What does it mean to " improve quench with different head gasket thickness"?

simply put- Thinner gasket more squeeze, thicker gasket less squeeze. By added a thicker gasket the combustion area is increased and vise versa.

Then does increased "quench" mean the same thing as increased "compression ratio"?

THINNER GASKET MEANS MORE COMP.

Yes- any whay you can Decrease the combustion area can bring up your combustion ratio (I.E. thinner gasket , decking the block, milling the heads...)

There's a flat area on the piston that matches a flat area on the head. That's your quench area. You need about .040 between the two so they do not hit at high rpm. You also need to get them as close as possible so that the air fuel mixture is sqeezed out causing turbulence in the rest of the chamber causing less detonation.

In most cases, from the factory, the piston is down in the block .010-.020. A Felpro blue teflon gasket is .042 thick. Added together you are above what most builders consider a good quench. Cometic gaskets are as thin as .020 (I don't have the catalog in front of me) and come in other sizes so you can choose your quench thickness. Or you can have the block zero decked, mill the deck of the block even with the pistons and use the Felpro gasket.

Experienced engine builders say that proper quench makes such a large difference that engines will keep running better right up to the point that pistons hit cylinder heads. Conversly, adding thicker head gaskets to lower compression will usually make detonation worse because you're making the quench distance to large.

PatriYacht,

Excellent explaination. Thanks!

Any idea what the stock quench distance is for a 496HO and how much can it be reduced without breaking something at high rpms?

I think it's about the same as other big blocks. You have to take the heads off and measure.

PatriYacht did a good job in his explanation. Also in Dennis Moores book on the BB Chevy Marine Engine he goes into quench in detail. As PY stated it is very important to making good horsepower. It is one of the examples of good engine building and good machine work makes power. Again, you have to choose your components to work together. Deck your block to work the the combination of pistons and gaskets.

What do you do if you have to run a dish piston?