can someone please explain this to me? i know basically what it is but i dont fully understand it. How do i determine proper quench for my engine? how do i adjust it? also how do i measure it? i have searched some but i cant really find much on it, maybe i just suck at searching. If someone has a link or something that would be greatly appreciated. I am going to be assembling my engine soon and i want to get all the detils right!

The quench or squish area (of the head) is the flat part of the cylinder head, inside the cylinder bore, but not part of the combustion chamber. When the block is decked to the rotating assembly, so a flat top piston, when at TDC, lines up flush with the deck, it has a corresponding flat area. When a .039-.042" compressed head gasket is used (ideal quench distance), it creates a very small space between the top of the piston and the bottom of the cylinder heads flat area. When the piston comes up the bore, this creates a sandwhich effect, which squishes the air, causing what is similar to a shockwave effect, of air, throughout the cylinder. This helps cool the combustion chamber.

To get your quench dialed in, you'll need to know the total height of your rotating assembly: crankshaft radius (stroke divided by 2) + connecting rod length + piston compression height = height of rotating assembly. An uncut SBC block is generally 9.025" crank centerline to top of deck pad. So if your rotating assembly comes out to be 9.010", you'd want to cut .015" off the deck. Also obtainable, by having the piston slightly down in the hole at TDC, and using a thinner head gasket. For example, if your piston was .020" down in the hole at TDC, you could use a .020" head gasket to obtain optimal quench.

wow! you totally cleared all of this up for me!!! man i thought it was rocket science heh. thanks alot for the info!! how much difference does having a proper quench make in your typical street engine? once again thanks for a very informative post!

I like having quench in my motors. It helps the combustion to move faster. Faster flame travel. In a nutshell, with proper quench, you can move the flame from one side of the piston to the other, faster. This means you won't need as much ignition timing. If you have a big quench, say .050" or greater, flame travel is slower. Thus, you need more timing to get the combustion to its fullest. Anything you can do to increase the speed at which the combustion process takes place, the later (or closer to TDC) you can light the fuse - less ignition advance.

One of the downsides of a lot of advance, is the lovely ping you get from too much ignition timing (the ignition timing that's required to get a complete combustion - with an incorrect quench). And I'm all for anything that reduces the chance of marbles in a coffee can.

There is no downside to having maximum air movement in the cylinder, when at TDC.

Above is correct, I like to run my motors with anywhere from .035-.040 of quench area. For instance, I just put heads on the motor in my car. The shortblock is a run of the mill gm crate 350, knowing that a stock 350's piston sits .025 in the hole (always measure this anyway) I ran a felpro .015 shim head gasket. This puts my total quench area at .040 witch is ideal for a good clean and fast combustion process. With lightly milled 64cc heads this put me at 10.3to1 compression. Not to much but not too little just enough to have to replace back tires every 3 months

On the same note though....I have seen motors ran with way over .040 of quench (like in the .060ish range) and not have any problems.

allright it makes alot of sense now, thanks alot for the input guys i will definetly be paying attention to this. it sounds like the shim head gasket is the way to go in my case as well, as the 350 is stock and im putting heads on it with 64cc chambers. of course i am still going to measure everything out though. again thanks for the informative posts!

this enlightened me too, thanks.

After a ton of machine work to my 400, I don't feel like setting it back up to deck it again. We decked it to get it perfectly straight with the crank centerline, and thought after measuring the pistons that they would be right around 0 in the hole, but after measuring every hole is .010 down.

So with that said, my ideas of using a felpro 1014 gasket (4.200 bore, .039") is kinda out the door.

Anyone have any idea of a gasket around .028" or so, with a 4.200" bore ?

I'm somewhat considering trying a pair of (local) bowtie 14011034 heads, 58cc. If I can get them to flow 260/185 they might be a good head for the 412.

Static compression be around 10.9, with a DCR at 8.66 with my cam. I think i'd want to get the quench under .040" with those numbers, especially with Iron heads..


-- Joe

I dont know if its available, but apparenly the composite MrG gasket was .028, though I dont know if they have one with a 4.2" hole.
Might also consider a MLS like Cometic. Little pricey, but seal good on fresh parts... and they can make you up whatever thickness you want.

Talk about interesting timing for this subject. I have just been looking into this with the existing motor for my car as I'm going to replace my 041 heads with some AFR 180's. One concern I have is dome top pistons and the heart shape chamber, the other is the quench/squish distance.

I measured my piston crown to block deck height at 0.065" and my head gasket at 0.050"
making my quench/squish 0.115"

Does this seem a bit excessive? I think that there is no real way to reduce this without total rebuilding and blueprinting the motor.

Any thoughts from anyone?

If your piston is .065" in the hole something is off.

-- Joe

I'd dearly love to talk to the bloke who built the motor but he has sinced passed away. I bought the motor about from him about 15-16 years ago and I have only put about 10,000 K's on it. You can still see the cross hatch marks from honing apart from along the thrust lines on the top and bottom of the bore.

I was planning on putting my ACCEL HI-FLO TPI setup onto this motor.

Think I may just find a good engine builder and get it re-built our turn it into a 383.