Iron or light-alloy casting which fits tightly to the cylinder block and contains the combustion chambers, valves, valve seatings and spark-plugs. An efficient cylinder head
will improve performance more than would be expected by many people.
With no moving parts other than the valves
, the cylinder head is involved in all aspects of a four stroke engine's operation - induction, compression, ignition, and exhaust
. One way of improving a cylinder head
is called porting and polishing. This very time consuming process enlarges and maximises the finish of the intake and exhaust
ports, and allows a more efficient flow of gas. Big valves
and multi-valve heads are also beneficial as they improve swirl patterns of the fuel/air mixture in the combustion chamber and thereby increase the amount of fuel that is burnt and turned into horsepower. A head can also be shaved — that is, a few thousandths of an inch of metal are scraped from the head surface to reduce the space in the compression chamber, raising the compression ratio and power in most cases.
Cylinder heads can also be fitted with roller rockers. Roller rockers take the place of the standard cast iron rockers which transfer the movements of the camshaft, through the pushrods, to the valves
(in an overhead valve engine). When the camshaft lobe lifts a pushrod, the rocker is forced to rock — hence the name — and open a valve. Cast iron rockers, while cheap to manufacture, have inherent problems. They are prone to breakage during extended periods of high engine speeds. They also scrape across the valve tip during the opening process, a method which causes a great deal of friction, heat, and can shorten the lifespan of valves
and valve guides. Roller rockers, on the other hand, actually have a roller bearing in their tip, which rolls across the valve tip, drastically reducing friction.