Sport Compact Camshafts

Sport Compact Camshafts

Webster’s Dictionary defines the word “cam” as a disk or cylinder having an irregular form such that its motion – usually rotary – gives a rocking or reciprocating motion to any contiguous part. In other words, when rotated, a cam will impart its irregular, out-of-round or eccentric shape onto the given corresponding object. The camshaft then, is simply a rod-shaped device that hosts one or more cams and is able to react against multiple objects all at once. And while cams may be found in any number of mechanical devices ranging from furniture hardware to high-tech machinery, the camshaft remains almost exclusive to the four-stroke internal combustion engine.

The camshaft’s primary job is to open and close a series of poppet valves within the cylinder head in an orchestrated fashion and does so directly relative to engine speed. As a result, the camshaft dictates an engine’s power curve and at what point in the RPM range the engine will produce maximum horsepower and torque. For a camshaft to work as intended, it must always be rotating. To accomplish this, it’s either driven by gears, belts, or chains that are linked directly to the crankshaft via a pulley. As the camshaft spins, so does the crankshaft at a 2:1 ratio – two camshaft revolutions for every single crankshaft revolution.

The series of cams, which will be referred to as lobes from now on, are then rotated as part of the camshaft thus reacting with the engine’s intake and exhaust valves via a series of rocker arms or shims, causing the valves to open and close in a timed fashion. In theory, the camshaft sounds simple enough, but is perhaps another one of the most complex and misunderstood components of the modern-day four-stroke automobile engine.

A basic understanding of the four-stroke piston-based engine cycle – introduced and patented by Dr. Nikolaus Otto in 1876 – is important prior to understanding the relationship that the camshaft shares with the rest of the engine. The four-stroke cycle’s name is indicative of the four piston stages inherent within these types of engines – intake, compression, power, and exhaust. For every 720 degrees of crankshaft rotation, each of the engine’s pistons travels upward and downward twice. This means that each cycle – intake stroke, compression stroke, power stroke, and exhaust stroke – requires 180 degrees of crankshaft rotation to complete.

The cycle begins with the intake stroke, during which both air and fuel are introduced into the cylinder through the open intake valves. During the compression stroke, both the intake and exhaust valves start off closed as the piston begins to rise. Once the piston reaches the top of the bore a spark ignites, thus beginning the power stroke. The explosion of air/fuel pushes the piston back down the bore. When the piston reaches the bottom of the cylinder the exhaust valves open. The exhaust gases are forced out as the piston rises in the bore. The process creates an explosion only every other revolution of the crankshaft. The valves, so carefully controlled by the camshaft, introduce air and fuel into the cylinders and then let the exhaust gases out, allowing the successful completion of the piston cycle.

Of course this is all just an oversimplified version, as in real-world applications the precise beginning and end of each cycle is a bit foggy due to the specifications of the camshaft and other physical and chemical constraints. Rather than functioning in 180-degree increments, any given engine’s valves remain open far longer in order to promote proper cylinder filling and exhaust gas release.

To understand the camshaft, you need to understand at least these four properties: lift, duration, overlap, and timing. In camshaft lingo, lift refers to the maximum distance that a valve is opened away from its respective valve seat. Imagine a room full of people trying to exit with only one doorway. There are two ways to make this happen: the first is to open the door as wide as possible, allowing multiple persons through simultaneously, the second is to leave the door open for a longer period of time. Lift can most closely be associated with the former since it has nothing to do with time.

To understand why lift is relevant in respect to engine performance, it’s first important to look at the relationship the valves share with the cylinder head. While the valves are necessary for the cyclical process of the engine, the valve’s head (necessary for sealing with the valve seat or “closing” the valve) restricts airflow and must be moved as far away from the valve seat as is practical to allow air to pass through. The farther away the valve head is from the valve seat, the more efficiently the air will be able to enter the combustion chamber. Increasing valve lift (up to a point) will usually introduce more air and fuel into the combustion chamber and increase the rate at which spent exhaust gases may exit.

Generally speaking, adding lift will usually result in increased power. However, you also need to be able to open and close the valves as quickly as possible without causing valve float or damaging the valve seats due to excessive impact – the farther you open the valves, the farther they must travel during each engine cycle. Different camshafts open and close the valves at different rates (sometimes referred to as having different ramps) to keep valvetrain operation as smooth as possible. Stronger valve springs may also be necessary to keep the lifter riding on the cam and the valve where it’s supposed to be. More on that later.

But despite the increased airflow that usually comes from additional lift, some engines may not make any more power. For one thing, many economy-minded engines are designed in such a way that increased lift doesn’t translate into an increase in usable airflow. Other engines may have piston-to-valve clearance dilemmas where an increase in valve lift will result in either a hole in the piston, a bent valve, or both. Still, other engines may be equipped with valve springs that are unable to withstand the additional pressures imposed by opening the valves more. In other cases, the rest of the engine may not be optimized to use that extra airflow. For instance, adding a high-lift cam to an otherwise stock engine will probably hurt more than it helps.

