The cylinder heads might be the most important part of an engine in terms of producing horsepower, but unless something opens up the valves, the heads will flow no air at all. And zero airflow equals zero horsepower. The responsibility of opening and closing the valves at precise intervals falls on the camshaft, which makes it the second most important component in the overall horsepower equation.
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By controlling how much, how long, and when the intake and exhaust valves open and close, the camshaft determines how much horsepower an engine makes and the RPM range in which that power is concentrated. The camshaft also profoundly impacts gas mileage and emissions quality, which probably aren’t very high on the priority list for hot rodders, and it also affects idle quality and low-RPM drivability, which are huge concerns for any streetdriven vehicle.
What makes proper camshaft selection so critical is that going too big or too small can ruin an otherwise perfect engine combination. Too conservative of a cam won’t allow an engine to take full advantage of the cylinder heads’ airflow capabilities. Too aggressive of a cam can compromise low-speed drivability so badly that you wonder why you spent so much time and money maximizing the cubic inch total in the first place. As with cylinder head design, camshaft theory is an extremely complex science that involves dozens of inter-related variables. It’s quite possible that there are even fewer true camshaft experts than there are cylinder head gurus, and that’s really saying something.
All LS-series small-blocks come equipped with a hydraulic roller camshaft from the factory. Thanks to improvements in modern valvetrain technology, this combines the convenience and low maintenance of hydraulic lifters with the high-RPM potential once reserved for mechanical lifters.
The good news is that you don’t need to know how to design the perfect lobe profiles in order to pick the ideal camshaft for your stroker motor project. Just learning the basics of camshaft theory will get you in the ballpark, and consulting with camshaft manufacturers and engine builders will get you the rest of the way there.
Camshaft manufacturers have invested thousands of hours into designing hundreds of off-the-shelf cam grinds that complement the vast majority of engine applications. Oftentimes, an off-the-shelf grind out of a catalog works remarkably well, and at roughly $400, it’s not terribly expensive to experiment with different camshafts, if necessary. If you do need to spec out a custom cam, chances are its design is based upon an off-the-shelf grind that’s been slightly modified to suit the specific demands of your application. Either way, the consumers are the direct beneficiaries of the massive R&D efforts of the major camshaft manufacturers, such as Comp Cams, Lunati, Isky, GMPP, and Edelbrock. Just learn the basics, and you’ll be on your way to selecting the perfect camshaft for your stroker buildup.
The camshaft determines how long the valves stay open and how far they move off their seats, as well as when these events take place. These factors profoundly impact how much power is produced, in addition to where that power is concentrated in the powerband.
Although LS enthusiasts looking for maximum high- RPM power usually upgrade from a hydraulic roller cam to a solid roller cam, a solid flat-tappet cam is also a very capable performer. It’s lighter and offers superior high-RPM valvetrain stability compared to a hydraulic roller cam, which yields greater power potential. Likewise, its flat noses offer quicker initial lift acceleration than a roller design, making it an appealing option for racing classes that place restrictions on maximum duration or lift. (Photo courtesy of Comp Cams)
Cam Effects
Although camshaft dynamics is an extremely complex subject, a few basic universal truths of cam theory can help simplify understanding the role a cam plays in overall engine performance. Replacing a stock camshaft with a larger aftermarket unit having longer duration and higher lift almost always yields an increase in horsepower. Likewise, the larger the camshaft, the higher in the RPM band an engine produces peak horsepower and torque. And because horsepower is simply torque multiplied by RPM, moving the torque peak higher in the RPM range increases horsepower every single time.
The drawbacks of big camshafts are that they increase emissions output and decrease low-RPM torque, throttleresponse, and intake manifold vacuum at idle. Again, tailpipe emissions and gas mileage probably aren’t big concerns for the typical hot rodder, but compromised low-speed torque requires shorter gearing, and an engine that doesn’t produce adequate idle vacuum won’t be able to actuate a power brake system. Furthermore, extremely aggressive camshaft grinds also accelerative wear and tear on the rest of the valvetrain components. Combating this with a heavier-duty valvetrain drives up cost considerably.
That said, like every other aspect of engine building, choosing the right camshaft is all about balance. Having an irrational phobia for duration and lift is a good way to guarantee that an engine never produces respectable power and torque. For instance, it makes no sense whatsoever to invest thousands of dollars in a set of top-notch cylinder heads and a forged rotating assembly capable of handling 9,000 rpm if the parts combination is going to be hampered by a dinky hydraulic roller cam that isn’t capable of fully exploiting an engine’s airflow and RPM potential. Furthermore, different drivers have different tolerances for low-RPM surge and choppy idle quality, so what’s considered an aggressive cam or a tame cam is purely subjective. Although a high school kid may find a lopey 230-at-.050 cam charming and intoxicating in a 346-ci motor, an older and more mature enthusiast might find the same cam unstreetable in a 396.
