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Topic Review (Newest First)
04-23-2004 09:15 AM
camaroman7d Ben, I think you will be happy you did. Glad we could help some.

04-23-2004 08:20 AM
Originally posted by lust4speed
So if the mixture is directed to one side of the piston, does the resulting combustion push harder on that side of the piston? Probably not, but scary thought.
The balance of pressure across the piston top shouldn't be upset enough to cause uneven wear. The momentary increase in pressure on the "chamber" side is offset by the momentary increase in pressure between the piston and head. Remember the piston is moving 2000-4000 feet per second. The sound barrier is just under 1000 feet per second I think. The pressure spike created when it comes within 0.035"-0.050" of the head could be roughly equal to an explosion such as combustion.

04-23-2004 06:07 AM
brainsboy Royce and others,

I understand it now, I found a few more sites and did more reading.... Thanks for all the answers and posts. Sounds like its worth the extra effort to just go ahead and build for a tight quench.


04-23-2004 01:25 AM
camaroman7d Lust4speed, I often thought the same thing. I suppose if the flat part of the piston and head was larger (pushing the charge WAY over to one side), I suppose that could be an issue.

04-22-2004 11:01 PM
lust4speed So if the mixture is directed to one side of the piston, does the resulting combustion push harder on that side of the piston? Probably not, but scary thought.
04-22-2004 07:58 PM

I read that site that someone posted about quench, and with what you said i understand more about quench. I guess this is what i learned today!!!! I am under the idea that the roll of quench is more important to detonation, but as a added benifit you pick up some horse power......

that was a good example you gave. It drove home what the web site stated about quench. What you said about the "D" shaped pistons is what i was trying to say but i guess it didn't come out right.... L.O.L.

It's nice to have a place to go and ask tough questions and have a disscussion with out people slamming you...... I love this place!!!!

What's in store for the class tomorrow?????

04-22-2004 02:12 PM
camaroman7d 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 Very good way to look at it.
04-22-2004 02:02 PM
blndweasel 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
04-22-2004 12:49 PM
mstngjoe 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.)
04-22-2004 11:47 AM
brainsboy 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.
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.

04-22-2004 11:20 AM
camaroman7d Ben,
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.

04-22-2004 10:41 AM
firestone Here is another good article on the topic of quench.

04-22-2004 10:40 AM
brainsboy 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.
04-22-2004 10:16 AM
mstngjoe 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.
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