While increasing valve lift is an effective method of introducing more air into the cylinders, holding the valves open for longer (more degrees of crankshaft rotation) is equally as effective. Getting back to the doorway analogy, duration may be most closely associated with the amount of time the door remains open. Unlike lift, which is measured in units of distance (usually thousandths of an inch or millimeters), duration is measured in degrees of crankshaft rotation. You can’t simply qualify duration in units of time, because at lower engine speeds, the valves are open for longer than at higher engines speeds, despite equal durations. This is the result of the crankshaft and camshaft turning slower at lower RPM. Therefore, adding duration keeps the valves open for longer at all engine speeds.

As is the case with increasing valve lift, an increase in duration will also yield additional airflow and potentially additional power. Also as with increasing valve lift, adding duration will only add power up to a point. This, of course, will vary depending upon the engine. As duration increases, and the amount of time the intake and exhaust valves remain open increases, so do the chances that the intake charge may become contaminated with exhaust gases. Intake dilution, as it is most commonly called, occurs when the exhaust gases merge with the intake charge. In some cases, a portion of the intake charge will exit through the exhaust valves as well. This almost always results in a poor idle and decreased performance at lower engine speeds. At higher engine speeds, the duration is the same, but the actual time that the valves are opened decreases and performance generally increases.

Engines featuring long-duration camshafts can usually be identified by their rough and lumpy idle and are usually associated with race cars. Camshaft lift and duration are not interdependent, but they do complement one another. A given duration value combined with a lift value will result in the camshaft’s profile, otherwise referred to as its rate of lift or angle. Visualize a hill with the elevation at the top representing lift, the distance from the base of the hill on one side to the base of the hill on the other side as duration, and the curve of the sides of the hill as the ramp. If you add lift but the duration remains constant, the hill gets steeper. If you add duration but keep the same lift, the decent and ascent will be more gradual. Much consideration must be taken when designing a given camshaft profile. Too steep a ramp can often result in consequences such as valve floating and valve seat damage. Excessive camshaft wear is but another indicator of too aggressive of a profile. Of course, the consequences vary depending upon whether the subject of the steep profile is either the intake or the exhaust valves.

On the intake side, a steep closing ramp is less of an issue since the cooler intake charge will help ensure a proper seating. The exhaust side is a different story. The extreme temperatures present within the exhaust ports and around the exhaust valves make the seats and valves themselves more prone to damage due to excessively steep camshaft profiles. As far as opening profiles are concerned, a number of factors, including valvetrain weight, will dictate an acceptable angle. Generally, an angle that starts out smooth and conservative and works its way up to an aggressive profile will impact the valvetrain minimally. Camshafts like these, that feature altering-profiles are said to be asymmetrical. Camshafts are typically rated and measured at .050-inches of lift, a U.S. industry standard when it comes to cam specifications. This method was adopted basically as a way to provide a more meaningful comparison between different camshafts since measurements can vary widely depending on lift values.

The phenomenon mentioned earlier – the point at which both the intake and exhaust valves are opened simultaneously – is commonly referred to as overlap. Overlap generally occurs for only a short period of time, specifically the point in time in which the intake valve is opening and the exhaust valve is closing. This is precisely why it poses less of a problem at higher engine speeds. Overlap happens when the piston is at top dead center. While, in theory, overlap may seem counterintuitive, the process actually helps draw exhaust gases out of the combustion chamber more efficiently. This process of assisting in exhaust gas evacuation, referred to as scavenging, can potentially create additional horsepower by allowing for a more efficient cylinder fill by evacuating unusable gases more quickly. However, in some cases, too much overlap will not only assist in evacuating the exhaust gases more efficiently, but can also draw the potential intake charge right into the exhaust, wasting both air and fuel.

Referring back to Otto’s 720-degree engine cycle, this all assumes that the valves close and open in 180-degree increments – something that may very well be possible if the valves possessed the ability to obtain full lift instantaneously. Since the laws of physics won’t allow a poppet valve to do this, and they do require time to reach full lift, the theoretical 180-degree cycle is expanded upon. Due to the nature of the cycle, opening the intake valve early positions it at full lift at the ideal point in the engine’s cycle. By closing it late, additional air may be drawn in while the piston dwells for a short period of time at bottom dead center. Since the pressure on the exhaust side of the equation is much greater than the intake, opening the exhaust valve earlier will prove even more desirable. Closing the exhaust valve later also has its benefits but must be taken on cautiously as this is the point at which valve overlap and intake dilution may occur.

