Just 20 years ago, the prospect of a production pushrod engine turning 7,000 rpm—while being backed by a 100,000-mile factory warranty—seemed absolutely preposterous. Nonetheless, that’s exactly what GM did with the LS7, which it introduced in the 2006 Corvette Z06. Needless to say, valvetrain technology has elevated the OHV platform far beyond what anyone dreamed of just a few short years ago.
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As is often the case, the push to improve valvetrain durability starts at the highest levels of professional motorsports. Most sanctioning bodies limit maximum displacement as a means of trying to regulate horsepower levels, and teams inevitably reach a point where maximum RPM, rather than cylinder head airflow, is the limiting factor in power output. Consequently, he who turns the most RPM stands the best chance of winning the race, which explains why NHRA Pro Stock motors are now turning more than 11,000 rpm. Perhaps even more impressive are NASCAR Sprint Cup engines, which turn 9,500-plus rpm reliably for 500 to 600 miles each race. As the lessons learned on track have trickled down into the hot rod market, RPM is now limited more by the size of an enthusiast’s bank account than by the durability of the valvetrain components. The good news is that valvetrain hardware is more durable and affordable than ever.
In essence, the valvetrain is the link between the camshaft and the cylinder heads. Without the valvetrain, there is no valve actuation, and without valve actuation, there is no horsepower. This simple truth helps put the importance of the valvetrain into perspective. The more precisely the valvetrain translates the motion of the camshaft to the valves, the more horsepower it produces. In addition to precision, an optimized valvetrain must perform reliably over extended periods of time. This is far easier said than done, as the valve actuation process is nothing short of violent.
It all starts at the crankshaft, which transfers rotational motion to the camshaft through a crank gear, cam gear, and timing chain. As the camshaft turns, its eccentric lobes push up on the lifters. The reciprocating motion of the lifters then pushes upward on the pushrods and rocker arms, which then pivot like a see-saw to push open the valves. All the while, the entire valvetrain is working against the force of the valvesprings, which attach to the valves with spring retainers and locks.
All of these moving parts make precise and reliable valve actuation extremely challenging, especially when the cumulative mass and deflection of all the components are taken into account. The more the valvetrain deflects, the smaller the cam appears to the engine, as the motion of the cam lobe isn’t precisely transferred to the valve. Consequently, camshaft manufacturers must consider the valvetrain mass and inertia as a whole when designing cam lobe profiles, as the entire valvetrain must work as a single system.
The goal is to get the valve motion to match the designed cam motion. During the initial opening of the valve, the lifter, pushrod, rocker arm, and valvespring are compressed into action. The lobe design has to be quick yet smooth to prevent transferring bad harmonics into the system, which causes springs to surge and potentially destroy them. As the lifter runs up the ramp to the peak of the lobe, the valve is opening farther, the parts are compressing more, and the dynamic loads are getting higher, placing tremendous stress on the system. At maximum lift, the valvetrain is fully compressed and struggling to rebound against the force imparted on it.
At high RPM, the inertia of the entire valvetrain going over the nose of the lobe resists the return spring force, and the lifter tries to hang in the air instead of following the cam profile. To prevent this, the mass of the valvetrain and spring pressure must be sufficient enough to allow the lifter to follow the cam all the way down the ramp. If not, the lifter bounces, which damages the valve seat and sends harmonics through the system that destroy the springs as well as the needles in the roller lifters. This reduces the volumetric efficiency of the engine, as the valve doesn’t seat properly and robs horsepower.
As daunting as all that may seem, selecting the right valvetrain components for a stroker motor is rather straightforward, because camshaft manufacturers have already done most of the homework for you. A cam’s duration and lift specs determine the type of valvesprings that will need to be used. After spring pressure and max valve lift have been established, the balance of the valvetrain components can be selected based on durability and the target RPM range of the engine combo. If that’s not easy enough, cam manufacturers often publish a list of matching valvesprings, retainers, locks, rocker arms, pushrods, lifters, and timing sets to go along with their off-the-shelf camshafts.
The inherent challenge of designing valvetrain components is trying to make them as light as possible to reduce valvetrain inertia while also making them as stiff as possible to reduce deflection. Unlike an overhead cam engine that positions the camshafts on top of the cylinder heads for a more direct actuation of the valvetrain, an OHV motor must transfer the reciprocating motion of the cam lobes from the center of the block all the way up to the cylinder heads by using lifters, pushrods, and rocker arms. More moving parts means more weight, which is a big problem when the entire valvetrain must reverse its direction of travel every time the valves open and close.
By nature, aggressive cam lobe profiles require stiffer spring pressure to keep the lifter seated, but this also increases stress on the valvetrain. That, in turn, increases the potential for deflection, and the stiffer valvetrain hardware required to resist this deflection can increase mass. It’s an ugly cycle, but having too much valvetrain deflection and mass leads to valve float, a condition where the valvetrain can no longer control the motion of the valves. When this happens, the valves open and close erratically, crash into the valve seats, and limit how many RPM an engine can turn. In extreme cases, the valves can slam into the pistons and destroy an entire engine.
In order to prevent valve floating, taking a single-system approach to valvetrain setup works best. For example, because installing stiffer valvesprings increases the loads placed on the rocker arms, pushrods, and lifters, it’s imperative to make sure the rest of the valvetrain is up to par when upgrading just one of the components in the entire system.
