Call me stupid, what exactly is it. Getting ready to start my 454 build soon. Gotta start learning.
Dave

The total distance from the flat part of the piston to the flat part of the cylinder head

Quench and sometimes called squish is the real thin clearance distance that is between the flat part of the head and the piston tops flat at TDC.

It keeps the piston from banging the head when they get close.

Tight quench like is mentioned blows the fuel/air mixture out of this area as the piston comes up on the compression stroke. This causes EXTREME fuel/air mixing which promotes good ignition and burning and thus helps to stop any preignition tendancies of the mixture that can cause knock.

It is well documented that loosening up the quench can cause an engine's fuel octane requirements to have to be increased b/c of less eff. combustion characteristics due to the lessening of this mixing action.

That is why it is not a good idea to try to lower an engines CR. by using thicker headgaskets!! But rather to still hold quench tight and either change combustion chamber size to one that is larger or lower the pistons dome height, use flat-tops or go to dished ones.

Changing a headgasket from say a .022" thick shim with a CR. of 10.5 to 1 down to say a .050" thick composite headgasket and 9.25 to 1 CR. MAY actually cause the motor to require a higher octane fuel b/c of the poor mixing action due to the wider quench distance.

This stuff is the biggest reason some/most(?) guy's have found for using flat-top pistons, or pistons with minimum dome height and small chambered, (SB's), or closed chambered heads, (BB's), b/c of the slightly better flame front creation even though larger, open chambered heads do breath better.

AND this is an on-going discussion thing b/c there are guy's that do both AND both work for them!!

AND I'm not gonna go there!!!!!

Hope this is of help.

pdq67

I think "Squish" is a more definitive term, because it "squishes" the air/fuel mixture out into the chamber.

When I think of "Quench", (outside of engines), I think of using water to cool hot iron. Or using water to quench one's thirst. So it usually involves some sort of "cooling off".

Originally posted by 71metbu:
Call me stupid...

Dave OK, stupid. ;) (You told me to.)

Chad, I think the term "quench" comes from the cooling of the piston as it approches the head on the exhaust stroke. My understanding is a lot of heat passes to the head at this time. Quenching the piston.

Originally posted by Wolfplace:
The total distance from the flat part of the piston to the flat part of the cylinder head ===
Dave,

Just thought I would post it again to make life simple in terms we use regarding engines :D

Squish is the sound you hear when you raise 6 wolves & walk through the yard without caution if you don't clean it at least twice a week
:(

BTW, Be glad to call you "stupid" if that's what blows your skirt up smile.gif
but,,,Not asking would be what I consider stupid graemlins/thumbsup.gif

So can anyone explain why engine rebuilders use .039 and .041 composite gaskets rather than .015-.022 steel shims? is it just because Fel-Pro only provides the .039-.041 in the gasket kits? And why would Fel-Pro only provide the composites if shims might actually be better all around?

Ad to this rebuilder pistons are usually lower "in the hole" than stock pistons were. So we end up with horrible quench, very low compression ratio, and it might still need premium fuel to run?

and, I'd rather you guys call me "young" instead of stupid, even though they usually mean the same, LOL.

Here's a good question then...I'm putting together a 454 that wasn't decked,but the heads have been shaved once by me, and maybe once by the previous owner(not sure) How do I find out what my quench/squish distance will be?
I know my gasket will be .39 compressed but how do I find the rest of the numbers to figure out the calculation?

Originally posted by sapper92310:
Here's a good question then...I'm putting together a 454 that wasn't decked,but the heads have been shaved once by me, and maybe once by the previous owner(not sure) How do I find out what my quench/squish distance will be?
I know my gasket will be .39 compressed but how do I find the rest of the numbers to figure out the calculation? Wow, you can shave your own heads. Cool!

The thing you have to know is how far down "in the hole" your pistons are at TDC. Add this to your compressed gasket thickness, and you have your quench.

The head "shaving" doesn't affect quench, only compression ratio.

I guess this is the answer to the original question then. Makes me feel stupid for giving an overly complex answer. tongue.gif

And remember, the only "stupid" question is the one NOT ASKED!! Imho...

pdq67

Quench...I tend to think that is something that really does happen to a certin degree.

With the piston @ TDC the quench / squish flat areas are high in metal surface area and low in fuel volume relitive to the rest of the combustion chamber.The tighter this quench/squish number is the less A/F volume can possibly be there.Any A/F that happens to remain in that quench/squish area would likely have some of it's temprature removed by the large flat surface's that just happen to be in one of the least active and therefore cooler areas of the combustion chamber.