Varying tastes and tolerances aside, what can’t be disputed is that larger-cubicinch engines reduce the adverse effects of a more aggressive camshaft. For example, a camshaft that struggles to idle and suffers from very poor throttle response in a small-displacement motor idles like stock and yields tire-shredding low-end torque in an engine that’s 100 ci larger.
Camshaft grinding is an extremely precise process in which tolerances are held to thousandths of an inch. In order to endure higher valvespring pressures and steeper ramp acceleration rates, solid roller camshafts are usually ground from a durable billet core.
Ideally, the effects of altering duration and lift would be independent from each other, but this isn’t possible, because both are determined by the shape of the cam lobe. Increasing lift increases the distance the lifter must travel around the cam lobe, thereby increasing duration. A good rule of thumb is to select duration based on an engine’s operating RPM range, and lift should be based on a cylinder head’s airflow at high lift.
Expanding upon that example, let’s say there are two engines, with identical camshafts, cylinder heads, intake manifolds, and compression ratios, but one measures 383 ci and the other measures 427 ci. The peak horsepower output between the two is similar, but the 427 produces far more low- and mid-range torque and manifold vacuum while peaking at a lower RPM. That equates to a far more streetable package that places less stress on the valvetrain components, enhancing durability, and enables running taller gearing for improved gas mileage. Consequently, camshaft selection must be closely matched to an engine’s displacement and intended usage. Simply changing the camshaft can transform driving characteristics from that of a stock-caliber rebuild to a lowRPM street cruiser to a dual-purpose street/strip machine, or a high-RPM race engine.
Lobe Profile
At the risk of pointing out the obvious, a camshaft is a shaft fitted with eccentric cam lobes, and lobe lift is the difference between the radius of the cam lobe’s base circle and the height of the eccentric. On an OHV engine like the LS-series small-block, the camshaft is mounted inside the block, and one lobe is designated for each valve, for a total of 16 lobes. The eccentric shape of the cam lobes enables them to convert the rotating motion of the camshaft into reciprocating motion. It’s this reciprocating action that pushes up on the lifters, pushrods, and rocker arms, thereby opening and closing the valves. How far the cam lobes push open the valves is referred to as “lift,” and the length of time the valves stay open is called “duration.”
The shape of the cam lobes establishes lift and duration, and consequently, they are interdependent. For instance, grinding down a cam lobe to reduce lift also reduces duration. Likewise, the maximum amount of lift that can be ground into a cam is ultimately limited by cam duration. That’s because increasing lift without increasing duration creates a steeper lobe profile, and there’s a physical limit to the rate of ramp acceleration that both the camshaft and lifters can handle. Under ideal circumstances, camshaft design would allow for isolating the effects of duration and lift from each other in terms of how they affect power, but that simply isn’t the case. Even so, understanding the relationship between duration and lift is a useful tool in the camshaft selection process.
The distance between the peaks of the intake and exhaust lobes establishes a camshaft’s LSA. Although duration and lift dictate how much power an engine will produce, the LSA is a fine-tuning tool that changes where in the RPM range horsepower and torque is concentrated.
If duration, lift, and LSA were the only factors in camshaft dynamics that influenced performance, then one manufacturer could reverseengineer another company’s R&D efforts by simply duplicating their published camshaft specs. The truth of the matter is that there are far more variables involved in designing a camshaft. Two camshafts with identical duration and lift figures can have vastly different acceleration curves. The challenge for manufacturers is figuring out how hard the lobes can push the valvetrain without creating excessive deflection and introducing valve float. (Illustration courtesy of Comp Cams)
Duration
Of the multitude of variables that go into designing a camshaft, duration has the most profound impact on power production. Because a camshaft rotates at half the speed of the crank, duration is expressed in degrees of crankshaft rotation. This represents how long the valves stay open in relation to crankshaft rotation. For instance, a cam that has 250 degrees of duration at .050-inch lift stays open for about 250 out of the 360 degrees that it takes the crank to make one complete revolution. At low RPM, when there is plenty of time to fill the cylinders with air, short-duration camshafts perform very well. However, as RPM increase, and the amount of time available to fill the cylinders decreases, a short-duration cam literally chokes off an engine’s air supply, and horsepower plummets accordingly.
