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For all pushrod V-8 engines, the camshaft lives in the lifter valley, underneath the intake manifold and above the crankshaft. |
The lifters and pushrods transmit cam lift to the rocker arm, which increases the lift by using mechanical leverage. |
A camshaft uses eccentric lobes to create lift thats imparted to tappets, or lifters. These are connected to pushrods that actuate the rocker arm, opening the valve against spring pressure. Valvesprings are used to close the valve. |
A set of gears and a chain drive the camshaft from the crankshaft. Note that the cam spins at half crankshaft speed since the cam gear is twice the size of the crank gear. This particular timing set is a billet set from Crane. Most cam companies offer several different timing sets to suit your budget. |
There are two basic styles of camshafts. The cam in the foreground is a roller cam, while the cam in the background is a flat-tappet variety. Within each style, you can also choose from either a solid or hydraulic lifter. |
This illustration gives you a better idea of how the eccentric of the lobe moves the lifter from the base circle up through the lift curve and then back down again. Maximum lobe lift is limited by the diameter of the main journals the cam rides on. If the lobe is taller than the journals, you wont be able to install the cam. |
Hydraulic lifters use a small piston inside the lifter body that rides on a chamber of pressurized oil from the oil pump. The pushrod rides on the top of this small piston, and since the oil expands to eliminate any clearance, no maintenance is required. This same principle applies to both flat and roller tappets. |
Roller cams are becoming more popular, especially in factory engines. This photo illustrates how Chevy uses a cast retainer over two roller tappets and then a stamped-steel spider to keep the retainer in place. |
Aftermarket roller lifters use either a horizontal tie-bar or a vertical tie-bar to prevent the lifters from rotating. If a roller lifter is allowed to rotate inside the lifter bore, it will quickly destroy the cam and the lifter when the edge of the roller cuts into the cam lobe. |
The camshaft is unquestionably the most complex component in the internal combustion engine. The good news is that despite its complexity, the terminology can be easily understood if we digest it in bite-sized chunks. The camshaft's role in the engine is to control the valve timing, ensuring that the intake valves open at the proper time to feed air and fuel into the engine. The second part of this operation is to give the exhaust sufficient time to escape out of the combustion space before the whole process starts over again. It's the size, shape, and placement of all those eccentric bumps on the camshaft that make it all happen.
Brutal Basics
For the purposes of our discussion, we'll deal strictly with the small- and big-block Chevy and with pushrod engines in general, but these basics apply to any four-stroke engine from a Briggs & Stratton to a Formula 1 screamer. In pushrod V-8 engines, the camshaft is located directly above the crankshaft and is driven at one-half engine speed by a chain-and-gear assembly located behind the timing chain cover at the front of the engine.
The camshaft rides on five main journals and operates the valves by using 16 eccentric lobes accurately machined into the camshaft to time the opening and closing of the valves. These lobes move lifters that are positioned in the block in the lifter valley. Pushrods then actuate rocker arms up and down. The rocker arms use a leverage ratio to increase the lift of the cam lobe. Stock rocker ratios for the small-block Chevy are 1.5:1, while the big-block uses 1.7:1. Performance aftermarket rocker arms amplify this ratio from 0.10 to as much as 0.30 or more.
There are two basic styles of camshafts: flat- and roller-tappet. Flat-tappet cams are more popular and less expensive and use a tappet, or lifter, with what appears to be a flat follower. In reality, there is a slight convex curvature to this surface, which spins the lifter in its bore, preventing excessive wear.
Flat-tappet camshafts are generally constructed of cast iron and are induction-hardened to prevent wear. Roller camshafts use a roller follower that, as the name implies, allows the follower to roll over the face of the cam lobe. The reduction in friction is minimal, but the design of the roller tappet does allow a far more aggressive lift curve with the same amount of duration.
With both flat-tappet and roller cams, the engine builder also has the option to specify either a hydraulic or solid lifter design. Solid tappets are the simplest to understand since they represent a solid link between the camshaft follower and the rocker arm. Solid lifters require a clearance, or lash, that's measured between the rocker arm and the valve tip. A hydraulic lifter does not require this lash because it uses a small piston located inside the lifter to hydraulically compensate for heat expansion, maintaining a zero lash at the valve stem tip that does not require adjustment.
Cam Specifics
Now that we know what the cam is and where it resides in the engine, the best thing to do next is learn some basic camshaft terminology. We'll start by looking at the basic camshaft lobe and its various characteristics. A camshaft lobe is an eccentric that converts rotating motion into linear (up and down) movement. To do this, a lobe, or bump, is created from a true circle known as the base circle of the camalso known as the heel. As the lobe rotates, the lifter follows the rise of the lobe, which moves the lifter upward. The maximum amount of rise is known as lobe lift. This is multiplied by the rocker arm ratio to create valve lift. So if we had a lobe lift of 0.300 inch with a rocker ratio of 1.5:1, then valve lift would be 0.450 inch. Increase the rocker ratio to 1.6:1 and the valve lift jumps to 0.480 inch.
