Editing Quench

==Introduction==
[[File:Squishband.jpg|thumb|350px||]] 
Quench (or squish, or "squench") is sometimes referred to as ''mechanical octane''. It decreases the need for octane by promoting a more homogeneous air/fuel charge in the combustion chamber and it also helps promote flame travel.

==Quench vs. squish==
The terms ''quench'' and ''squish'' actually have different technical meanings. Quench also refers to the passing of heat from the combustion chamber into the surrounding metal, some of which finds its way into the cooling system. The more quench that is in effect, the more heat passes into the cooling system and vise versa. On one hand, having a quench-type combustion chamber and piston shape and tight quench distance may be looked at as a detriment to power production (heat IS energy, after all). But in the case of the IC engines we are working with, the loss of heat energy is more than offset by the decrease in the tendency to encounter detonation- which will kill power at a much greater rate and amount than the loss of some combustion chamber heat to the quench effect.

==What is quench?==
Quench is the name given to the turbulent mixing of the air/fuel mixture caused by the close proximity of the piston to the quench pad(s) of the cylinder head. As the piston approaches top dead center (TDC), the clearance between the crown of the piston and the quench pad of the cylinder head diminishes to about 0.040" (steel rods, aluminum rods require more distance). This squeezes or "squishes" the air/fuel mixture from the area where the piston and head are closest, to the area where the combustion chamber is located. This action creates turbulence to achieve a more homogeneous mixing of the air/fuel mixture. A quench distance of ~0.040" will allow a high performance engine to run without detonation using less octane than would otherwise be needed. Of course this is providing that all the other important areas are also covered, like the static and dynamic compression ratios, correct air/fuel ratio, correct plug heat range, good ring and valve guide seal, etc.
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==How to arrive at the target quench figure==
One way to arrive at a ~0.040" quench distance is to cut the block decks to zero piston deck height and to use a head gasket that compresses to around 0.040". This allows a quench (or squish, or "squench") measurement of 0.040".

While that is a straight forward way to go about it, there are better ways: Use a thinner head gasket and cut the deck only enough to get a flat surface with the correct finish for the head gasket to seal against. By doing it that way, more deck thickness is maintained, resulting in a potentially better head gasket seal due to the deck being thicker.

==Parts stack height==
When the parts that make up the reciprocating assembly are selected, these parts have to fit into the SBC block deck height of ~9.025". When calculating the height of the parts that make up the reciprocating assembly, use 1/2 of the stroke. To that, add the rod length, head gasket thickness, piston compression height. 

The height of the parts needs to be right at 0.040" less than the deck height of the block.

In a running engine, the oil clearance will create a slightly longer stack- a 0.003" rod bearing oil clearance will add something slightly less than 0.003". In this article, oil clearance will NOT be added into the stack height. If desired the oil clearance may be added; easiest way to do this would be to either add the oil clearance to the rod length, or simpler yet, just add the oil clearance after the stack height is calculated. The added height from the oil clearance would only be an issue if the engine is being built with a marginal amount of quench (<0.035" for steel rods); if built with the "ideal" 0.040" quench, the oil clearance can be basically ignored.

Another consideration is piston "rock". At TDC as the piston transitions from upward to downward movement, the piston will tip on its wrist pin. This causes one edge of the piston to be a small amount higher than the other edge. The exact amount will vary with how much piston to wall clearance there is; more clearance means more piston rock. Forged pistons generally have a looser piston to wall clearance than cast pistons, but newer design forged pistons have tighter clearances than was used in days gone by. This is another thing that's basically accounted for if a 0.040" quench distance is maintained. Only if less than 0.035" would this possible be an issue. 

The bottom line to all this is it's best to maintain an adequate quench figure of 0.040". There's nothing to be gained by going tighter, and a 0.040" quench distance will avoid unseen problems for the most part. If the quench is less than 0.040", be sure to double check clearances to be sure there is no contact between the piston and head- for obvious reasons.  

