You will see that this Summit Racing cam offers duration in two different tappet lift specs of 0.006-inch and also at 0.050-inch. That orientation is designed into all four-stroke engines. That means the crankshaft has to spin twice for the cam to turn one Keep in mind that the camshaft turns at halfĮngine speed because the cam drive gear is twice as large as the crankshaft All camshaft specs are expressed in degrees of crankshaft rotation.Ĭrankshaft degrees are used because that’s the easiest way to measure these The number of degrees of crankshaft rotation that each valve that spends off The next spec we’ll deal with is duration. Reading a Cam Card: Duration & Advertised Duration That’s the lift the engine will see at the valves. Note that the card calls this ratio out at 1.7:1. This is accomplished through the rocker ratio. On that same line in this cam card, the lift at the valve has increased from 0.321-inch at the lobe, to 0.545-inch at the valve. Reading a Cam Card: Rocker Arm Ratio (Rocker Ratio) You will often see where the lift will be different for the intake versus exhaust. Note that in this particular case, both the intake and exhaust lobes generate a lobe lift of exactly the same amount: 0.321-inch.
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On our sample Summit Racing Pro LS cam card above, the first line is called Lift and is detailed as either intake or exhaust lobe lift at the camshaft. This simple upward movement of the cam lobe can be measured in terms of inches (or millimeters) of lobe lift. Summit Racing’s got a handy camshaft timing calculator that can help you determine the right cam specs for your performance needs. As the cam lobe turns and the lifter follows the rise of the eccentric, the lifter moves upward, converting this rotating motion into an upward movement connected by the pushrod to the rocker arm. This is accomplished by use of a cam follower or lifter that starts on the round portion of the eccentric, which is called the base circle. A camshaft is really a device used to convert rotating motion into linear (up and down) movement. Let’s begin by describing exactly what a camshaft does. (Image/Summit Racing) What Does a Camshaft Do?
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A cam card from a Summit Racing Pro LS series camshaft. We’ll use a timing card from one of Summit Racing’s Pro LS series camshafts as our example, but these same numbers and relationships will apply to any camshaft from any manufacturer for any four-stroke engine. The camshaft has been described as the “brain of an engine,” so let’s break down each of these specs to decipher how a cam exerts its control over an engine. That’s vitally important when it comes to choosing the right camshaft. These may seem obscure and even perhaps baffling, but in fact these cam specifications lay out a simple way of determining exactly how a particular camshaft will operate in the engine. All aftermarket and production camshafts use a series of terms and specifications to identify their operating characteristics.
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One area of confusion is certainly a camshaft timing card. What is a Camshaft Timing Card or Cam Card? But just like learning a new language, if you break each one down into smaller pieces, everything becomes easier to understand.
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The world of high performance engines can be a very confusing place, where terms and specifications seem to be full of obscure references that appear to make no sense.