Though it has been explained many times to countless gear heads (myself included), it still appears that many people do not understand what quench is and why it is so important. I thought I would take a stab at providing the fullest explanation I could with some illustrations.

To begin, we always say quench when referring to a distance from piston to head. For me to fully explain quench I will use two phrases 1.) Quench and 2.) Quench effect (could also be called squish). Quench effect is the actual process of squishing the air and fuel mixture into the combustion area while quench is nothing more than the piston "in the hole" depth (or deck clearance) plus the compressed head gasket thickness. You could also say that quench is the distance from the flat portion of the piston to the bottom flat surface of the head. Seems pretty simple right? Now I’ll start with the confusion.

Quench is NOT affected by the volume of a dome/dish/valve relief or the head combustion chamber size. Only the distance from the top of the piston (flat portion) to the bottom of the head affects quench. The quench effect will vary depending on how much “quench area” there is on your setup, but that will be discussed later. Here is a picture to illustrate what quench is (piston deck clearance + compressed gasket thickness is your quench).



The reason valve dishes, relief’s, and domes do not count for measuring quench is because it’s the distance measured between the flat area on the piston surface and head surface that comes within the desired quench (for my explanation I will use a quench of .05”). This quench area forces all of the A/F mixture in the cylinder into the actual combustion chamber when the piston is at TDC (top dead center). For example, if you are looking at the surface of a dished piston you will always notice a fairly large flat area opposite of where the sparkplug would protrude into the chamber. This area extends out and surrounds the dish itself... this is the potential quench area for the piston. The “actual quench area” is where the distance between the flat surfaces of the piston and head are at or EXTREMELY near our .05” quench. Look at the picture of the head and piston below to see the flat areas I’m talking about.



If you are looking at a domed piston the same type of quench area can be observed around the dome. The flat top piston may seem different because the whole surface is flat except for valve relief’s but it just has a lot more potential quench area than a domed or dished piston. You could say that the head will determine how much “actual quench area” you will have because pistons are designed around the head chamber. The only reason for domes and dishes are to change the volume of the combustion area. Here is a picture of an O.E. flat top, dished, and dome piston from left to right.



Now lets look at what happens in this "quench area". The arrows represent the motion of the A/F mixture.



Notice in the picture above that the A/F mixture in the quench area is being forced into the combustion chamber. The tighter the quench, the more velocity the A/F mixture gains while being forced into the combustion chamber. The higher the velocity of the mixture the more "mixed" the A/F mixture will become leading to a higher percentage of burned A/F. More turbulence is derived from this higher velocity as well. While turbulence is a bad thing when talking about induction, you want as much turbulence as possible in the combustion process. The more turbulence experienced while the piston is ascending to TDC makes for an extremely homogenous A/F mixture which = more efficient burn = more power with less unburned A/F.

Head chambers are designed around creating the most efficient burn cycle possible. The greater your quench, the more area you are essentially creating for "pockets" of the air and fuel (A/F) mixture to get into. Because these pockets of A/F mixture are not in the primary combustion area, they may not ignite with the rest of the mixture. These unburned pockets are essentially a ticking time bomb because they have a tendency to [detonate] before the piston is back at TDC. Usually when this happens the piston is moving upwards while the detonating pocket of A/F mixture tries to force the piston back down. This is why detonation is so damaging to your mechanically personified creation (your engine… lol sorry, I get carried away writing stuff like this). Detonation places added (unwanted) stress on the piston and works against the rest of the engine, robbing you of power and causing headaches.

Here are some excerpts from Chevy High Performance that I though would help you to determine what quench is right for your application.

1.) "Tightening the quench area often results in the reduction of ignition timing requirements. This can then lead to a reduction in negative work (the cylinder pressure rising while the piston is still approaching TDC). This often is evidenced by a gain in low- and mid-range torque."

