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6sally6 01-15-2010 08:02 PM

What is quench?
What is squish?
What is the difference,and how do get the best of both?

SSedan64 01-15-2010 08:10 PM

There is no difference in the dimension. Generally you want .040"-.045" Quench or Piston to Head clearance.

timothale 01-15-2010 08:27 PM

combustion chamber design
Engineers changed the shape of the cylinder heads to try to meet the emission stds starting in the 70's. In the older 60's engines the combustion chamber was a smaller somewhat D shaped area and there was a thin area outside the D between the head and the top of the piston. the thinking was that this area cooled and that the fuel did not burn in this area . some pistons were made dished and some combustion champers changed to be O shaped to meet the new smog regulations. A smaller D combustion shape will have more turbulance which help keep the fuel and air mixed,, smaller combustion chambers will have a faster burn. producing more pressure closer to the top of the stroke. It's gets complicated trying to get more power and economy and burn cleaner.

cobalt327 01-15-2010 08:49 PM

I think Bogie also had a post on this some time ago. You might do a search for it...

cranky1 01-16-2010 10:08 AM

I like to go tighter than the commonly accepted .040-.045 but it also depends on what you are trying to accomplish. Here's an article from KB pistons Chief Engineer, John Erb. I've read a lot of articles on the subject and this one helped me to understand better what was going on. It's long but worth the read imo.

What is the most, exact precisely defined occurrence in all piston engines? It isn’t ignition timing, combustion, crank indexing, or valve events. It is Top Dead Center. You can’t build an engine with an error at Top Dead Center because TDC is what everything else is measured from. Spark scatter, crank flex and cam timing can move, but TDC is when the piston is closest to the cylinder head in any one cylinder. The combustion process gets serious at Top Dead Center and about 12 degrees after TDC, most engines want to have maximum cylinder pressure. If maximum cylinder pressure occurs 10 degrees earlier or later, power goes away. Normal ignition timing is adjusted to achieve max cylinder pressure at 12 degrees after TDC. If your timing was set at 36 degrees before TDC that is a 48 degree head start on our 12 degree ATDC target. A lot of things can happen in 48 degrees and since different cylinders burn at different rates and don’t even burn at the same rate cycle to cycle, each cylinder would likely benefit from custom timing for each cylinder and each cycle. Special tailored timing is possible but there is an easier way—“Magnificent Quench”. Take a coffee can ½ full of gasoline burning with slow flicking flame. Strike the can with a baseball bat and you have what I would call a “fast burn”, much like what we want in the combustion chamber. The fast burn idea helps our performance engine by shortening the overall burn time and the amount of spark lead (negative torque) dialed in with the distributor. If you go from 36 degrees total to 32 degrees total and power increases, you either shortened the burn time or just had too much timing dialed-in in the first place. If you have really shortened the burn time, you won’t need so much burning going on before Top Dead Center. Now you can retard timing and increase HP. Did you ever have an engine that didn’t seem to care what timing it had? This is not the usual case with a fast burn combustion but an old style engine with big differences in optimum timing cylinder to cylinder will need 40 degrees of timing on some and others only need 26 degrees. If you set the distributor at 34 degrees, it is likely that 4 cylinders will want more timing and 4 cylinders will want less ( V-8). Moving the timing just changes, which cylinders are doing most of the work. Go too far and some cylinders may take a vacation. Now what does quench really do? First, it kicks the burning flame front across and around the cylinder at exactly TDC in all cylinders. Even with spark scatter, the big fire happens as the tight quench blasts the 32 degree old flame around the chamber. Just as with the coffee can, big flame or small flame, hit it with a baseball bat and they are all big instantly. The need for custom cylinder-to-cylinder timing gets minimized with a good quench. The more air activity in a cylinder you have the less ignition timing you are likely to need. When you add extra head gaskets to lower compression you usually lose enough quench that it is like striking the burning coffee can with a pencil. No fire ball here and that .070-.090 quench distance acts like a shock absorber for flame travel by slowing down any naturally occurring chamber activity. A slow burn means you need more timing and you will have more burn variation cycle-to-cycle and cylinder-to-cylinder, result more ping. Our step and step dish pistons are designed not only to maximize quench but to allow the flame to travel to the opposite side of the cylinder at TDC. The further the flame is driven, the faster the burn rate and the less timing is required. The step design also reduces the piston surface area and helps the piston top stay below 600 degree f (necessary to keep out of detonation). All of our forged pistons that are lower compression than a flat-top are step or step dish design. A nice thing about the step design is that it allows us to make a lighter piston. Our hypereutectic AMC, Buick, Chrysler, Ford, Oldsmobile and Pontiac all offer step designs. We cannot design a 302 Chevy step dish piston at 12:1 compression ratio but a lot of engines can use it to generate good pump gas compression ratio. Supercharging with a quench has always been difficult. A step dish is generally friendly to supercharging because you can have increased dish volume while maintaining a quench and cool top land temperatures. You may want to read our new design article for more information. ".

