Thread: Understanding squish and quench....

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I wrote this on another forum for a young fellow who is just beginning to understand the workings of the internal combustion engine. It turned out so well that I decided to share it with my friends on CHR.....

the youngster wrote:
I'm gonna get the measurements for this "302" block cause it's been decked before, let's just say it was decked 10 under putting it at 0.015 what piston compression height would I expect? Thanks for everything I wanna do this right the first I've learned my lessons on past builds lol.

I wrote:
If a stock block (9.025" block deck height) were decked 0.010", then the new block deck height would by 9.015". If you then used a stock 350 crank with a radius of 1.740", a stock 5.7" rod and a standard compression height 1.560" piston, the stack would be 9.000" and the block deck height would be 9.015", leaving the piston down in the bore by 0.015" with the piston at top dead center, so that the piston deck height would be 0.015". You might then choose a gasket that compresses to 0.028" to give you a squish/quench of 0.043". A fellow should know the different thicknesses of available head gaskets before he begins whittling on the block or choosing any parts for the build. I just happen to know off the top of my head that there are at least 2 GM composition gaskets that compress to 0.028", 10105117 and 14096405.

You might wonder what all this noise of squish/quench is all about. Let me explain......
With the intake valve open and the piston descending in the bore, atmospheric pressure pushes the air/fuel mixture slug into the cylinder until the intake valve closes. Problem is, not all the fuel/air mixture is the same density. Some of it is large droplets of pure fuel that will not burn and some of it is a fog of finely mixed density that burns easily. This is where electronic fuel injection shines, it delivers a much more flammable mixture into the cylinder than a carburetor does. Neither, however, is perfect. What we need is a method of further breaking up the large droplets of liquid fuel into a flammable mixture that will burn and contribute to turning the crankshaft.

Enter squish.

As the piston comes up to top dead center on the compression stroke, the fuel/air mixture that has been pushed into the cylinder by atmospheric pressure is trapped between the piston crown and the underside of the cylinder head. Some of it is in the combustion chamber and some of it is in an area opposite the chamber where there is a flat area on the head and a (ideally) flat area on the piston crown. The larger the area on the piston crown, the better. A perfectly flat crown with small valve reliefs might be the best design possible from a squish standpoint. If a fellow needed a piston that would allow a lower static compression ratio, requiring a dish in the piston, then a D-cup type piston crown might be the best design. As the piston approaches top dead center, the fuel/air mixture is "squished" from between the piston crown and the underside of the cylinder head with a great deal of force. It is blown over into the chamber area of the head where the turbulence tends to further break down the large droplets of fuel into a burnable mixture. This action not only helps to produce more power, it also makes the motor more "detonation-resistant" when using fuels that might be very close to detonating because of static compression ratio, fuel quality and cam timing figures.

By copious experimentation, it has been determined that the best measurement for squish is 0.035" to 0.045". You need to understand that what you put together and measure is not necessarily where everything stays when the motor is running. Even as stout as it looks, the crankpin on the crank yields a little on the upswing, bringing the piston closer to the head at top dead center. The rod will stretch a little and grow from the heat of operation (being constantly splashed with hot engine oil) and the piston will stretch a little and also grow a little due to heat of combustion. Each motor is different, of course, and uses different designs and different components, so each motor would be a little different on the minimum squish you could use before crashing the piston crown into the bottom of the head at speed. I have read of fellows who have found the part number that is on the crown of the piston, imprinted on the underneath of the cylinder head when the motor was disassembled. Now, in my opinion, that would be the optimum squish, just short of disaster.

Ford Motor Company found out about squish when they built the 1972 BB cylinder heads without a squish pad. You may want to google and read about that debacle. They had to lower the static compression ratio down into the 7's to prevent the motors from detonating.

Enter quench.

Many people substitute the word quench for the word squish. They are, however, two completely different things. Quench is the action of bringing the piston into proximity of the cylinder head so that heat from the piston crown can be transferred to the relatively cooler metal surface of the cylinder head and then into the water jacket, to be carried away by the radiator to atmosphere. Did you ever see a blacksmith plunge a hot part into water or oil? That's quench. It is the cooling of a part. In this case, it is the cooling of a piston crown. Please do not use the word quench to describe squish. It's not the same thing.

I write the whole thing as squish/quench so that everybody knows what I'm talking about, because not everyone understands the difference.

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