The concept of time in relation to cylinder filling might seem awkward at first, but it’s actually very easy to conceptualize. Consider that at 2,000 rpm, the intake valves open and close roughly 17 times per second. At 6,000 rpm, however, that figure increases to 50, giving the incoming air charge far less time to fill the cylinders with air each time the intake valves open and close. Because longer-duration camshafts hold the intake valve open longer, thus improving cylinder filling, they improve horsepower and torque output at high RPM. Additionally, they also extend the RPM at which peak power is produced. For example, swapping out a 220-at-.050 cam in a 408 stroker motor with a 240-at-.050 cam increases the horsepower peak from about 5,500 rpm to 6,000 rpm.
On the other hand, the same longduration camshaft that works so well at 5,000-plus rpm sacrifices low- and midrange torque compared to a shorterduration cam. In order to maximize cylinder filling, it’s common for the intake valve to stay open even after the piston passes BDC on the intake stroke. As a result, the intake valve doesn’t close until after the piston begins traveling back up the bore during the compression stroke. That might sound like a bad idea at first, but at high RPM, the intake air charge packs enough energy and velocity to continue filling the cylinder even after the piston passes BDC.
Unfortunately, that’s not the case at low RPM, when the incoming air charge simply lacks adequate velocity to do so. Consequently, a portion of the intake air charge is pushed back past the intake valve and into the intake manifold. The subsequent drop in cylinder pressure accounts for the loss in low-RPM torque in a long-duration camshaft. To compensate for this, it’s very common practice to increase the static compression ratio to increase cylinder pressure and minimize low-RPM torque loss. At the end of the day, duration determines both the power potential of an engine and the range of RPM in which it produces that power.
At maximum valve lift, the piston is nowhere near TDC. With very aggressive cam grinds, the piston often travels halfway down the bore at maximum intake valve lift. As a result, piston-tovalve clearance is more a function of duration than it is maximum lift.
Overlap
During the four-stroke cycle, there is a brief period when both the intake and exhaust valves are open at the same time. As the piston travels up the bore during the exhaust stroke, the intake valve opens before it reaches TDC. This gives the intake charge more time to fill the cylinders at high RPM and increases the scavenging effect imparted by the exiting exhaust gases. At high engine speeds, the inertia of the combustion gases escaping into the exhaust port helps pull additional air through the intake port and into the cylinder.
On the other hand, overlap isn’t always a good thing. At low RPM, the scavenging effect of the exhaust is insignificant. Consequently, when the intake valve opens near the end of the exhaust stroke, when residual cylinder pressure is still present in the cylinder, exhaust gas flows past the intake valve and reverts back into the intake manifold. This is what gives performance camshafts a lopey idle.
Overlap is essentially the distance, in cam degrees, between the peaks of the intake lobe and the exhaust lobe. These peaks, known as the intake and exhaust centerlines, are measured in degrees of crankshaft rotation and establish the lobe separation angle (LSA) of the cam. For instance, if a cam has an intake centerline of 108 degrees ATDC and an exhaust centerline of 112 degrees BTDC, averaging the sum of both figures yields an LSA of 110 degrees. In other words, the distance between the peaks of intake and exhaust lobes in such a cam would be 110 degrees of camshaft rotation. Decreasing the lobe separation by moving the intake and exhaust lobe peaks closer together increases overlap. As with duration and lobe lift, the LSA of a camshaft can’t be changed without regrinding the lobes.
Adjustable timing sets allow altering the installed centerline of a camshaft, thereby advancing or retarding the valve events in relation to crankshaft rotation. Comp Cams’ adjustable billet timing set has nine keyway slots in the crank gear for up to 8 degrees of latitude. (Photo courtesy of Comp Cams)
Although camshaft duration determines the operating RPM of an engine, changing the LSA can be used to further fine-tune the operating characteristics of an engine within its powerband. It seems simple enough, but tightening or widening the LSA as a tuning tool can get tricky, because its effects are dependent upon duration.
On a small cam with roughly 210 degrees of intake duration at .050-inch lift, a tighter LSA improves top-end horsepower at the expense of idle quality and low-end torque. That’s because generous overlap improves scavenging at high RPM, but it also increases reversion at low RPM. Conversely, widening the overlap with a short-duration camshaft tends to improve idle quality and low-RPM torque at the expense of top-end power.
However, gauging overlap solely by a cam’s LSA can be a bit deceptive. As duration increases, overlap increases, even if the LSA isn’t changed. For example, a 260-at-.050 cam has much more overlap than a 225-at-.050 cam, even if both are ground on a 112-degree LSA. In other words, the actual overlap of a camshaft—measured in crankshaft degrees—takes precedence over the lobe separation angle.