Lastly, in regards to valve timing, it’s important to understand what the lobe-separation angle and cam centerline are. The lobe-separation angle is the physical distance, measured in camshaft degrees, between the intake and exhaust lobes. On a single overhead cam engine, this measurement is fixed, while on dual overhead cam engines, it can be altered depending on cam timing. This, combined with lift and duration values, will determine how much overlap there will be. If lobe separation values remain equal, but lift and duration are altered, overlap will change too. The centerline of the cam then is just its position in relation to the crankshaft. When looking at the number one cylinder’s intake camshaft lobe for example, you have to find its centerline with the number one piston at TDC. The centerline value is important for figuring out when the valves are opened or closed.  

Of all the automobile engines that have been produced over the years, including the oddballs such as the flatheads and the rotaries, two have proved to withstand the test of time: the overhead camshaft engine and overhead valve engine. For the most part, most production vehicles today – and certainly all those in this book – feature overhead camshaft engines. Like the name implies, the overhead camshaft engine’s camshaft lies above the cylinder head or heads instead of being housed in the block. By positioning the camshaft or camshafts in the head, bulky pushrods and lifters may both be eliminated. The results are reduced valvetrain inertia, more reliable and higher engine speeds and certainly increased performance.

In the overhead camshaft engine, the camshaft lobes ride on the rocker arms or an inverted cup referred to as a bucket, transferring motion onto them. The rocker arm or bucket pushes upon the corresponding valve causing it to open away from its seat. The complexity of the overhead camshaft design is intensified when concerning the drive belt or chain. Since the camshaft is positioned farther away from the crankshaft in comparison to overhead valve engines, a series of elaborate belt tensioners, guide plates, idle pulleys, and covers are required. This all adds to the difficulty level when concerning removing and reinstalling the cylinder head. None of this is to disparage the overhead valve engine. A number of high-powered eight-cylinder domestics like the Corvette still rely on this age-old technology and are arguably better for it.

The term “overhead valve engine” actually has two meanings. It was named such in the early 1940s in order to differentiate itself between those engines with valves positioned beside their cylinders. Today, all engines feature overhead valves, and the term now applies to those in which the camshaft is located within the engine block. To avoid confusion, and perhaps more appropriately, many refer to these overhead valve engines simply as “pushrod engines.” In the pushrod engine, the camshaft orchestrates its corresponding valves, which are located above the camshaft in this case, via a series of lifters, pushrods, and rocker arms. The camshaft imparts itself against the lifter, which raises the tube-shaped pushrod, which reacts against the fulcrum-like rocker arm.

Finally, the rocker arm actuates the valve in a downward motion. This system requires more parts and occupies more space compared to an overhead-cam engine. Because of the extra bulkiness and weight, pushrod valvetrains are less likely to handle high-RPM duty in stride. Pushrods also cause restrictions upon cylinder head port size and shape. Often times, a compromise must be made between port size and location at the expense of positioning the pushrod into place. As a result, airflow can suffer. This isn’t to say that the overhead valve engine doesn’t have its benefits. By using only a single camshaft and placing it in the engine block, packaging, overall engine weight, simplicity, and under-hood space are improved. By positioning the camshaft closer to the crankshaft, the camshaft drive is also shortened, resulting in more accurate timing in some cases – an undeniable benefit.

In an engine with a single overhead camshaft, the camshaft must actuate both the intake and exhaust valves. Single overhead camshaft V-6 and V-8 engines have one camshaft per cylinder head. The camshaft lies above the valves in these engines and actuates them by either shims or rocker arms.

All things being equal, the single overhead camshaft engine will theoretically produce more torque than those engines featuring two camshafts per head. The reason has to do with the absence of parts, resulting in less valvetrain inertia translating into more usable energy. Of course, things never are equal, especially when you’re considering a DOHC engine versus a SOHC engine, where much more than an extra camshaft and additional valvetrain components are involved. Matters like these are seldom equal and it’s important to consider the potential increased valve diameters and valve angles inherent with the dual overhead camshaft engine before making any comparisons. Also, as the metallurgy and technology of the dual overhead camshaft engine becomes more sophisticated, this becomes less true.

It wasn’t that long ago that single overhead camshaft engines typically only had the ability to accommodate two or three valves. The absence of a third or fourth valve decreased the available airflow and performance and power production were both sacrificed. Of course, more and more modern-day single camshaft engines incorporate a fourth valve, but the result is a more complex valvetrain featuring more complicated rocker arms and camshaft lobe profiles. But the main problem with the SOHC design is that it’s impossible to alter intake and exhaust lobe phasing without grinding an entirely new cam. Yes, it’s possible with an adjustable timing gear in many cases to advance or retard valve events, but whatever goes for the intake, goes for the exhaust since they are dependent on one another.