This small area does help provide turbulance in the combustion process but squeezing out the A/F that was there and forceing the A/F into the larger area of the chamber to be consumed.... The later the ign timing IE at idle with say the plug fireing @ 10 deg TDC the piston is the less squish / quench area there is....If at full throttle or whenever full advance is in...say 38 deg BTDC then the piston is further down the when the spark plug dues it's thing.... Since the goal tends to be to have Peak Combustion Pressure somewhere around 20 ATDC the helps add the necessary turbulance to mix and spread the A/F and the flame front... to some varing degrees.... Either way you look at it at least with respect to Chevy combustion chamber designs this quench/squish area plays an important role in preventing secondary ign..... The more quench / squish gap you have the more A/F volume will be present and the more chances for auto ignition in this end gas region ..as well as less turbulance...
The less squish/quench gap there is ( within reason) the more turbulance and less A/F volume in that squish area then the less likely for auto ignition to take place in this end gas region of the combustion chamber.

Originally posted by Slowpoke70:
So can anyone explain why engine rebuilders use .039 and .041 composite gaskets rather than .015-.022 steel shims? is it just because Fel-Pro only provides the .039-.041 in the gasket kits? And why would Fel-Pro only provide the composites if shims might actually be better all around?

Ad to this rebuilder pistons are usually lower "in the hole" than stock pistons were. So we end up with horrible quench, very low compression ratio, and it might still need premium fuel to run?

and, I'd rather you guys call me "young" instead of stupid, even though they usually mean the same, LOL. Enrique
Ok, "youngster",,,,,

The main reasons rebuilders use the composite gasket is most of them did not have the equipment to get a sufficiently smooth surface to seal a shim gasket.
This is no longer the case in a quality shop & using a shim is a very viable option. I have used 1094 felpros in 13.0+ circle track engines with aluminum heads with no problems what so ever.
If you use the stock "destroked" rebuilder pistons shame on you smile.gif
There are just to many good options out there now to accept these pistons in any kind of performance engine.
In my opinion if you are doing a performance engine you should insist on the quench being done properly. It may vary a little from cylinder to cylinder because of small differences in stroke, rod length & compression height (the distance from the centerline of the pin to the flat of the piston)but you can get it pretty good. I normally use 9-9.005 deck height on a small block which comes out to zero-.005 down with the standard FelPro gasket.
There are a ton of combos now that will give you good quench. Use one ;)

Of course, the other option is to lower the piston enough so quench is no longer an issue,, it becomes part of the chamber.

This is becoming way to complicated for a simple question graemlins/clonk.gif

Hi Al graemlins/waving.gif

O.K., so another way to think of it would be to say it's the clearence between the top of the piston to the bottom of the head, at tdc. That's pretty much what I was thinking. So it will be really important to get my heads spec'ed out before I start thinking about pistons. guess it's time to load up all of my engine parts, and head for the machine shop to get them hot tanked, inspected and measured.

Wolfplace, got a 120 pound Akita/white german shepard "puppy", that probably mines the back yard as bad as 3 of your wolves. I don't go out there as the wife wanted another kid, so I got her a dog. Told her I was through with diapers and land mines! graemlins/thumbsup.gif

C.J., I'll be waiting ;)

Thanks to all for the information, Dave

Hey Mike graemlins/beers.gif

He, He!! Daipers AND land mines!!!

AND the wife for no apparent reason bought a brand new fold-up stroller for my daughter and new Son-in -Law off QVC..

I chidded my baby last week going to work about when I was gonna be a "papa" and she said, not for a while, but we all know how that works, don't we DAD's???

pdq67

The head "shaving" doesn't affect quench, only compression ratio Why would having your heads shaved not affect quench? It lowers your head to your piston deck does it not?

No, flat is flat, shaving the heads only reduces the combustion chamber volume. Reguardless how much is shaved off the surface stays flat.

The head "shaving" doesn't affect quench, only compression ratio It will affect Piston to Valve clearance though...

I guess what I should of asked is "Whats the difference between taking 10 thou off the head or taking it off the deck?" Both cause the flat part on the piston to sit closer to the flat part of the head, right?

I'm not trying to be argumentative...just looking for clarification smile.gif

Originally posted by sapper92310:
I guess what I should of asked is "Whats the difference between taking 10 thou off the head or taking it off the deck?" Both cause the flat part on the piston to sit closer to the flat part of the head, right?

I'm not trying to be argumentative...just looking for clarification smile.gif Because as stated above "flat is flat" Think of it this way, you can shave the heads until they disappear but you never got the head closer to the piston. The valves maybe but not the surface that you shaved. Now shave the block. Whatever amount you take off the block, you move the head closer to the piston which decreases the amount of quench area until you got to zero at which point the piston would hit the head.

Jeff