This fact is important to remember as duration at .050 inch tappet lift increases. The same 112-degree LSA that idles so smoothly in a 210-at-.050-duration cam will yield a very choppy idle in a 260-at-.050-duration cam. The substantial overlap in the 260-at-.050-duration cam will not only sacrifice low-end torque, it can also dilute the intake charge enough to reduce high-RPM horsepower. Such a cam usually still produces excellent peak horsepower numbers, but the power curve drops off very sharply after that point. Consequently, with long-duration camshafts, a wider LSA can result in a broader, more flexible powerband that drops off much more gradually after peak power. And in competitive racing classes, power after peak is almost as important as peak horsepower.
Between 1997 and 2006, all LS-series small-blocks used cam cores that attached to the timing gear with three bolts. However, GM began phasing in single-bolt camshafts in 2006. Since VVT-equipped Gen IV small-blocks use a single cam bolt that doubles as an oil control valve, GM presumably started phasing in single-bolt cam cores throughout the LS lineup to cut down on production costs. All VVT-equipped LS motors have a single-bolt cam, but not all LS motors with single-bolt cams have VVT. Fortunately, swapping out a three-bolt cam for a single-bolt setup, and vice versa, is as easy as pairing the camshaft with a matching single-bolt timing set.
Studying the specs of factory Gen III/IV camshafts thoroughly reinforces this point. The 5.7L LS1 used in 2001–2002 F-bodies came equipped with a 196/207-at-.050-duration camshaft ground on a 116-degree lobe separation angle. On the other hand, the 7.0L LS7 utilizes a 211/230-at-.050 cam with a 120.5-degree lobe separation angle.
Compared to the typical aftermarket cam, factory LS-series camshafts have far wider lobe separation angles to improve idle quality and clean up emissions output. This is particularly important in the LS7, as its duration specs are quite aggressive in the world of factory cams, which was necessary in order for GM engineers to achieve their target horsepower and operating RPM range. To compensate for the inherent increase in overlap that its longer-duration specs yield, the LS7 cam is ground on a substantially wider LSA than the LS1’s. The LS7’s wider LSA also helps mask the detrimental effects on idle quality and emissions output that its longer duration naturally creates. Because idle and emissions quality aren’t major concerns in a hot rod application, aftermarket cams can get away with much more overlap and tighter lobe separation angles.
Measuring Duration
Camshaft manufacturers publish both advertised duration figures and duration at .050-inch tappet-lift specs. The advertised duration figure is always bigger, and although it seems strange, there’s a good reason why two different duration specs are necessary.
Due to the acceleration rate of a cam lobe’s ramp, it’s difficult to determine the precise moment at which the lifter starts climbing the ramp. As a result, camshaft manufacturers start to measure duration at a predetermined amount of lifter, or tappet, rise. For example, Comp Cams begins measuring duration once the lifter rises .006 inch above the base circle. Obviously, the lower this figure is, the longer the duration specs appear to be, even though the duration hasn’t actually changed. This makes the cam look bigger on paper than it is. Because camshaft manufacturers can measure advertised duration at any lift point they choose, it makes it very inaccurate to compare advertised duration specs among different manufacturers. Recognizing this problem, camshaft manufacturers agreed to use duration at .050-inch lifter rise as the industry standard for measuring duration. Doing so allows engine builders and enthusiasts to accurately gauge the duration figures of camshafts among different manufacturers. As a result, it’s widely accepted that advertised duration numbers aren’t nearly as important as duration at .050 figures.
Although that may be the case, it doesn’t mean that the duration figures below .050 inch tappet lift are entirely irrelevant. Low-lift numbers between .001 and .020 inch tell an engine builder a great deal about engine vacuum and throttle response, and high-lift numbers greater than .200 inch are more indicative of power potential. The .050-inch number is relatively easy to measure with a dial indicator and degree wheel, which explains why it’s the universal industry standard. Additionally, it does the best job of predicting the operating range of a given lobe in a specified application.
It’s important to remember, however, that duration at .050-inch tappet lift isn’t the same as the actual amount of time the valve stays open. The actual duration of the intake valve—or how long the valve remains unseated between its opening and closing events—is affected by tappet lift below .050 inch, as well as rocker arm ratio. This is one of the reasons why two camshafts with identical duration, lift, and LSA can perform very differently on the dyno and on the street.
Lift
Compared to duration, cam lift is relatively straightforward. Lobe lift is simply the difference between the radius of the cam’s base circle and the height of the eccentric portion of the cam. For example, a factory 2001 LS6 cam has a base circle radius of .760 inch, and the distance between the base circle centerline and the highest point on the intake cam lobe is 1.068 inches. Subtracting the base circle radius of .760 inch from the cam lobe height of 1.068 inches nets .308 inch of lobe lift. This simple illustration explains why high-lift camshafts typically have smaller base circles.