By John Erb
Chief Engineer
KB Performance Pistons

cobalt327 01-16-2010 01:37 PM


Originally Posted by 6sally6
What is quench? What is squish?
What is the difference,and how do get the best of both?

Damn good question 6s6.

Instead of trying to put my thoughts into words, I will instead defer to oldbogie on this most interesting and important aspect of engines and engine building.

I cite oldbogie here, because, well, IMHO he’s ‘da man’ when it comes to the technical aspects of IC engines. He’s got a lot of knowledge in the years he has invested in the automotive field, as well as countless hours divulging that info on this forum.

The following is but a few of his remarks in re quench/squish. I made no effort to cherry pick the posts that I have made excerpts from- the following are from the first five results that came up.

If you’d like to read more, do what I did and search on this forum using “bogie quench squish”. There are at least 42 results…

Bogie on Quench/Squish Post #10
“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 .040 min to .060 max. for best effect. This is hard to achieve with the factory's round dish under this area adding another .080 inch to the gasket and clearance stack up.

“The squish happens as the piston closes to the top of the chamber. This ejects the mixture toward the spark plug doing 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 advance is the result. End product is better fuel economy and more power as well as lower emissions.

“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, it's 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.” Post #13
“OEM pistons are as usual a bone of contention. even with the Vortec 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 up on octane to keep from blowing the heads off. 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 CCs 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.” Post #9
“With aluminum heads you can up the compression by at least a full point many be a point and a half. But the factory L-31 piston follows GM keep it cheap as possible formate of a circular dish. These things are the bane of performance as they lead to a less tan ideal compression ratio for the fuel being used because of their lack of adequate squish and quench. As you can see from the picture that COBALT was so kind to include, the dish is .080 inch deep. Add to the dish's depth the typical production head gasket of about .020 inch crushed and the typical Chevy having another .025 inch between the top most part of the piston and the block's head deck. All told that adds up to .125 inch between the bottom of the piston cup and the head's squish, quench deck. The optimum squish/quench is achieved at .040 inch from the pistons head surface to the heads squish/quench deck. That's a long way from the .125 (at best) the factory lets you live with. You make up the difference in squish/quench function with the octane's you buy at the pump. Now some part of the OEM piston, that being a rim around the outer edge does get close to the head but it is too small to be of much value.”


“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.

“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 and 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 sexy as EFI, the basics seldom are sexy, but the foundation is what keeps it together.” Post #13
“Squish/quench is one of these that gives gifts, or not, that functions in weird ways, a large surface provides plenty of squish at low speeds to push the mixture toward the spark plug, a good thing; but provides too much quench, for which you pay for at the pump. At high speeds the squish function is unnecessary but quench needs to be optimized to hold off detonation.

“For an engine operated at low to moderate speeds the Singh grooves reduce the blast out effort of squish and reduce the effect of quench by giving the flame front a path into the tightly approaching decks of the piston and head which may speed the burn and provide better power and mileage. At low speed with light load operations this is probably useful. At high speeds the chamber is no longer dependent upon squish to stir the mixture so the function of Singh groove at this point is moot, but they still let the flame front
pass into the quench area, but time for events may be overrunning the difference in less quench. Still this may reduce detonation tolerance.” Post #12
“In theory a flat top piston in a wedge chamber gives the best flame characteristics, that is fastest 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, these 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. The D 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 through 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 is used to optimize the compression ratio that available fuels can tolerate.

“A digression to squish and quench. All engines 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's and pent's it's located around the outside diameter of the bore pushing the mixture toward the middle. 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. 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 counter act 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.”

cobalt327 12-07-2012 09:07 PM

Should this thread come up during a search for info, more info can be seen on the Crankshaft Coalition wiki under the title Quench.

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