3/17/2009 - Charts and Guides
Understanding how a camshaft works—and all of the terms associated with camshaft operation—can be daunting, even to people who have built a few engines in their day. To help you get a solid foundation in camshaft-ese, we put together a short Q&A listing of camshaft terms and what they mean. Understanding the basics of camshaft operation will help you choose the right cam for your engine and intended use.
If you want more in-depth information about camshafts and how to choose the right one, read the Thump Sticks story in the Street & Strip Tech section.
Q: What is lift?
A: Lift refers to how far an intake or exhaust valve is lifted off its seat at the cam lobe's highest point. Total valve lift is controlled by camshaft lobe lift and rocker arm ratio. The rocker arm ratio multiplies the cam's lobe lift. For example, if the camshaft's lobe lift is 0.400 inch, adding a 1.5 ratio rocker arm increases total valve lift to 0.600 inch.
Q: What does lift do?
A: The intake and exhaust valves open to let air and fuel into the cylinders and allow the exhaust gasses to escape. In general, opening the valves quicker and further will improve the engine’s ability take in the air/fuel mixture and increases output. Increasing valve lift can yield more power without much change to an engine’s overall power curve. However, an increase in valve lift is almost always accompanied by an increase in duration (the amount of time the valve is open).
Q: What is duration?
A: Duration is the amount of time a valve remains open. It is measured by the angle, in degrees of crankshaft rotation, that the valve stays off its seat during the lift cycle of the cam lobe. Because airflow past the valve is negligible until the valve is approximately 0.050 inch off the seat, the most accurate way to measure duration is from the point where the valve is 0.050 inch open (off the seat) until it is 0.050 inch away from closing.
Q: How does duration affect an engine?
A: By keeping the valves open longer, a cam with more duration can increase high-rpm power. Increasing the duration also increases the rpm range where the engine produces power. Increasing duration without a change in lobe separation angle will result in increased valve overlap.
Q: What is lobe separation?
A: Lobe separation is the angle in camshaft degrees between the maximum lift points of the intake and exhaust valves. It is affected by the placement of the intake and exhaust lobes on the camshaft.
Q: What does lobe separation do?
A: Lobe separation affects valve overlap, which in turn affects an engine’s power curve, idle quality, idle vacuum, and rpm range. Duration, lobe separation, and valve overlap all affect each other. For example, if you want to increase valve overlap without changing duration, you can decrease the lobe separation angle. Increasing lobe separation angle without changing valve overlap requires increasing the duration.
Q: What is valve overlap?
A: Valve overlap is the angle, in degrees of crankshaft rotation, that the intake and exhaust valves are both open. This occurs at the end of the exhaust stroke and the beginning of the intake stroke. Increasing lift, duration, and/or decreasing lobe separation increases valve overlap.
Q: What does valve overlap do?
A: At high engine speeds, valve overlap allows the gasses exiting the cylinder through the exhaust valve to help pull a fresh air/fuel mixture into the cylinder through the intake valve. Increasing valve overlap helps high-rpm power, but reduces low-rpm power and idle quality.
Q: What is intake centerline?
A: The intake centerline is the point of highest lift on a camshaft’s intake lobe. It is expressed in crankshaft degrees After Top Dead Center (ATDC). In an assembled engine, the camshaft’s intake centerline is measured by the number of crank degrees between TDC and the point of maximum valve lift.
Q: Do I need to degree my camshaft?
A: Degreeing is the process of timing the valves’ opening and closing events with crankshaft rotation to achieve optimal performance. The intake valve closing event is the most important because it controls the amount of air and fuel allowed in the cylinder. Advancing the cam (by crankshaft degrees) closes the intake valve sooner, which helps low-rpm torque at the expense of high-rpm horsepower. Retarding the cam does the opposite—more high-rpm horsepower at the expense of low-rpm torque.