Reducing the size of the base circle while leaving the cam lobe height unchanged increases the lobe lift. That’s because decreasing the base circle diameter increases the distance between the top of the cam lobe and the base circle radius. Using this approach, let’s imagine that the 2001 LS6 cam’s base circle radius was reduced from .760 inch to .720 inch while its 1.068-inch cam lobe height remained unchanged. This would effectively increase lobe lift from .308 inch to .348 inch. Furthermore, smaller base circles are also necessary to prevent the connecting rods from contacting the camshaft in engines with stroker crankshafts.
Single-bolt camshafts are also used in newer non-VVT LS-series engines. Aftermarket companies offer camshafts and timing sets for both three- and single-bolt configurations. An engine equipped with a single-bolt camshaft can be converted to a three-bolt arrangement, and vice versa, by swapping over to a matching timing set. (Photo courtesy of Comp Cams)
In the aforementioned example, marginally increasing lobe lift might seem rather insignificant. However, valvetrain dynamics suggests otherwise. As the cam lobe pushes up on the lifter and pushrod, the rocker arm acts as a see-saw and converts this upward motion into downward motion. During this process, it also multiplies the lobe lift. Consequently, valve lift is the product of lobe lift multiplied by the rocker arm ratio. With the exception of the LS7, all factory Gen III/IV small-blocks utilize a 1.7:1 rocker arm ratio. Therefore, increasing lobe lift from .308 inch to .348 inch increases valve lift from .524 to .591 inches, which is substantial in anyone’s book.
While duration specs are based on the target operating RPM range of an engine, lift is based upon airflow through the cylinder heads. This makes it very easy to select the proper amount of lift for a camshaft. For instance, if a cylinder head achieves peak airflow at .650-inch lift, it should be matched with a camshaft that has at least .650-inch valve lift. In extreme applications, however, things can get more complicated. Some engine builders contend that it’s sometimes possible to increase horsepower by opening the valves beyond the point where the cylinder heads back up. Such an application might have a .800-inch lift cam even though the cylinder head airflow starts dropping off at .700-inch lift. The justification is that, beyond a certain point, the flowbench can’t accurately replicate the operating conditions inside an engine. In other words, in extreme high-airflow, high-RPM conditions, the piston sucks down on the intake charge much harder than the electric motor in a flowbench can draw air in through the ports.
Because GM’s slick VVT system is hydraulically actuated, it requires channeling oil through the center of the camshaft. To facilitate this, VVT cam cores are drilled hollow.
Nonetheless, one aspect of lift that can’t be disputed is how much of it can now be packed into a relatively shortduration camshaft. Compared to cams of just 20 years ago, modern bumpsticks allocate much greater lift over much shorter duration cycles. In the past, relatively long-duration camshafts were necessary in order to hit a target valve lift to reduce stress on the valvetrain. That’s because steeper lobes place greater loads on the lifters, pushrods, rockers, valvesprings, and the lobes themselves. Cylinder heads of the day rarely flowed well beyond .500-inch lift, so this wasn’t a big deal. However, as cylinder head technology improved, as evidenced by factory LS-series castings that flow well past .600-inch lift, it became necessary to improve valvetrain durability. Fortunately, camshaft and valvetrain manufacturers met the demand, and now it’s possible to stuff tons of lift over a steep, short-duration lobe. This gives both the drivability that was once missing in large cams and the power that was difficult to achieve with short-lift cams.
Piston-to-Valve Clearance
When planning a new engine build, or upgrading to a larger cam in an existing combo, it’s critical to check for adequate piston-to-valve clearance. As the term suggests, there must be enough clearance between the valves and piston crown near TDC in order to prevent severe damage to the valvetrain, cylinder heads, and short-block.
Generally, piston manufacturers recommend a minimum of .080-inch clearance between the intake valves and piston crown and .100-inch clearance for the exhaust valves when using steel connecting rods. Due to the increased stretch of aluminum rods, they require an additional .030 inch of clearance. Hot rodders instinctively examine the maximum lift figures of a cam to try to determine if there will be adequate piston-to-valve clearance, but the issue at hand revolves more around duration than lift. The reason for this is simply because when the piston is at TDC, the intake valve is nowhere near peak lift. In fact, at TDC, the intake valve is just starting to move off its seat.
