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Old 10-10-2012, 10:55 PM
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Quote:
Originally Posted by 57Chevy3100 View Post
Sorry for my lack of knowledge but what is squish?
Squish is the clearance between the piston crown and the underside of the cylinder head with the head gasket in place and the piston at top dead center.
The tighter you can run the squish, the more fuel tolerant (knock resistant) the motor becomes. What happens is that as the piston comes to top dead center, the space between the piston and the head diminishes to a certain point and that clearance is called squish. The piston "squishes" the fuel/air mixture out from between the piston crown and the underside of the head and "jets" it across the chamber just as the plug fires. This "squishing" homogenizes the fuel/air mixture and eliminates lean and rich pockets of air and fuel and results in a cleaner burn which in turn makes more power, plus makes the engine less detonation sensitive for the fuel you're using.

I'm just gonna throw some figures out here, so don't take them to the bank. Let's visualize that the motor has a squish of 0.080" and detonates on 87 octane fuel. If we cut the block decks to reduce the piston deck height or use a taller piston compression height and reduce the squish down to 0.040", the motor may run on 87 without any detonation. This is just a "what-if" set of figures, so don't hold me to them.

David Vizard, famous engine builder, has said that the motor will pick up power and run on lesser fuels the tighter he builds the squish. I think he got all the way down to 0.027" before the piston crown began kissing the underside of the head on a small block Chevy. The parts don't keep their same dimensions as they had sitting on the shelf when the engine is under operating conditions and getting hot. The crank bends a little, the rods stretch a little, the piston gets taller from expanding as a result of heat, etc., etc. So, if you build the motor with a (for instance) 0.027" squish while it's sitting there on the engine stand, then the heat and stretching as a result of running will close up the gap to zero. Most small block Chevy builders will shoot for a squish dimension of between 0.035" to 0.045".
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