The dual overhead camshaft engine simply makes use of two camshafts per cylinder head – one to control the intake valves, the other the exhaust. Dual overhead camshafts were originally implemented so that more than two valves could be introduced into the cylinder head packaging. Rather than fitting a single overhead camshaft cylinder head with two large and heavy valves, the dual overhead camshaft head offers comparable, if not better, airflow with four smaller, lighter valves. The product is reduced valvetrain inertia resulting in more horsepower, torque, and increased reliability.  

The dual overhead camshaft engine has a number of benefits in comparison to the single overhead camshaft engine: first, all the DOHC engines on sport compacts have four valves per cylinder, and secondly it allows for better placement of the valves for improved flow. On older engines, the dual overhead camshaft design was also preferred as it allowed more room for optimal spark plug location, but modern-day single overhead camshaft engines have since overcome this obstacle and feature spark plugs located in ideal locations. Perhaps the most enticing advantage the dual overhead camshaft design offers is its ability to independently adjust camshaft timing for both the intake and exhaust valves. In this situation, overlap may be increased or decreased by independently controlling the amount of time either the intake or exhaust valves remain opened. The result is often improved torque, horsepower, and, in some cases, better emissions.

In order to make use of two camshafts instead of one, the tradeoffs are few and include those of weight, cost, and the complexity of parts. Installing camshafts in a dual overhead cam type head can also be more challenging, especially when concerning lining each cam into position and aligning the belt or chain. This is generally a bit more involved than when working with SOHC cylinder heads.

Perhaps more so than any other component of the four-stroke engine, a one-size-fits-all camshaft is far from a reality. Due to the nature of the four-cycle engine, a camshaft’s lift, duration, and overlap characteristics must be specifically tailored for each application.

Most camshaft theory, including that on these pages, assumes that the proposed engine relies on naturally aspirated induction. The naturally aspirated engine, one which receives no help with introducing air into the cylinder, relies heavily on increased lift, longer duration periods, and significant overlap, as well as optimized cylinder head ports and intake and exhaust systems. Assuming that the intake and exhaust systems are up to snuff, each of these factors will contribute to a more powerful engine and more desirable power curve.

Opening the intake valves early is seldom necessary on camshafts suited for turbocharged and supercharged engines. The boost pressure present in the intake system means the incoming air needs little help making its way into the cylinders. Valve overlap in a boosted engine will pose negative results: as the high-pressure incoming air charge seeks the path of least resistance, it will tend to blow right past the cylinder into the exhaust valves, bypassing the whole combustion process. As such, properties like lift and duration when concerning turbocharged engines’ intake camshafts are rarely altered significantly. In high-boost applications, many forced induction camshafts are designed not to open the intake valves until the piston begins to head downward just as cylinder pressures drop.

Turbocharged engines can use some help on the exhaust side. Exhaust backpressure is always the enemy, but this is especially true in a turbocharged engine, since the exhaust gases spin the turbo and build boost. In general, boosted engines need more exhaust flow because the air is boosted coming in, but on the way out, the pressure is dropped, thereby creating more volume of exhaust air. Lofty lift and high duration values are also often not required, as engines like these are able to produce higher-RPM power and torque due to the nature of forced induction. When installing a camshaft featuring larger lift values, it’s often recommended to upgrade to stiffer valves springs to help alleviate the additional seat pressure already realized due to the added boost.  

Optimal camshaft timing for a nitrous-fed engine will certainly differ than that for a naturally aspirated engine. When selecting the optimum nitrous camshaft, be aware that it might not be that great while you’re running off the juice. For nitrous, the intake side of the camshaft is rather irrelevant. Increasing duration and/or lift will do little and may often put a damper on performance. As for the exhaust, the contrary is true. In order to deal with the excess exhaust gases created by introducing the additional oxygen into the cylinder, additional exhaust duration is very helpful for making power. It’s important to get all of the exhaust gases out, as quickly as possible, to make room for the fresh intake charge.

To be blunt, the answer is no. The perfect camshaft is defined as one that would maximize engine efficiency and power output throughout the entire RPM range. Not to mention the fact that it would also do so without sacrificing drivability, reliability, emissions, or fuel consumption. And, oh yea, it would do all this – move the valves up and down that is – instantaneously, without friction and vary throughout the RPM range of the engine. The scenario is impossible, even if half of these factors were of no concern. The key is selecting a camshaft that’s best for the given application. Some camshafts are designed to allow for maximum top-end power production, others for low-end. Still others are designed for more mild power increases across the entire powerband. And in the case of the majority of production vehicles, emissions and fuel consumption concerns almost always trump those of horsepower and torque production. Your best bet is to choose what’s important to you can select camshafts based on that.

Source -- http://www.cartechbooks.com/techtips/sportcompactcamshafts