Verifying this line of thinking is as easy as looking at the intake centerline angle of a cam’s published specs. To illustrate the point, let’s take a look at one of the most aggressive LS-series camshafts in Comp Cams’ catalog, its XFI286R113 grind. This beastly solid roller cam boasts duration specs of 251/256-at-.050 and .660/.655 inch valve lift. Despite the fact that it packs a massive amount of lift, especially for a small-block, a quick look at the intake centerline angle reveals that the intake valve doesn’t reach peak lift until 110 degrees ATDC. At that point, the piston is nowhere close to TDC and has actually descended about halfway down the bore. That means that even with a stock 3.622-inch LS1 crank, a piston would be nearly 1.811 inches down the bore. Worrying about whether or not the cam’s peak lift—in this case .660 inch—is enough to smack into the piston is awfully silly, considering that the piston would be almost 1.811 inches down the bore. And that’s before you even take the angularity of the intake valve into account. This resoundingly reinforces the point that piston-to-valve clearance has very little to do with peak valve lift.
To feed oil to the VVT system’s phaser assembly, oil is routed from a groove cut into the number-2 cam journal to the oil control valve that bolts inside the cam snout. This simple arrangement allows supplying hydraulic pressure to actuate the system without any modifications to the block, which makes it very easy to retrofit VVT to non-VVT engines.
On the other hand, it’s safe to assume that a cam with 251 degrees of intake duration, regardless of lift, would probably cause piston-to-valve interference issues, unless big valve reliefs were cut into the piston. During the time it takes the crankshaft to make one complete 360-degree revolution, a 251-degree cam leaves the intake valve open for roughly 70 percent of that cycle. That means that the intake valve is closed for just 109 degrees, or 30 percent of time, for each revolution of the crank. Additionally, the actual valve duration is even longer than the duration figure at .050- inch tappet lift.
Unfortunately, although long-duration camshafts increase the potential of piston-to-valve interference, there is no single spec that gives a definitive answer on whether or not it will be an issue. Variations in block deck height, cylinder head casting tolerances, head gasket thickness, combustion chamber depth, and piston shape all affect piston-tovalve clearance. Consequently, the only way to accurately check for it is during the engine assembly process. A builder checks it by placing a piece of clay on top of the pistons, bolting the cylinder heads down, and then rotating the crank over by hand several times. Removing the cylinder heads and inspecting the clay clearly reveals whether or not piston-tovalve interference is present.
Valve Events
Cam duration and lift determine how long and how much the valves open. Granted they’re the two most important variables in the horsepower equation when it comes to camshafts, but they don’t offer any insight as to when the valves open and close. In a four-stroke internal-combustion engine, there are four valve events: intake valve opening (IO), intake valve closing (IC), exhaust valve opening (EO), and exhaust valve closing (EC). When each of those events takes place, it is collectively known as valve timing, and each plays a role in the shape of the power curve.
Because lift and duration can’t be changed without regrinding a camshaft, the only parameter that can easily be tweaked once the lobe profiles have been finalized is the cam timing. Although when the valve events take place in relation to the position of the crankshaft, and therefore pistons, may seem rather inconsequential, engine builders have proven otherwise over the decades. Of the four valve events, intake valve closing most profoundly impacts horsepower output. In fact, some engine builders say that intake closing is more important than the other three valve events combined.
The phaser assembly in VVT engines replaces the cam gear. It features an internal rotor and stator assembly, which enables the camshaft to be advanced or retarded. An oil control valve that doubles as the cam bolt adjusts hydraulic pressure in the phaser assembly, which advances or retards the camshaft.
The opening and closing of the intake valves determine how much air can be drawn into the cylinders on the intake stroke. With a typical performance camshaft, it’s not uncommon for the intake valve to close up to 60 degrees ABDC. Extremely long-duration cams delay intake closing even farther. This isn’t ideal at low RPM, as the pistons push the air/fuel mixture back past the intake valve and into the intake manifold, which hurts low-end torque and idle quality.
On the other hand, delaying the intake closing point is exactly what an engine needs at high RPM, and simple physics dictates why this is the case. Although the piston tries to push the air/fuel mixture back past the intake valve as it travels up the bore, at high RPM the inertia and velocity of the intake charge exceeds the upward pressure exerted by the piston. So even though the piston is moving up the bore after BDC during the intake stroke, the inertia of the intake charge continues filling the cylinder with air. By nature, long-duration camshafts delay the intake valve closing point, which is largely responsible for the increase in horsepower they yield over shorter-duration cams.
One downside to GM’s VVT system is that excessive valvespring pressure can overwhelm the hydraulic pressure inside the phaser assembly, sending it into full mechanical retard. To prevent this from happening, Comp Cams and Mast Motorsports offer restrictors that can be fitted into the phaser assembly to limit the movement of its internal rotor to 20 to 30 degrees.
