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.