==Some excerpts from posts by Hotrodders forum member, oldbogie==
*From Hotrodders forum thread [http://www.hotrodders.com/forum/350-290-help-170440.html 350/290 Help]:
<Blockquote>  “Squish/quench is a method of building mechanical octane into the engine. These two functions are provided for by the closing of the piston's flat crown surface to that of a similar surface of the combustion chamber opposite the valve and spark plug pocket. The closing distance should be 0.040 minimum to 0.060 maximum for best effect. This is hard to achieve with the factory's round dish being under the quench pad area of the head. The round dish's depth is added to the gasket and piston deck clearance stack up. </Blockquote>  
<Blockquote>  “The squish happens as the piston closes to the top of the chamber. This ejects the mixture toward the spark plug. This does two things. First is it mixes the fuel and air providing a smooth and complete burn; better mileage and power result. Second, it puts nearly all the mixture in front of the spark plug which increases its density, making it easier to light off and causing it to burn faster; fewer miss and late fires and more early pressure with less ignition advance is the result. End product is better fuel economy and more power as well as lower emissions. </Blockquote>  
<Blockquote>  “The last function is quench, like it sounds it dampens the end burn. The end burn is where detonation occurs, the last mixture furthest from the plug is subjected to high pressures and temperatures that cause it to self ignite ahead of the burn, its explosion is the ping you hear when the pressure wave slams into the metal parts. The quench is an area of little volume and a lot of surface area, so it sinks the heat of the late burn, delaying the point where the unburnt mixture explodes. This lets you push the engine harder, at cruise you can operate at higher temps which increase thermal efficiency, and at WOT it holds off detonation which raises the RPM operating limit, assuming the cam will sustain more RPM and bottom end is strong enough.”</Blockquote>  


*From Hotrodders forum thread [http://www.hotrodders.com/forum/another-hp-torque-estimate-350-a-170908.html Another HP/torque estimate on a 350??] 
<Blockquote>“OEM pistons are as usual a bone of contention. Even with the Vortec head, GM just couldn't let go of the circular dish in the crown. This does a lousy job of squish and quench forcing you to buy fuel with more octane content to keep from blowing the heads off with detonation. To the rescue is the D-shape dish of the aftermarket. These function like a flat top for squish and quench while putting all the dish under the valve pocket where you select the dish volume to dial in the compression ratio for the fuel you want to use. A commonly used source is Keith Black, it's on the web.”</Blockquote>
 

*From Hotrodders forum thread [http://www.hotrodders.com/forum/l-31-compression-ratio-piston-type-170262.html L-31 compression ratio and piston type?]
[[File:L31 PISTON DISH DEPTH .080 002.jpg|thumb|right|350px|L31 OEM round dish piston design]]
<Blockquote>“With aluminum heads you can up the compression by at least a full point, maybe a point and a half. But the factory L-31 piston follows GM's keep-it-cheap-as-possible format by using a circular dish. These pistons are the bane of performance as they lead to a less-than-ideal compression ratio for the fuel being used because of their lack of adequate squish and quench. As you can see from the picture on the right, the dish is 0.080 inch deep. Add to the dish's depth the typical production head gasket of about 0.020 inch crushed and the typical Chevy having another 0.025 inch between the top most part of the piston and the block's head deck. All told that adds up to 0.125 (1/8) inch between the bottom of the piston cup and the head's squish/quench deck! The optimum squish/quench is achieved at 0.040 inch from the pistons head surface to the heads squish/quench deck. That's a long way from the 0.125 (at best) the factory lets you live with. You make up the difference in squish/quench function with the amount of octane you buy at the pump. Now, ''some'' part of the OEM piston (the rim around the outer edge) does get close to the head, but it is too small to be of much value.”</Blockquote>
<Blockquote>“Squish and quench are functions that build what's called mechanical octane into the engine, they also improve off idle and high axle ratio cruise performance as well as optimizing performance against the available octane fuels. The same parts perform both functions, which are merely separated in cycle time. Squish happens first, as the piston closes to TDC the mixture on the far side of the chamber is ejected by the close closing of the piston and head decks toward the spark plug. This stirs the mixture and increases its density before the spark-plug making it easier to light off and faster to burn.</Blockquote> 