The maximum lift point on the cam is called the nose, while the inclined areas leading up to and away from the nose are called the ramps. For solid lifter camshafts, a small clearance ramp is included on the opening side ramp to gently remove the lash before the cam begins to open the valve. This prevents valvetrain abuse.
At some point on the opening ramp, the lobe begins to create lift. Depending upon the company's definition of "advertised duration," some point on the lift curve of the opening ramp is used to determine where cam lobe duration is measured. For hydraulic-tappet camshafts, some companies use 0.006 inch of tappet lift, while others use 0.010 inch on the opening and closing points of the lobe as the start and stop points for measuring advertised duration. Camshaft duration numbers are always expressed in crankshaft degrees.
Since there are so many different measuring points for advertised duration, a common measuring point became necessary so cams from different companies could be accurately compared. The industry established 0.050 inch of tappet lift as that standard measuring point. Keep in mind that this shortens the duration figure considerably from the advertised numbers. For example, an Isky 270 Megacam offers 270 degrees of advertised duration, but the 0.050-inch tappet-lift duration figures measure 221 degrees.
At this point, it's worth looking into the effect of duration on engine power. Stock camshafts offer relatively short duration and low lift numbers since the factory is after a crisp, smooth idle and excellent part-throttle operation. Most OEM cams are in the neighborhood of 190 degrees of duration at 0.050-inch tappet lift with valve lift around 0.400 inch. If we increase duration, the intake valve is now open for a longer period of time during the induction cycle. This added duration tends to affect engine power by decreasing idle vacuum and shifting the power curve to a higher rpm. This reduces low-speed throttle response and power while increasing power at the higher engine speeds. Too much duration, especially in stock-type engines, will kill power everywhere, and you will end up with an engine that is extremely lazy.
In the old days of camshaft design, most cams were designed with exactly the same duration and lift on both the intake and exhaust lobes. But decades of engine research have determined that many engines (depending upon cylinder head airflow considerations) prefer more duration on the exhaust lobe than on the intake lobe. These are called dual-pattern cams, while the term single-pattern refers to cams with the same lift and duration on both the intake and exhaust.
Now that we have the lift and duration numbers covered, let's move on to a few other more complex areas. Most cam lobe illustrations present the lobe as being symmetrical, which means if you folded the lobe down the middle and laid the opening and closing ramps on top of one another, they would be the same. But today, most cam designs are actually asymmetrical, meaning that the two ramps are not the same. This is especially true with intake lobes that are designed to slow the intake valve as it nears the seat to prevent it from slamming closed. This violent closing rate is usually what causes valve float.
Each cam lobe also has a centerline. Many camshaft companies use the intake lobe centerline of cylinder No. 1 as a way to determine how the camshaft is phased with the engine. This intake centerline is expressed in terms of the number of crankshaft degrees after top dead center (ATDC). For example, a typical street cam will have an intake centerline between 106 and 112 degrees ATDC. Engine builders have found that advancing the centerline (i.e. 106 degrees ATDC) helps low-speed power while retarding the centerline (i.e. 112 degrees ATDC) hurts low-speed power and improves top-end power.
Of course it's also correct that the exhaust lobe has a centerline that positions it in the four-stroke cycle. Therefore, there is a relationship between the position of the intake and exhaust lobes that is usually described as the lobe separation or lobe displacement angle. This is the angle (in cam degrees) between the intake and exhaust lobe centerlines. This separation angle is used to indicate the relative closeness of these two lobes.
Looking at the valve timing graph that resembles two camel humps, you can easily see the distance (expressed in degrees) between the two lobe centerlines. This angle is established when the camshaft is machined and cannot be changed unless you grind a new cam. Looking at the middle of this graph, you can see that there is a short period of time, when the exhaust lobe is almost closed and the intake valve is just opening, that both valves are open. This is called valve overlap. That small triangle makes it easier to see the amount of overlap present in this particular camshaft.
Generally, it is this amount of overlap combined with the amount of intake duration that can make a camshaft "lumpy," giving it that distinctive chop, or rough idle. If we lengthen the duration of the intake lobe by opening the intake sooner, the amount of valve overlap increases. Or, it's possible to increase overlap by merely shifting either the intake or exhaust lobe centerlines closer together. Conversely, we could also decrease overlap by moving either one or both centerlines apart. As you can begin to see, there are a ton of variables that can be tried when it comes to experimenting with cam timing.
This has been a very basic introduction to cam timing and how all the different points of camshaft design interrelate to create this most complex engine component. Now that you know a little more about camshafts, you can take the next step and check out our camshaft selection story for big- and small-blocks. This story will help you get closer to choosing the proper camshaft for your next Bow Tie street-power project.