If you look at the combustion chamber of any small block head, the chamber only accounts for a little more than half of the bore. The remaining portion of the chamber is flat. With a flat top piston, the distance between the top of the piston and the flat portion of the head is defined as the quench area. This distance varies depending on the deck height of the piston (positive above the deck or negative below the deck) and the head gasket thickness. A performance piston-to-head clearance is usually between .040 and .045 inch. This quench are is too small to support combustion, serving instead a more useful function. As the piston approaches TDC, this tight area tends to squirt or squish the air/fuel mixture in this area into the open combustion space. This violent action produces turbulence that acts like a very efficient blender. This blending action helps homogenize the air-fuel mixture and supports more efficient combustion.
Dyno testing has proven that improving the piston to head quench area will improve power even when the compression ratio remains the same. It is even possible to optimize the quench and decrease the compression without suffering detonation problems! This is because improving the homogenization of the mixture in the chamber reduces the tendency for lean areas in the chamber to promote detonation. The ideal combination is to run a .005-inch negative deck height along with a composition head gasket with a compressed thickness of .038 to produce a .043-inch piston to head clearance for a steel rod engine. Optimizing the quench will often reduce the required ignition lead, further reducing the possibility of detonation while also reducing unburned hydrocarbon exhaust emissions. Just for the record, if you are going through this same drill with a domed piston make sure you still retain a safe piston to dome clearance of at least .040 inch.
Domed pistons are the classic small block piston design. Chevrolet built thousands upon thousands of production engines with domed pistons back in the lost days of 103 octane pump gas. Domed pistons are usually reserved for race engines these days for a number for good reasons. Since it's possible to generate well over 11:1 compression with flat top pistons, it makes little sense to stuff a dome into a large combustion chamber to accomplish the same goal. Conventional wisdom also claims that domes merely get in the way of an efficient combustion process and lead to reduced power even with higher compression. This is especially evident with pistons that require a "fire slot" in the dome to give the spark plug sufficient area to create a flame front. Domed pistons also increase weight at the very top of the piston, which increases the g-forces on the piston. The best way to reduce weight is to opt for"hollow dome" pistons where the dome area has been relieved to reduce weight.
These parameters establish the basic configuration of not only forged pistons but cast and hypereutectics as well.
