In an
internal combustion engine, the combustion chamber is the space where
combustion occurs. Usually it is defined by the space left clear in the
cylinder head with both valves closed and the
piston at
top dead center.
The relationship between a cylinder's swept volume and the size of the combustion chamber is called the compression ratio. A smaller combustion chamber produces a higher compression ratio. This produces more power, but also higher tempratures, and greater danger of pre-ignition or pinging. This means that high octane fuel may be required.
Combustion chamber size may be changed in two ways. First of all, the head may be milled, reducing its height and shrinking the combustion chamber accordingly. Milling is expressed in thousandths of an inch, and .020 inches is a large mill. This is a common hot rodding trick. Second, the piston shape may be changed. A domed piston, shrinks the compression ratio, producing more power. A dished piston increases the combustion chamber, reducing compression. Auto manufacturers install all types of pistons, according to their desired application. Larger combustion chambers are favored for many applictions, because they allow more flexible fuel choices.