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brainsboy 04-22-2004 07:03 AM

why is QUENCH important????
I was told and remember reading that a tight quench is important to keep from detonation. Anything from .050 to .035 would be ok. I am currently working on a project that I dont want to buy custom pistons for and found a piston that will work but it sits .105 below the deck. The motor still comes out to 10:1 compression so that part is ok. So here is my question, Why do you have to keep a tight quench???? Whats the difference between this piston and one that is .010 deck height with 22cc dish built in? Either way they both come to 10:1 compression, I just dont understand why quench would make any difference. Both have 10:1 compression, just one has the outside edge higher and one is a flattop .105" low. I have searched all over the internet and no one really talks about it, or explains why.

As long as I have the compression I need can I run with pistons .105 below the deck height?

Ben 04-22-2004 07:54 AM

Check the archives of this Forum. This subject has been discussed in depth. One thing an engine with excessive quench area does is accumulate carbon on the piston tops. Also they are victim to a lazy flame pattern (incomplete/slow combustion of the charge). This is because a normal "squish" area creates a turbulance across the piston top to help keep deposits from building. The carbon deposits then will glow red hot under high load conditions causing pre-ignition, detonation and related problems. A good example is some of the early flathead engines. It was common to have to pull the heads and de-carbonize them. The deposits would get so bad they hit the cylinder head.

brainsboy 04-22-2004 07:57 AM

So then why doesnt a 22cc dish piston have the same problem?

engineczar 04-22-2004 08:28 AM

pistons with a full dish do have that problem, that's why pistons with a D shaped dish make more power compared to full dish.

k-star 04-22-2004 08:59 AM

It is my understanding that the only job of proper quench is prevent detonation. When the spark lights the mixture the flame is to burn across the top of the piston in a controlled fashion. Like a stone thrown into a pond when you get the rippled effect. The more chamber you have above the piston (IE piston .105 down the hole) the less ideal the chamber is to prevent detonation..... The idea in piston design is to help the flame travel. That would be my idea of why a "D" piston would work better because at the point away from the spark plug the quench is right making a better burn....

How much it actually effects real world operation of a motor i have no idea. I can tell you i bought a short block from a friends car. The motor was to have been built by a well know engine builder. It had a 14:71 blower on it. Those pistons were .190" down in the hole, flat tops with one small valve cut on the intake side.( big block chevy) The car ran awsome and as far as i know had no retard on the standard timing (not sure if it was boost retarded or not but i am sure it had to be) There was no signs at all of any detonation in the short block.......

So i know that motors will work that way. Not sure if this helps you out or not... If i am off on my understanding of quench someone jump in here.....


camaroman7d 04-22-2004 09:43 AM

The quench or squish also cools the combustion chamber. As the mixture is "squished" it quenches the chamber (this is why it is called quench).

Ben, you would be VERY wise to choose a combo that will give you a .035 - .045 quench. I know you are going to push the limits with your new application. Sure a tight quench is better for making power as well but, detonation is even more of a reason to keep it tight. A tight keeps the charge in a controlled area, not floating all over the combustion chamber. This way when it ignites it is controlled (one flame front), if not then there is a MUCH higher chance of having more than one flame front (due to the charge being all over the chamber) and if you have more than ond flame front this is where detonation comes from. You will be much better off with a slightly higher compression ratio and a tight quench than a lower compression ratio and a loose quench.


mstngjoe 04-22-2004 10:16 AM

Here's some info on quench from Speedomotive:

What's the part # of the piston you're looking at? What's the chamber size on the heads?

A measurement of .105 below the deck is a lot in most circumstances.

brainsboy 04-22-2004 10:40 AM

Royce and others,

Why would flame front be any different for a flat top .105" down or a normal piston .010 down with a 22cc dish, they both have a total of 86cc's chamber volumes at TDC. To me it seems as both would be the same. How much does quench really effect in the real world? Wouldn't both these pistons respond the same? Royce I hope I don't sound like Im arguing, Im just trying to understand the physics involved here.

K-star mentioned his friend ran .190 down. with out problems.

If anyone has built motors with .060" + quench please post here and give me your opinions.

firestone 04-22-2004 10:41 AM

Here is another good article on the topic of quench.


camaroman7d 04-22-2004 11:20 AM

I understand where you are coming from and I am not taking it as an argument.

