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This covers the basic bottom end assembly and how it works. In our next installment we will get into the operation and function of the cylinder head and some of the other important engine subsystems.

In this weeks installment we will continue to discuss some of the engines main internal parts and how they work. It might seem boring now, but it is essential to understand the workings of these parts so when we begin to modify them, we have a better idea of what there function is.

In this installment we cover the all-important top end of the engine. The cylinder head and all of its parts are important because it controls what goes into and out of the engine. The head, like your head is the brain of the engine. Lets dig right in for this week’s lesson.

The Cylinder Head

The Cylinder head is, on most Imports, an aluminum casting that caps off the top of the block and contains the spark plugs, valves and the valve train. The head must contain the explosive force of the igniting fuel air mixture so the explosion of said mixture could only drive the piston down the bore instead of blowing out of the bores top. The cylinder head has the combustion chambers cast into it. This is the area where the valves and sparkplug are located. When looking at the underside of the cylinder head, the side that bolts to the block, the combustion chambers are the depressions that line up with the bores. It is in these chambers where, when the piston is at the top of its stroke; the fuel air mixture is ignited, kicking off the power stroke. The cylinder head also has cooling jackets filled with circulated water to help keep the combustion chambers from getting too hot.

On a modern engine the head also contains the intake and exhaust valves. The intake and exhaust valves are spring loaded poppet type valves. The springs hold the valves shut, but allow them to be opened with a push. The intake valves open to admit the explosive mixture of fuel and air into the combustion chamber. They then close to allow the engine to build up compression as the piston, driven by the crank comes up to TDC or top dead center, this is what engine builders call the event when the piston is at the top of the stroke. When the spark plug ignites the fuel air mixture and the piston is driven down, and the exhaust valves open near the bottom of the pistons downward travel, allowing the burnt waste gasses to escape to prepare the combustion chambers for the next charge of fresh fuel and air.

The valves are opened and closed by the camshaft or camshafts, which are basically rods with off center bumps or lobes on it that spins in the cylinder head at 1/2 of the crankshafts speed. The lobes of the camshaft push the valves open and closed so that air and fuel can be admitted with burnt exhaust being expelled. Sometimes the cam can work directly on the valve. Many motorcycles and racing type engines are like this. Typically the camshaft works the valves through a rocker arm, which is like a miniature teeter totter. One end of the rocker arm rubs on the rotating camshaft with the other end pushing the valves open and closed.

The engines found in imports are typically called overhead cam engines. This means that the camshaft is contained within the cylinder head on top of the valves. This is opposed to overhead valve engines like low revving, old school domestic V-8. s that have the camshaft located in the middle of the block, connecting to the valves with, lifters, long pushrods and rocker arms. Overhead cam engines are better for the typical high rpm, small displacement Import motor, because they have simpler, lighter, more direct acting valve trains. These valvetrains work better at high rpm because their lower inertial mass allows them to follow the camshaft. s lobes with more accuracy. If the engine has only one camshaft that controls both the intake and exhaust valves it is called a SOHC or single overhead cam motor. The Honda D16 that is found in the ever popular Civic and the Nissan VG30 are examples of SOHC motors.

Many Import motors have dual overhead cams, which means that there are separate cams for the intake and the exhaust valves. The advantage with this is that the cam can be placed very close to the valve allowing the cam. s lobes to either work directly on the valves or through a very small rocker arm. This reduces the inertial mass of the valve train to a minimum, which helps high rpm operation even more. Just about all of the higher performance Import motors use dual overhead cam valvetrains also known as the DOHC configuration. The Nissan SR20DE and the Acura B18C are prime examples of DOHC motors.

The Intake System

The intake system consists of the manifold, which basically is a series of pipes that connect the throttle body, which is the valve the controls the amount of air that can reach the insides of the motor, to the intake ports of the head. The throttle controls the amount of air the engine can suck in, thus controlling its speed and power. When the throttle is shut, the air is very limited so the engine must idle. When it is wide open, the engine takes in all the air it can so it can produce its maximum level of power. The manifold usually contains the fuel injectors, which are electro-mechanical valves controlled by the ECU, or engine control unit, a small computer which is the engines brain. The ECU controls the amount of fuel being injected into the engine by modulating the open and closed time of the injectors. When the throttle is fully opened, allowing the maximum amount of air possible into the engine, the ECU will command the injectors to stay open longer so they can inject a proportionally greater amount of fuel to create a bigger volume of explosive

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