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by: Cobalt327
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[edit] Introduction


For this discussion, we will be using the term "quench" to denote the distance between the cylinder head deck and the piston deck at TDC, as well as for the overall action/effect of the combination of squish and quench.

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.

[edit] Quench vs. squish

The terms quench and squish are often used interchangeably. But they 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.

[edit] What is squish?

Squish 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.

From corvetteonline.com
While the terms “quench” and “squish” are often used interchangeably by many manufacturers, quench and squish are not the same thing, nor are they produced by the same set of conditions. The Society of Automotive Engineers (SAE) has defined squish as the gases trapped between the piston dome and head that are ejected across the combustion chamber at high speed by the near-collision of the piston dome and head, causing turbulence and mixture homogenization. For our purposes, if the squish area is too close, there is a pumping loss and if the area is too far apart there will be lower squish velocity and less turbulence.

Quench on the other hand, is the ability to lower temperature of the end gases trapped between the piston dome and head by conduction. This prevents a second flame front from igniting the air/fuel mix prematurely. Members of the SAE acknowledge that for motors with 3.5” to 4.5” cylinder bores, a quench distance of 0.035” to 0.040” work well and result in near zero clearance due to thermal expansion, rod stretch and piston rock-over.

[edit] 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 other 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.

[edit] Parts stack height

When the parts that make up the reciprocating assembly are selected, these parts have to fit into the deck height. For a Small Block Chevy (SBC) this value is ~9.025". When calculating the height of the parts that make up the reciprocating assembly, use 1/2 of the stroke, add the rod length, and piston compression height. For a stock 350 SBC, this would be 1.74" + 5.7" + 1.56" = 9.00". If the block is uncut, the decks will be about 9.025". Using a head gasket of 0.015" will give a quench figure of 0.040". If the decks are cut the thickness of the head gasket can be used to compensate as needed.

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 to 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.

[edit] Piston design

For a wedge shaped combustion chamber like used in many engines, a flat top piston is the best design for promoting the quench effect. However there are times when a dish has to be used. In those cases a reverse dome, also called inverted dome, or D-cup piston (below left), will be the best compromise.

KB P-N 135 SBC 383 PISTON.jpg

Next to a D-cup piston would be a wide quench band round dish piston as seen above right. This isn't as good as a D-cup, but it is better than a stock-type SBC piston seen below. The stock piston has a too-narrow quench band due to the large dish plus the 45 degree large chamfer around the piston OD.

Round dish sbc piston.jpg

[edit] Some Hotrodders forum posts

[edit] Compression ratio calculators

[edit] Dynamic compression ratio

Warning Note: Some dynamic compression rtatio calculators (like KBs) ask for an additional 15 degrees of duration be added to the IVC @ 0.050" lift point figure. This works OK on older, slower ramped cam lobes, but the faster lobe profiles may need to have 25 degrees or more added to be accurate.

Warning Note: If the intake valve closing (IVC) point isn't known, it can be calculated:

  1. Divide the intake duration by 2
  2. Add the results to the lobe separation angle (LSA)
  3. Subtract any ground-in advance
  4. Subtract 180

This result does not need to have any amount added to the IVC point, like the KB calculator calls for.

[edit] Static compression ratio

[edit] Resources

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