<Blockquote>“As the burn proceeds from the plug, the temperature and pressures go up very quickly. The so called end burn on the far side of the chamber wants to spontaneously ignite creating an explosion, colliding flame fronts, and high pressure waves. The mechanical way of reducing this is to have an area on the far side of the chamber that has a lot of surface area to its volume to quench the explosion by being a heat sink. These two functions work together to improve the burn giving more power and economy and to delay the onset of detonation, especially under high loads and part throttle operation. Certainly not as high tech as EFI but the foundation is what keeps it together.”</Blockquote>
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*From Hotrodders forum thread [http://www.hotrodders.com/forum/favorite-streetable-piston-style-survey-169616.html Favorite streetable piston style survey]
<Blockquote>“In theory a flat top piston in a wedge chamber gives the best flame characteristics, that is fastest flame travel speed across the chamber with excellent squish and quench characteristics. Pop up domes get in the way of the flame front, slowing it down which is compensated for with excessive ignition advance, which then introduces problems with detonation and preignition combined with high fuel consumption and emissions. A circular dish piston reduces compression to that tolerable by the fuels used, these are typical of OEM low manufacturing cost solutions. But the circular dish design pistons have poor squish and quench as too much of the piston crown is too far from the head's squish/quench deck to be effective. This results in a tendency to detonate and preignite combined with poor power and excessive fuel consumption and high emissions. The D-shaped dish piston keeps the flat top's fast burn rate, eliminating the use of excessive advance and its problems; it brings the flat top's excellent squish and quench characteristics making for much greater detonation and preignition resistance, this is often referred to as mechanical octane. Like the flat top, it pushes the mixture into a pocket in front of the spark plug giving a more reliable light off and thorough burn for good power and lowest fuel consumption with that power. The dish is available in several volumes and with the deck clearance space, head gasket volume, and combustion chamber space, it is used to optimize the compression ratio that available fuels can tolerate.</Blockquote> 
<Blockquote>“A digression to squish and quench. All engines, be they hemi, pent, or wedge use this in some design form. It is an area where the piston and head close very closely together. With hemi and pent heads. it's located around the outside diameter of the bore, pushing the mixture toward the middle where the sparkplug is usually located. On a wedge chamber it is found on the side opposite the sparkplug and valve pocket, pushing the mixture toward the sparkplug. These parts, or features of parts, perform two functions; one is squish the other is quench. They are separated by time in the cycle of compression to power. Squish happens first on compression as the flat surface of the piston closes toward the matching surface of the head. This ejects the mixture toward the sparkplug with great force both stirring the fuel and air together and increasing the density of the mixture directly in front of the spark plug. This both improves the chance of the plug lighting a burn (reduces miss and late fires), and it speeds the burn so cylinder pressure is optimized for piston position to press on the crankshaft with the greatest force possible (best power and use of the energy you pay for). At what is called the "late burn" part of the cycle is where detonation is like to occur.</Blockquote> 
<Blockquote>"The temperatures and pressures ahead of the flame front are getting very high to where the remaining mixture is entering the "diesel" zone where it's happy to just blow up. To counteract this tendency is now the quench function of the close fitting parts of the combustion chamber. This is now a zone with a lot of surface area to volume, so it acts as a heat sink, delaying the point where the temperatures and pressures become so great that the mixture explodes instead of burns. These days of restricted octane fuels has made this feature very important as you can no longer just throw more Tetra-Ethyl-Lead at the problem.”</Blockquote>



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[[Category:Engine]]