A VVT timing cover is easily identified by the big hump in its upper section, which houses an electric solenoid that controls oil flow through the phaser assembly. The solenoid—which interfaces with the camshaft position sensor—operates via pulse width modulation, since it must react instantaneously to inputs received by the PCM.
The oil control valve fits behind an electric solenoid mounted in the timing cover. When the solenoid applies pressure to the valve, oil exits out of the valve’s multiple orifices and into the phaser assembly. By manipulating pressure, the solenoid can put more pressure on one side of the phaser assembly than the other, causing it to rotate.
To put into perspective how late the intake valve closing point can be delayed in a long-duration camshaft, let’s re-examine Comp Cams’ XFI286R113 solid roller grind. This 251/256-at-.050 cam has an intake valve closing point of 74 degrees ABDC. That means the intake valve doesn’t close until the piston is almost half way up the bore during the compression stroke. Expanding upon this example, many race-only cams have more than 280 degrees of duration at .050-inch tappet lift, pushing the intake valve closing point even farther into the compression stroke. To put it succinctly, with the tremendous airflow potential of modern cylinder heads, and the highRPM potential of today’s short-blocks and valvetrain hardware, never underestimate the effects of inertial charge filling and the dividends in horsepower they offer.
In contrast, the other three valve events also affect power production, but they aren’t nearly as important. The exhaust opening point is most commonly accepted as the second most important valve event, as it determines the LSA of a camshaft. Retarding exhaust opening improves bottom-end torque, and advancing it allows spent fumes to exit the cylinders sooner, which generally improves high-RPM power. Because the intake valve opens before TDC during the exhaust stroke, it affects overlap. An earlier IO increases overlap, thereby sacrificing low-end torque for top-end power. Delaying the IO does the exact opposite. Likewise, the exhaust valve closes after TDC during the intake stroke.Consequently, an early EC decreases overlap and boosts low-end torque, but it doesn’t allow sufficient time for exhaust gases to escape out of the cylinder at high RPM, decreasing top-end power. A late EC has the opposite effect. Although IO, EO, and EC all alter horsepower and torque production in some way, their effects are largely inconsequential compared to IC.
Timing Tricks
As with duration and lift, the four valve events can’t be changed independently of each other without regrinding a camshaft. They can, however, be changed at the same time. During engine assembly or dyno tuning, advancing a cam involves turning it a few degrees clockwise in relation to the crankshaft, thereby advancing when the valve events take place; retarding the cam involves turning it a few degrees counterclockwise to delay the valve events. In other words, advancing or retarding the cam simply changes the installed intake centerline in relation to the crank.
GM’s VVT system uses surprisingly few VVT-specific parts. All you need to retrofit VVT onto any Gen III or IV small-block is a timing cover, a phaser assembly, a 58-tooth reluctor wheel, and an oilcontrol valve from an L92/L99, all of which can be purchased for well under $300. Other requirements include a VVT-specific camshaft and a computer from a factoryequipped VVT car that has the necessary programming tables to control the phaser assembly. A VVT retrofit is quite justifiable if you’re building an LS motor from scratch to drop into a muscle car, since most of the retrofit-specific hardware is stuff you’d have to buy anyway.
Programming the VVT system requires obtaining a PCM from a VVTequipped car and reprogramming it with software from HP Tuners or EFI Live. Another alternative is to use Mast Motorsports’ stand-alone M-90 PCM and wiring harness, which has the necessary tables to control the factory VVT system.
For example, if a cam is ground on a 109-degree intake centerline, but it’s installed at a 112-degree intake centerline, it has been retarded three degrees. To facilitate quick-and-easy timing adjustments, most aftermarket timing sets have multiple keyway slots ground into the crank sprocket. Advancing the cam generally improves low-end torque and throttle response while sacrificing peak horsepower, and retarding the cam decreases low-end torque while increasing peak power. This is because advancing the cam closes the intake valve sooner, and retarding the cam delays the intake closing point.
In theory, changing all four valve events in unison isn’t an ideal situation. Ideally, the intake and exhaust lobes should be phased independently, but that requires either a DOHC valvetrain or a trick cam-in-cam layout like the one in the 2008-and-up Dodge Viper V-10. Fortunately, the other three valve events are so inconsequential compared to intake closing that it’s nothing to split hairs over.
