Airflow in and out is what building horsepower is all about. Every month we attempt to deliver a killer airflow engine combination and uncover the mystery of a particular performance part. This month we take a hard look at camshaft design.
Rather than building a specific engine combination and handing the answers out, we'll go over camshaft principles and offer the information necessary to choose a cam for a winning combination. While diving right into the opening and closing points in relation to lobe separation angles and intake centerlines would produce the answer most seasoned veterans are looking for, we also understand that it's equally important to review basic terminology and principles before such technical answers can be explained.
The camshaft's function is to time each valve opening and closing point in relation to the piston and the combustion process to every degree of the engine's 360-degree rotation. In order to achieve accurate timing, the camshaft's timing gear has to operate at exactly half the speed of the rotating bottom end.
During the camshaft's installation, it is extremely important to make sure the crankshaft and camshaft's timing gears are properly aligned by placing the dimple on the crankshaft timing gear in the 12 o'clock position while the dimple on the camshaft timing gear is positioned at 6 o'clock. Of course, this assumes the camshaft is being installed without any advance or retard. Before we go any further, it's important to define some common terms associated with camshaft measurement and construction.
Valve lift is probably the most mentioned term when it comes to bench racing and bragging about whose camshaft is bigger. In cam speak, there is only lobe lift, not valve lift. The cam lobe raises the lifter, which in turn raises the pushrod and actuates the rocker. From there, the rocker multiplies the lobe lift by the rocker ratio to determine the actual amount that the valve is pushed off its seat. Basically, as the camshaft rotates in the engine, the lobes lift the valvetrain and actuate the valves accordingly. One way to effectively increase the amount of valve lift without actually changing the camshaft is to utilize a higher-ratio rocker arm. For example, a lobe lift of 0.310 inch multiplied by the typical small-block Chevy 1.5:1 rocker ratio would provide a valve lift of 0.465 inch. On the other hand, simply changing to a 1.6:1 ratio would deliver an additional 0.031 inch lift to move the valve a total of 0.496 inch from its seat. Of course, with this additional lift comes additional duration as well, which brings us to our next subject.
The next most popular camshaft measurement is the duration of time a camshaft lobe keeps the valve off its seat. The purpose of a valve is to regulate the flow of air. In order to measure a camshaft's duration, you must use a degree wheel in conjunction with a predetermined lift point. The most common duration references are advertised duration measured at 0.050-inch lift. While the advertised numbers are supposed to be measured at 0.020-inch lift, some cam grinders measure farther down the lobe to make their camshaft look more radical than it really is. Because of this confusion, most engine builders use the 0.050-inch duration figure, as will we throughout the rest of this story.
Duration is probably the biggest factor in determining an engine's overall character. When the length of the duration is increased, the engine's maximum top-end potential will increase as well. This is due to the intake and exhaust valves being held open longer to move more air and fuel through the cylinder. While this long-duration technique is great for making upper-rpm power, there is also a negative effect that must be considered. When the valves stay open longer, it requires them to leave the seat sooner and close back down on it later, which causes an overlap condition. This allows the combusted cylinder pressures inside the cylinders to fall at low rpm, which in turn creates a loss of low-speed torque. >> As for upper-rpm cylinder-pressure bleeding, there isn't enough time for a considerable amount of pressure to disperse before the next engine cycle can begin.
Sooner or later, the question of ultimate valve timing manipulation will enter the picture and issues of piston-to-valve clearance will become a problem. While most enthusiasts believe that total valve lift is primarily responsible for piston-to-valve clearance, it actually plays only a small part. Pistons contact the valves due to extensive amounts of duration. Camshafts that typically measure less than 0.550-inch lift and 220 degrees of duration (at 0.050-inch lift) are usually safe with flat-top pistons, but even then they should be checked for acceptable clearance.
While valve duration is the amount of time a valve remains off its seat, overlap is a measurement of time that the intake and exhaust valves in the same cylinder are both open simultaneously. This overlapping of the valves is required to take advantage of an engine's maximum potential but comes with a few disadvantages. The first drawback: When a cylinder fires during its combustion stoke, both valves are closed in order to contain compression. However, to maximize exhaust potential, an engine builder may use a camshaft that opens the exhaust valve before the piston reaches bottom dead center (BDC) and closes slightly after the exhaust stroke passes TDC during the early induction cycle. The benefit here is a well-scavenged exhaust stroke but it comes at the expense of some bled-off cylinder pressure and a slightly contaminated intake charge. Along with a disturbed intake charge, the engine's vacuum signal is weakened because the piston's vacuum is not only drawing its intake charge from the induction system, it's also pulling dirty air from the open exhaust valve. This weakened air signal raises emissions and lowers idle engine vacuum, which plays a major role in the operation of engine-driven accessories. Along with the previously mentioned problems, excessive overlap also creates a weak torque curve, as the induction system is not functioning at its maximum potential when the cylinder is not being filled properly. In this case, low- and mid-speed torque are traded for peaky upper-end horsepower.
Lobe Separation Angle (LSA)
The next concern should be lobe separation angle. We know that the intake valve opens to create power and the exhaust valve follows to keep that power building. Lobe separation angle (LSA) basically describes the amount of lifter movement time between the intake at its maximum lift point and the exhaust at its maximum lift point. Of course, this is relative to the camshaft duration, which determines how long it takes the lifters to travel to and from their maximum lift points.
Refer to the diagram of two camshaft lobes. Points A and B are the beginning and end points of measurement for the camshaft's LSA, C and D refer to the intake's 0.050-inch duration measurements, and E and F indicate exhaust duration.
Notice that points E, G, and D form a triangular area, which is called the camshaft area of overlap. Once all of these points are understood, it's easier to see how a wider (higher numerically) LSA would create a smaller area of camshaft overlap. Now assume we tighten (numerically lower) the LSA. Points A and B would move closer together and cause overlap points E, G, and D to form a larger triangular overlap area. This means that the LSA can be used to alter the overlap of the intake and exhaust valves, and a wider LSA will offer the best idle vacuum and emissions, and allow the camshaft to carry out its power curve for a longer period of time.
This doesn't necessarily mean the engine will make more power the higher it is spun, but rather that the engine will not nose over quite as quickly as a more peaky engine would. As for tightening the LSA, valve overlap will become more evident and provide a number of benefits. Improved peak power potential is one and a stronger vacuum signal at maximum airflow efficiency is another. The strong maximum efficiency signal is also a way of saying that responsive power (peak torque) at or near the same maximum airflow efficiency will improve. As far as making the right choice, a street-driven vehicle should look for a wider LSA, while a race car can benefit from a tighter one. Either way, once the principles of camshaft design are understood, they can be used in conjunction with one another to develop the ultimate grind.
Intake/Exhaust Centerline
Anyone other than a camshaft engineer or competitive engine builder rarely discusses the subject of intake and exhaust centerlines. The reason for this is because they are both ground into the camshaft as initial timing points and allow an engine builder to phase the camshaft in with the rotating assembly. Earlier, we discussed how a camshaft must spin at half the speed of a four-stroke bottom end in order for it to operate the valves and deliver the spark at the correct time. The engine builder only needs to know the specifications in order to degree the camshaft in with the bottom end and check its accuracy to make sure the cam was ground correctly. Most builders only check the intake centerline because the exhaust centerline is phased off the intake, which means that if one is correct then the other is too. Unlike LSA, these centerline specifications are predetermined and ground into the camshaft at the time of manufacture. This means they cannot be altered unless the camshaft is completely reground.