"There is plenty of discussion about the net effect of squish and quench. While it’s doubtful that this will ever amount to more than a few horsepower in any street application, it does offer some distinct advantages when it comes to increased engine efficiency, better fuel mileage, and driveability. If you’ve ever wondered why certain engines run better than others, this could be one reason why."

2.) "All of the respected engine builders who we’ve talked to are firm believers in minimizing the quench clearance. According to Ken Duttweiler, the tightest quench he recommends is around 0.050-inch. He has built engines with far tighter clearances than this, but much of this depends on the piston-to-wall clearance. All pistons tend to rock slightly as they transition through TDC and this rocking motion reduces the piston-to-head clearance. Smaller-diameter pistons with tight piston-to-wall clearances don’t rock nearly as much in the cylinder bore compared to larger-bore pistons with wider piston-to-wall clearances.

Since piston clearance plays such a big part in piston-to-head clearance, it is possible to run a piston-to-head clearance tighter than 0.040-inch if you feel brave. Noted horsepower hero John Lingenfelter says that clearances of 0.037 to 0.040 inch are possible, but you must know what you’re doing. The late Smokey Yunick also recommended a quench clearance of 0.040 inch as a safe but critical clearance."

I want to intervene here because one variable they do not mention here and should be is the pistons material. Hypereutectic pistons have a lower thermal expansion rate compared to a forged piston so they can have tighter initial piston-to-wall clearances and will not "rock" in the bore as much - especially when cold. Forged piston engines are "looser" when cold and can have substantial piston rock that needs to be considered when choosing quench (I’m sure you have heard people talk about piston noise when first starting a forged piston engine). Because forged pistons have a higher thermal expansion rate than hypereutectic pistons, they have a higher initial (cold) piston-to-wall distance but will yield a tighter piston-to-wall clearance (within the hypereutectic piston range) once normal engine operating temperature is attained. Of course this is all based on thermodynamics and some other fun jazz.

Continuing on with the Chevy High Performance article -

3.) "All the engine builders we spoke to mentioned that tightening the quench (reducing the piston-to-head clearance) to get it under 0.050 inch will increase the static-compression ratio, but this tighter clearance also creates a more powerful squish effect. This additional turbulence creates a more homogenous “soup” in the chamber, reducing the harmful effects of lean air/fuel ratio pockets. With all other variables being equal, this contributes to creating an engine that is less prone to detonation."

In conclusion, to chose the right quench for your application you should consult with your engine builder or look at other experienced home builders proven combinations. Be realistic when choosing a quench distance as well. Based on what I have discussed with you all in this write-up you should understand that for a stock street driven motor up to a higher performance street motor there is no need for the risk of running .042” or tighter quench. For even hotter street/strip cars you could push it up to the edge with .037”. Flatout race cars… do what you please because you will put as much time on your engine in a season as you would in a week of running around in the daily driver. I guess this paragraph could be taken lightly for I am not building your engine, but I would highly recommend to consider what I have stated.

While the ideas are the same for pretty much any engine (loosely speaking), my analysis is primarily for 1st generation chevy's (Lxx/LTx series). 3rd generation engines (LSx/LQx series) are somewhat different (piston is actually out of the hole as opposed to being at zero deck height and below) but it is still the same principal.

All information was found through various online searches, articles, and my interpretation. Images were used from the Chevy High Performance article on the same subject HERE

And that ladies and gentlemen, is the down and dirty of quench and pretty much anything related . Hope this helps with some peoples quest on the ultimate street machine and I hope I didn't fluzzlebombard anyone's head.

 

If you have any questions/comments please let me know so I can update the info as needed. Thanks

 

Nice job.

 

Very good explanation. I'm building a new bottom end and this is the main criteria that I considered in the planning and parts selection.

 

 

Thanks for the excelent explanation. Like it, I did copy and gave it to some friends that like to understand as well how things work

 

I guess I don't quite understand the relevance of quench. For a given compression ratio, you have some cc in the combustion chamber at tdc. The quench doesn't even really refer to the average height of this space, since you're saying valve reliefs and head cc's don't count. Now if we were pumping 1/4" BB's through the motor, I could see the relevance of having a quench height greater than 1/4"

I guess the bigger question, is with the same exact compression ratio, how does a hemi head (more quench) do vs the one shown (less quench), all else being equal? Hell, i haven't even seen a crosssection of a hemi head, I guess I better do that first.