Let me see if I can put it another way, If you don't have a quench (anything above .050 or so), then the charge is not going to be forced (squished) to one part of the chamber (usually towards the spark plug). This will invite uncontrolled burns to start. It is not only the shape of the top of the piston that matters but, it plays a big role. With a "dog bowl" dish there is no quench (no flat part on top to create a squish), so in this case it doesn't make a big difference but, this is also the reason "D" dish pistons are a better choice. The same could be said for open or closed chamber heads.

So basically a good quench controls where in the combustion chamber the charge will be, and with this you can control "when" it is ingited. Since the squish will cool the cobustion chamber as the charge rushes across the top of the piston the benefits are two fold. Now with your example of a large quench I don't think it would matter what the shape of the piston is, because you really don't have a quench effect anyway. The piston is not coming close enough to the head to create a squish, so you will have a "lazy" charge just waiting to be ignited by the first available source (hot carbon, etc...) since it didn't cool the combustion chamber there is even more of a chance for hot spots and uncontrolled ignition. As you can see it is a cycle and chances of pre-ignition and detonation are increased as the quench gets loose.

Now if you run high octane race fuel this would be less of an issue, even though you would make more power with a tighter quench (IMO) but, you won't have as big of an issue with the detonation.

To answer your question about the difference in the flat top .105 down and the D dish .010 down. While you will have the same 86cc's of volume, it is "where" the volume is that counts. With the flat top (.105 down)a lot of the volume will be spread out over the whole piston and all over the chamber. With the tight quench and the d dish the same 86cc's of volume is in one specific are (right next to the spark plug) and HOW it gets over to that area is the advantage, It is forced/squished over to that side. This is the effect we are looking for.

While K-star has/had a friend that ran a HUGE quench and "got away" with it, I don't know what the combination was/is so I really can't comment. If I had to guess I would say he had open chamber heads for one. Was this a street car running on pump gas? What was the compression? Seeing that it was a 14-71 blower on it I would say it was not a pump gas engine. The blower would also have a very "active" charge. Even at that how do we know if it was detonating or not? If he filled up with 91 octane what do you think would have happened?

I am looking at this from a street driving perspective, drag/race cars get away with a lot of things that will not live on the street or pump gas. So make sure we are comparing apples to apples. Last I remember you planned on running pump gas, correct? If not then you may be able to get away with that wide quench but I still say power will suffer.


k-star 04-22-2004 11:41 AM

Royce, you are right it was an open chamber head. They were bb-2's and he did run 110 or 116 octane in it.

Do you think that an increase in horse power is due to the quench in and of it self, or due to the fact that with the proper quench you can get away with a more aggressive tune due to the fact that it is less sensitive to detonation???? what would the power difference be between the 2 motors we are compairing????

That is a problem with the hemi head designed motor. The chamber above the piston at top dead center is so big and you have 2 areas on each side of the plug to get the burn to..... The orginal hemi was a tuners nightmare until they figured out what was going on. They needed so much spark lead to get it all burnt and then just a little more would put them into detonation and lift ring lands.


brainsboy 04-22-2004 11:47 AM

Well, I read the last article, it was very informative, I hadn't seen this article before, thanks for posting it. But it still doesn't make complete sense to me yet. :( ... I can understand the reason why it works on a flat top going from .060 to .040 . But I dont understand if you take these 3 pistons show here wouldnt all the quench or squeezing area be the same?

just hypothetical numbers here for example

piston 1 = .000 deck height -10cc 10:1 compression

piston 2 = -.050 deck height -5cc 10:1 compression

piston 3 = -.100 deck height flat top 10:1 compression

All have the same amount of compression area. Turbulence would be the same for all 3 pistons wouldn't it? They all have the same size cc chambers.. If these were all flat tops I can understand quench improves because you have less cc'ed area, but if you are limited to say 10:1 compression, arent all 3 pistons going to react the same? If nothing else piston 3 leaves more of the fuel air mixture closer to the head then piston 1 because its spread out over the entire piston top.

mstngjoe 04-22-2004 12:49 PM

I don't know what exactly you're working on, but.....if I put scenario #3 in my engine and 58cc heads, I'd have 8.2 static compression. My quench would be .144.

(BTW, I've got a small block Ford stroked to 331. Flat tops (-6cc), zero deck, 58cc chambers and running about 10.3:1 static cr with a .039 head gasket. Quench is .039.)

blndweasel 04-22-2004 02:02 PM

perhaps I can try to explain it in yet another way...

