.
Here is a great explanation that I found on if I'm not mistaken.

Edit: The above link is wrong. Here it is.

The shape of the piston and cylinder head have their greatest affects on combustion efficiency when the piston is within about 10 mm of Top Dead Center (TDC). When the piston is more than 10 mm away from TDC, the cylinder head and pistons are just distant and innefectual walls bordering a large open expanse. So the grooves do their work within this 10 mm range. On the compression stroke, they force the compressing air/fuel charge into the quench area via higher velocity jet streams to better vaporize and homogenize the fuel. On the power stroke, they guide the explosion through these same jet streams but in reverse to create vorticies, to clean out the ring lands, and to some greater or lesser degree further excite the flame front.
For all you Dijkstra fanboys:

Knuth also cites a letter sent to him by Dijkstra, in which the latter adds some nuance to this earlier statements: "Please don't fall into the trap of believing that I am terribly dogmatical about [the go to statement]. I have the uncomfortable feeling that others are making a religion out of it, as if the conceptual problems of programming could be solved by a single trick, by a simple form of coding discipline!"
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