 

"Quench" is the flat area, in the diagram above, between the top of the pistion and just the flat part of the cylinder head. It does not include the combustion chamber.

The way I picture this is as the piston comes up, this "flat area" becomes extremely small very fast and "squishes" the air/fuel mixture out of that area into the main combustion chamber. So if we have a tight quench area, we get a lot of force creating more turbulance in the combustion chamber, a good thing. But if this flat area is no closer than, say, .050, there is not nearly as much force and some combustion actually occurs within the flat area because there is still air and fuel in there. Sort of like popping a paper bag full of air. If we can flatten it completely, we get a big "pop". But if we cannot flatten it completely, not so much "pop." BTW "pop" is a technical term.

I have a 406 in process with the block milled to a "zero" deck. This means my "quench" area or "squish" will have the depth of the compressed head gasket as its dimension, something under .050. My main issue in doing this is to have clean burning combustion with minimal pollutants so the smog ****'s will love me.

 

Good explanation!

 

I'm rewriting the thing because I left out some stuff... lol this is harder that I though to make it realatively easy to comprehend.

 

By definition, a true hemi has a ZERO quench area. Squish and quench only apply to wedge headed engines.

RACE ON!!!

 

From the drawing, I'd say the magazine has confused "quench" with "squish". If so, the drawing is incorrect. I don't know how to draw on this medium, but squish is the band resulting from head overhang around the cylinder periphery that gives a couple of thou clearance between the piston's rim and the actual head. The importance is preventing combustion dead spots by forcing the outlying mixture into the center of the combustion chamber at TDC.

"Tightening the quench area often results in the reduction of ignition timing requirements. This can then lead to a reduction in negative work (the cylinder pressure rising while the piston is still approaching TDC). This often is evidenced by a gain in low- and mid-range torque."
This is better known as "increasing the compression ratio."

Let's not make this stuff too complicated.

Larry
code5coupe

 

Just rewrote everything.... spent a few hours. Hope this resolves many issues... and yes, the whole squish vs quench thing is in there

 

In everything I've ever read on the subject, quench and squish are interchangeable terms. If that's incorrect, someone please explain. Quench or squish improvements will increase CR, all other things being equal. But this is not the reason to strive for the ideal. If you have two engines with equal compression ratios, the one with the most nearly optimized quench will be the most efficient in all the ways and for the reasons stated already.

 

If you make it too simple, you miss the point. For instance, by simply raising the compression ratio, by other means, you don't get a "reduction of ignition timing requirements",you get a reduction of timing tolerance, a greater tendency to knock. Two very different things. If a minor increase in compression ratio is achieved by ONLY decreasing the quench distance, the odds are good that the higher compression ratio will be LESS likely to knock than before.

RACE ON!!!

 

squish = distance

quench = area

right?

 

I think that this is one of those situations where the terms are loosley used and is why people get confused when they are thrown around. I feel that we get to hung up on what something means as opposed to understanding what it pertains to. As long as you understand the process involved with quench, squish, quench area etc., and why it is importaint to an efficient engine it really doesn't matter what you call the distance, area, or anything else. I focused my writeup primarily on the entire process and gave detials of what happens and why. Call it what you will. If someone is confused, point them in the direction of this thread and tell them to read it until they understand. May take a few times, but they will get it.

 

Good explanation that seems to basically agree with everything I've read on the subject. The item I would like to see more discussion on in relation to this is the effect of piston shape and cylinder head shape on the burn effiency after you have this desired maximum quench effect. I have seen a couple of articles on measured cylinder head swirl but they don't usually go into piston shape effect on this swirl.

 

Good Explanation. Thanks for posting

 

that was very informative. thanks