Try this little experiment. take a big book. stand it up on one end on a table somewhere, and let it fall over to one side. You should notice a big wash of air come out from under the book as it approaches the surface of the table. It's elementary physics. As the surface of the book comes closer to the surface of the table, the air has to go somewhere, and it wooshes out wherever it possibly can.

Take a look at the combustion chamber on your cylinder heads. There's an open part next to the spark plug and around the valve seats, and if you were to scribe a circle around the circumference of where the piston bore meets up with it, you should notice that a little less than half of that area is completely flat and flush with the surface of the mating surface between the head and the block. So you understand that there is a flat part on the head, right? The flat portion of that head is like the table.

Now take a look at the face of your piston. If it's flat all across the top, the analogy is simple. The face of the piston is like the book. As the piston approaches top dead center, the air in between the piston face and the flat portion of the combustion chamber on the head is forced out... into the cavity portion of the combustion chamber on the cylinder head. Air is being forced from the flat portion to the open portion, creating movement of air (i.e. turbulence) It's the turbulent movement of air and gas that encourages an even burn throughout the combustion space. In fact, the piston face gets so close to the face of the cylinder head at its flattest part, that (correct me if I'm wrong) very little combustion occurs in this .035" space between the surfaces.

As soon as you start to add volume between the flat surface of the cylinder head and the piston, namely as you increase that .035" gap to, say, .060 or .100 by adding a full face piston dish, or setting the piston lower than the surface of the deck, you introduce an area where essentially "stagnant" air can reside.

If you take a bowl and perform the same experiment as with the book, and let the open face of the bowl fall over onto the table, you're going to notice much less air evacuating from under the bowl, because it's getting trapped inside. Understand why the bowl is producing less air movement / air turbulence / quench than the book? Same difference between full dish pistons or pistons set low in the block, and zeroed in flat tops that have a proper quench height.

D-shaped dishes work differently, because the piston still maintains a flat face at the area where quench occurs, at the flat portion of the cylinder head within the diameter of the piston bore. So essentially if you only look at the quench area of the piston, D-shaped dishes look identical to flat top pistons in this area. That's why both flat tops and D-dishes produce quench.

Does this help at all, or am I just rambling?

the blonde weasel
san diego, CA

camaroman7d 04-22-2004 02:12 PM

Keith I think the better quench make more power because of better combustion. The fuel mixture is nice and swirled and the fact that igniting at the right time and in the right place. I don't think in a race fuel application the difference would be as large as in a pump gas application.
I can't really follow your example because you are not taking the cylinder head into account (or at least I don't see how you are). The combustion chamber is in the head, not in the cylinder (if you will). So the design (size and shape) of the cylinder head is just as important as the quench and piston top.

In your example I don't see how you can still have 10:1 in all three examples, I understand it was just in theory but, I think too many factors were left out. The top example would have the best quench though. The reason is (assuming a closed chamber head) the charge would be forced to the "open" part of the chamber because that is the only place for it to go. In your example the heads would all have to be flat or open chamber for your conclusion to be true.

Think of it as stepping on the edge of a water balloon. All the water will rush to the other edge, now if you step on the center the water goes in all directions. Same basic principle (which I am sure you understand the theory behind quench/squish). Since you know what it is, you have to understand what it does. The effect of the fuel rushing across the top of the piston and combustion chamber, cools (quenches). If you have no squish then you have no quench. You can have the same compression, but without the squish you have no quench. Yes, the engine will still run but, you have lost the quench effect. This is how the different "squish" (quench) heights make a difference.

With a large quench you are basically stepping on the middle of the balloon the mixture goes in all directions (out of control). This is why .035-.045 is the goal we shoot for. Much more than .045 and you no longer have squish. Running tighter than .035 would be even better but, you have to give room for rod stretch etc... Some guys like to build race engines so the piston just kisses the head when the piston rock at TDC, they say is make a little more power. Of course this is not something you would want to do in an engine you want to live for a while.

Once you get much over .045 it no longer makes a difference. So if you have a choice of .090 or .140 it really doesn't matter neither will have squish/quench.

I don't claim to know it all, I can only explain what I have learned/believe.

I hope that helps some?


Edit: blndweasel, we were typing at the same time. Excellent exapmle :thumbup: Very good way to look at it.

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