Camshafts

Camshafts are used to operate poppet valves. It consists of a cylindrical rod running the length of the cylinder bank with a number of oblong lobes protruding from it, one for each valve. The cam lobes force the valves open by pressing on the valve, or on some intermediate mechanism, as they rotate. Camshafts can be made out of several types of material. These include:

Chilled iron castings: Commonly used in high volume production, chilled iron camshafts have good wear resistance since the chilling process hardens them. Other elements are added to the iron before casting to make the material more suitable for its application.

Billet Steel: When a high quality camshaft or low volume production is required, engine builders and camshaft manufacturers choose steel billet. This is a much more time consuming process, and is generally more expensive than other methods. However, the finished product is far superior. Camshafts are typically produced by forging steel, casting or CNC machining and, more recently, using hydroforming technology. Camshafts can be solid or hollow to reduce weight, such as camshaft used in the BMW N52 motor.




To understand how camshafts work, you have to know about four things- timing, duration, lift and position. In brief, here are the explanations:

Timing

The relationship between the rotation of the camshaft and the rotation of the crankshaft is of critical importance. Since the valves control the flow of the air/fuel mixture intake and exhaust gases, they must be opened and closed at the appropriate time during the stroke of the piston. For this reason, the camshaft is connected to the crankshaft either directly, via a gear mechanism, or indirectly via a timing belt or timing chain. Direct drive, using gears, is unusual because of the cost. Where gears are used in cheaper cars, they tend to be made from resilient fiber rather than metal, except in racing engines that have a high maintenance routine. Fiber gears have a short life span and must be replaced regularly, much like a timing belt. In some designs the camshaft also drives the distributor and the oil and fuel pumps. Some vehicles may have the power steering pump driven by the camshaft. With some early fuel injection systems, cams on the camshaft would operate the fuel injectors.

The timing of the camshaft can be advanced to produce better low RPM torque, or retarded for better high RPM power. Changing cam timing moves the overall power produced by the engine down or up the RPM scale. The amount of change is very little (usually < 5 degrees), and affects valve-to-piston clearances.



Duration

Duration is the number of crankshaft degrees of engine rotation during which the valve is off the seat. In general, greater duration results in more horsepower. The RPM at which peak horsepower occurs is typically increased, as duration increases, at the expense of lower rpm efficiency (torque).

Duration specifications can often be misleading because manufacturers may select any lift point from which to advertise a camshaft's duration and, sometimes, will manipulate these numbers. The power and idle characteristics of a camshaft rated at a .006" lift point will be much different from one with the same rating at a .002" lift point.

Many performance engine builders gauge a race profile's aggressiveness by looking at the duration at .020", .050" and .200". The .020" number determines how responsive the motor will be and how much low end torque the motor will make. The .050" number is used to estimate where peak power will occur, and the .200" number gives an estimate of the power potential.

A secondary effect of increased duration can be increased overlap, which is the number of crankshaft degrees during which both intake and exhaust valves are off their seats. It is overlap which most affects idle quality, inasmuch as the "blow-through" of the intake charge immediately back out thru the exhaust valve, which occurs during overlap, reduces engine efficiency, and is greatest during low RPM operation. In general, increasing a camshaft's duration typically increases the overlap, unless the intake and exhaust lobe centers are moved apart to compensate.

Lift

The camshaft "lift" is the resultant net rise of the valve from its seat. The farther the valve rises from its seat the more airflow can be provided, which is generally more beneficial. Greater lift has some limitations. First, lift is limited by the increased proximity of the valve head to the piston crown and secondly, greater effort is required to move the valve springs to a higher state of compression. Increased lift can also be limited by lobe clearance in the cylinder head casting. Higher valve lift can have the same effect as increased duration where valve overlap is less desirable.

Higher lift allows greater airflow; although even by allowing a larger volume of air to pass through the larger opening, the brevity of the typical duration with a higher lift cam results in less airflow than with a cam with lower lift but more duration- all else being equal. On forced induction motors, this higher lift could yield better results than longer duration, particularly on the intake side. Notably though, higher lift has more potential problems than increased duration, in particular, as valve train rpm rises which can result in less efficient running or loss of torque.

Cams that have excessive valve lift, running at high rpm, can cause what is called "valve float," where the valve spring tension is insufficient to keep the valve following the cam at its apex. This could also be a result of a very steep rise of the lobe, where the valve is effectively shot off the end of the cam rather than following the cam’s profile. This is typically what happens when a motor over-revs. This is where the engine rpm exceeds the maximum design rpm. Sometimes an over-rev can cause engine failure when the valves become bent, as a result of colliding with the piston crowns.

Position

Depending on the location of the camshaft, the cam operates the valves either directly or through a linkage of pushrods and rockers. Direct operation involves a simpler mechanism and leads to fewer failures, but requires the camshaft to be positioned at the top of the cylinders. In the past, when engines were not as reliable as today, this was seen as too much trouble. But, in modern overhead cam engines, where the camshaft is on top of the cylinder, this is quite common.

Number of Camshafts

While today some engines rely on a single camshaft per cylinder bank, which is known as a single overhead camshaft (SOHC), most modern engines are driven by a two camshafts per cylinder bank
arrangement (one camshaft for the intake valves and another for the exhaust valves). Such a camshaft
arrangement is known as a double or dual overhead cam (DOHC). Thus, a V engine, which has two separate cylinder banks, may have four camshafts (commonly known as a quad cam engine).



In summation, the overhead camshaft design adds more valve train components that ultimately result in more complexity and higher manufacturing costs, but this is easily offset by many advantages over the older design- multi-valve design, higher rpm limits, and design freedom to better place valves, spark plugs (Spark-ignition engine), and intake/exhaust ports.

Maintenance

The rockers, or cam followers, sometimes incorporate a mechanism to adjust the valve lash through manual adjustment- a good example of this was the 426 Street Hemi that did not get hydraulic lifters until 1970. However, most modern auto engines have hydraulic lifters which eliminates the need to adjust the valve lash at regular intervals as the valve train wears, in particular the valves and valve seats in the combustion chamber.

Sliding friction between the surface of the cam and the cam follower which rides upon it can be considerable. In order to reduce wear at this point, the cam and follower are both surface hardened and modern lubricant motor oils contain additives specifically to reduce sliding friction. The lobes of the camshaft are usually slightly tapered and the faces of the valve lifters slightly domed, causing the lifters to rotate to distribute wear on the parts. The surfaces of the cam and follower are designed to "wear in" together, and therefore each follower should stay with its original cam lobe and never be moved to a different lobe. You can put new lifters on an old cam but never old lifters on a new cam. In some engines the followers have rollers which eliminate the sliding friction and wear but add mass to the valve train.

Camshaft bearings are similar to crankshaft main bearings, being pressure-fed with oil. However, overhead camshaft bearings do not always have replaceable bearing shells, meaning that a new cylinder head is required if the bearings suffer wear due to insufficient or dirty oil.

Performance Camshafts

There is a broad range of camshaft styles and grinds, from mild lobe patterns to highly aggressive profiles, to meet the performance demands of hot rodders. They can give a classic muscle car that lumpy, high performance idle or hold on for high-lift, high-rpm race performance. Aftermarket cams are available for street, street/strip, race, and truck applications. Options include hydraulic flat tappet, hydraulic roller, and mechanical camshafts from such “bumpstick” manufacturers as COMP Cams, Crane Cams, Isky, Trick Flow, Howards Cams and Lunati.