For instance, exhaust opening is commonly accepted as the second most important valve event, as it determines the LSA of a camshaft. Retarding the exhaust opening improves bottom-end torque, and advancing it allows spent fumes to exit the cylinders sooner, which generally improves high-RPM power with long-duration camshafts. This is exactly the opposite of what happens when advancing or retarding intake closing, which means that optimizing intake closing actually compromises the exhaust opening point. Nonetheless, the effects of intake closing are so much more profound that it really doesn’t matter much at all.
Variable Valve Timing
The obvious limitation of valve timing adjustments is that after an engine is built and installed into a car, the only way to make addition adjustments is to tear into the motor again. Furthermore, aftermarket timing sets typically limit the latitude of adjustment to roughly six degrees. New car manufacturers recognized this problem long ago, and as a result, variable valve timing (VVT) systems have been used in production cars for more than 20 years. GM got in on the action, too, with the Gen IV L92 smallblock in 2007. The L92 was the first production GM small-block to utilize variable valve timing, and the system has since been installed on the L99 and LY6.
The beauty of the system is its simplicity. GM’s VVT setup features a hydraulically actuated phaser assembly that’s integrated into the cam gear. Essentially a rotor that rotates inside of a stator, the phaser assembly uses oil pressure to move the camshaft in relation to the timing chain and crankshaft. Using instructions from the engine management computer and cam position sensor, an electric solenoid mounted inside the timing cover presses upon a hydraulic valve bolted into the nose of the cam. This manipulates oil flow into the phaser assembly to advance or retard the cam. GM’s VVT system is very flexible, and it allows advancing the cam 7 degrees and retarding it up to 45 degrees, for a total of 52 degrees of latitude.
Although GM uses VVT technology as a means of boosting fuel economy and cleaning up emissions, in performance applications, its primary purpose is to optimize the intake closing point. This allows advancing the cam at low RPM to boost torque and retarding it at high RPM to increase top-end horsepower for tremendous flexibility in broadening up the power and torque curves.
Perhaps the key to the system’s seamless performance is advances in modern electronics that make its technology possible. Because the phaser assembly is essentially a rotor that moves inside of a stator, hydraulic pressure is the only thing preventing the cam from twitching around erratically. Even when the cam phasing is locked in one position, the engine management software is constantly adjusting oil pressure into the phaser to keep the cam in a fixed position. Otherwise, the pressure exerted on the phaser from the valvesprings could force it into full mechanical retard, as the system relies on hydraulic pressure to overcome the force exerted by the valvesprings. In fact, once valvespring pressure exceeds 380 pounds of open pressure, the VVT systems starts losing control of the cam phasing beyond 5,000 rpm. Consequently, there is a practical limit to how aggressive cam duration and valvespring pressure can be when using GM’s VVT system in a performance engine build. Nonetheless, hot rodders have already used the factory VVT system in stroker builds, producing well in excess of 600 hp with ultra-broad powerbands that non-VVT motors can only dream of.
When installing a camshaft, it’s a good idea to cover it with a liberal coating of assembly lube. Although roller cams do not require a break-in period, the lube helps keep friction in check until oil starts to flow through the engine during the initial start-up period.
Single- vs. Dual-Pattern
With production cylinder heads, the exhaust ports always flow less than the intake ports. Unlike the intake ports, which rely on the pressure differential created by the piston to draw in air during the intake stroke, the exhaust ports benefit from the pistons physically pushing exhaust gases out of the cylinders during the exhaust stroke. Consequently, it simply isn’t necessary for the exhaust ports to flow as well as the intake ports. Nonetheless, to compensate for this disparity in airflow, engine builders often use camshafts with more exhaust duration than intake duration. These are referred to as dual-pattern cams, and bumpsticks that have the same duration and lift specs on both the intake and exhaust lobes are known as singlepattern cams.
As no surprise, GM uses dual-pattern cams on all LS-series small-blocks. However, there is a big difference between the amount of duration split used on GM’s cathedral-port and rectangle-port heads. For example, a stock LS2 features a 204/211-at-.050 cam, and a stock LS7 cam measures 211/230-at-.050. The reason why the LS7 needs 19 degrees of intake/exhaust split compared to the LS2’s meager 7 degrees of split is because the LS7’s exhaust is relatively weak. A stock LS2/LS6 cylinder head flows roughly 183 cfm on the exhaust side and 260 cfm on the intake side for an exhaust/intake ratio of 70 percent. In comparison, a stock LS7 head flows about 220 cfm through the exhaust ports and 370 cfm through the intake ports for an exhaust/intake ratio of 60 percent. As the two heads illustrate, the amount of duration split in a dual-pattern must always be matched to the flow rate of both the intake and exhaust ports.
Written by Barry Kluczyk and Posted with Permission of CarTechBooks
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