Modified Mustangs & FordsHow To Engine
Camshafts Explained - The Engines Personality
Understanding the Workings of Bumpsticks, Stopcocks, and Power
When you consider the workings of the internal combustion engine, it's truly remarkable that it works as well as it does considering its crude internal demeanor. We can huff cubic inches of air into cylinder bores and combustion chambers, and marvel at the display of power when the butterflies are pinned. From a distance, it all looks so easy. But making real power takes a whole lot of knowing what you're doing.
You can fit it with a larger carburetor or fatter fuel injectors. And you can top off the block with a pair of deep-breathing cylinder heads. But, if you can't get valve timing and function down to a pleasant melody that allows the fuel system and cylinder heads to sing in perfect harmony, you might as well funnel your working capital into the vitreous china fixture with the American Standard nameplate [that's a toilet for most of us--Jeff].
The camshaft is your engine's frontal lobe, so to speak. It determines your engine's personality more than any other single element. It sits in the middle of your V-8's valley between the cylinder banks in an overhead valve design, or above the cylinder heads in an overhead cam design. In either case, the camshaft's job is a busy one, and that is to properly time valve-operating events, which allow air and fuel to enter the combustion chambers, and hot exhaust gasses to leave when the extraction of power is over. And it gets to do this again and again at a wide variety of engine speeds. The greatest challenge for us as performance enthusiasts is to choose a camshaft that allows us to make the most of our engine package.
I'll Take A V-8!
For the sake of simplicity, we'll focus on the overhead-valve V-8 engine here. The camshaft has 16 lobes, which open and close 16 valves in a timed sequence with crankshaft and piston motion events. These lobes move lifters, which are positioned in the block and tied to pushrods, then rocker arms in the cylinder heads, which operate the 16 valves. Rocker arms use leverage to open the valves. As the cam lobe comes around to its high side and pushes the lifter upward, the pushrod moves upward against the rocker arm. The rocker arm pivots on a shaft or stud, which opens the valve. The valve is held close by spring pressure, which holds the valve face against the seat for cylinder sealing.
Each cylinder in a typical V-8 engine has two valves, one intake and one exhaust. And each cylinder needs two camshaft lobes to open the valves. One lobe opens the intake valve while another one close by opens the exhaust valve. As the intake cam lobe rotates against the lifter and opens the valve, the piston is beginning its journey downward in the cylinder bore. The open intake valve and descending piston work together to allow fuel and air to be drawn into the cylinder. As the piston reaches bottom dead center, the intake lobe is ramping down, which allows spring pressure to close the intake valve.
With our fuel/air charge poised for action above the piston at TDC, it is time for the piston to begin that journey back to the top of the cylinder. With both valves closed, this allows the piston to squeeze or compress the mixture. The ignition distributor, which is tied to the camshaft drive gear, fires the spark plug, which ignites the mixture. Contrary to popular belief, the mixture does not ignite with the piston at top dead center. Ignition begins before the piston reaches TDC.
The igniting mixture creates heat and pressure, which drives the piston downward in the cylinder bore, turning the crankshaft, giving us our power stroke. As the piston nears bottom dead center, the exhaust lobe is opening the exhaust valve to allow the super-heated gasses to escape ahead of the ascending piston. The camshaft has done its job. And it gets to do this 8 times with 16 valves.