Engine Maintenance and Modification Bradley Artigue

FIAT 124 Spider
Engine Maintenance + Modification
6. The Cylinder Head
If you open your hood and look at the center of the engine you’ll see four spark plugs.
The aluminum thing that your spark plugs are screwed into is the cylinder head. The
cylinder head on a FIAT 124 Spider is of the semi-hemispherical, dual overhead cam
(DOHC) type. It is called semi-hemispherical because the operation occurs along a 180
degree arc. The two camshafts sit on either side of the spark plugs and rotate in time
with the engine main crankshaft to draw in, burn, and expel fuel. A cylinder head has
three major components:
Camshafts are metal poles with lobes on them. The lobes press against valves
that open and close, allowing gases into and out of the engine. Lobes are
engineered into a specific pattern, resulting in the camshaft being the
mechanical brain governing the Otto cycle.
Valves cover ports between the intake and exhaust manifolds and the
combustion chamber. Valves are held closed by a high tension double-spring
assembly. The spring is held down by retainers installed in a donut-shaped cap.
The entire assembly is capped off by a bucket. The camshaft strikes a shim on
the top of the bucket, opening the valve. The shim is adjustable to maintain
proper valve clearance.
Spark plugs descend into the combustion chamber and create a spark that
ignites the air/fuel mixture.
We start this section with the intake of fuel from the fuel system and finish with the
expulsion of spent fuel into the exhaust system. Assume your engine is just starting the
intake cycle – the camshaft has depressed the intake valve, drawing air and fuel into the
combustion chamber (figure 41.1). As the camshaft rotates the valve closes. The piston
is pushed upwards, compressing the mixture (figure 41.2). Ignition occurs (figure 41.3),
exploding the mixture and forcing the piston down. Finally, the camshaft opens the
exhaust valve and the piston is drawn up, expelling the burned mixture (figure 41.4).
59