The cylindrical slugs of metal that ride the surface of the cam lobes are called lifters, tappets, or followers. They’re retained inside recesses, known as the lifter bores, in the block. Regardless of what you call them, lifters can be classified into four groups: hydraulic flat tappets, hydraulic rollers, mechanical flat tappets, and mechanical rollers. Unlike their flat-tappet counterparts, roller lifters have a roller wheel assembly integrated into the base of their bodies. This substantially reduces friction and allows for much steeper cam lobe profiles. In other words, for any given amount of duration, a roller lifter can handle much more lobe lift. This allows camshaft designers to lift the valves high enough to take advantage of the high-lift flow potential of modern cylinder heads while keeping duration short enough to retain excellent drivability.
All LS-series small-blocks are equipped with hydraulic roller lifters from the factory, and in fact, GM hasn’t installed flat-tappet cams in a production small-block in decades. In the distant past, roller lifters were more prone to failure, as the allocated valvespring pressure loads onto a smaller surface area. Additionally, the roller wheels ride on needle bearings, which present another area of potential failure. Nonetheless, roller lifter technology has improved to the point that these drawbacks have been mostly eliminated. For proof, look no further than any production GM small-block built today, whose roller lifters often last for 200,000 miles. Unless an engine is being built for an obscure racing class that bans roller lifters, LS-series small-blocks are rarely built with flat-tappet lifters.
That said, both mechanical and hydraulic roller lifters are used in the vast majority of stroker Gen III/IV engine combos. Mechanical lifters, or solid lifters, as they are sometimes called, are solid pieces of metal. Because the valvetrain components expand as they heat up, a valvetrain utilizing mechanical lifters must be set up with some slack to accommodate this growth. This clearance, or lash, is measured between the rocker arm and valve stem tip using a feeler gauge. Naturally, this slack makes for noisier valvetrain operation. Conversely, hydraulic lifters incorporate an internal cavity filled with oil and a piston. This hydraulic assembly enables much of the lash to be removed from the valvetrain, because the piston inside the lifter compresses as the valvetrain expands, which eliminates the clatter associated with mechanical lifters. In addition to quieter operation, hydraulic lifters eliminate the need for periodic lash adjustments that mechanical lifters require.
The reduced maintenance and quieter operation offered by hydraulic lifters are the primary reasons why GM uses them in all LS-series small-blocks. Those benefits aside, solid roller lifters offer clear performance advantages, particularly at high RPM. On a typical street/ strip engine, a hydraulic roller system often experiences valve float between 6,500 and 7,000 rpm. Hydraulic lifters are more prone to valve float, due to their greater mass and tendency to pump up at high-RPM. Furthermore, higher valvespring pressure goes a long way in reducing valve float, but there is only so much pressure the piston assembly of a hydraulic lifter can handle.
By comparison, a solid roller lifter’s lower mass and ability to manage greater valvespring pressure make it the clear victor in applications exceeding 6,500 rpm. Although the performance difference between a hydraulic roller system and a solid roller application with similar camshaft specs might be marginal up to about 6,000 rpm, beyond that point the horsepower advantages of a solid roller lifter can easily exceed 50 hp.
When it comes to valvetrain hardware, RPM usually costs money, and that’s definitely the case with a mechanical roller valvetrain. Not only are the stiffer valvesprings necessary to run a solid roller valvetrain more expensive, the rocker arms and the lifters themselves also cost more. Additionally a solid roller valvetrain’s steeper cam lobes and stiffer valvesprings increase the stress on the entire valvetrain, so stronger rocker arms are required, which adds cost.
For example, a set of mild beehive valvesprings for a hydraulic roller cam costs about $200, and super-stiff dual springs for a solid roller cam cost $400. Likewise, a set of $150 stock GM rocker arms will work fine in a hydraulic cam application, but a high-RPM solid roller combo often requires a $1,500 shaftmount rocker arm setup. At the end of the day, solid roller lifters offer irrefutable advantages over their hydraulic counterparts, but they require much more expensive valvetrain hardware.
Although flat-tappet lifters aren’t common in Gen III/IV engine builds, it’s worth noting that a solid flat-tappet lifter system can sometimes outperform a solid roller setup. These instances aren’t common, but certain race classes, such as in circle track, sometimes impose limits on valve lift or duration. In such a scenario, solid flat-tappet lifters can be advantageous over solid roller lifters, because they offer quicker initial lobe acceleration very early in the lift curve. Roller lifters can, indeed, handle higher peak ramp acceleration rates, but solid flattappets have the advantage very early in the lift curve. And, if a racing class limits peak valve lift to, say, .500 inch, then a solid flat-tappet cam might actually perform better than a roller cam.
Solid Rollers for the Street
In many hot rodding circles, solid roller camshafts and street cars don’t mix, and that reputation is well earned. Decades ago, solid roller lifters were plagued with reliability issues, as they were originally designed for high-RPM race use. The only way their needle bearings could be lubricated was from oil thrown up by the crank. Because race cars spend very little time at idle and low RPM, that oiling method worked just fine. However, when people tried to use the lifters on the street, the same lifters that lasted for several seasons in race cars were failing more quickly. Compounding the problem of using solid roller lifters in a street car was that most old roller cams were designed for very high spring loads. This kept everything under control at high RPM with an aggressive cam, but it greatly increased valvetrain load at low speed.
Camshaft manufacturers recognized these problems and pioneered several effective solutions. First, mechanical roller lifters were completely redesigned to include an integrated oil band with a small hole to feed oil down to the needle bearings. Furthermore, the steel used for the axle was greatly increased in strength and redesigned to distribute load more efficiently. Second, new lobe profiles and valvesprings were developed specifically for street use. These new profiles perform very well in the 2,000- to 7,000-rpm range while requiring far less spring pressure than older race profiles. Together, these changes make it possible to now run a solid roller cam in a street car without any of the past reliability issues. Although it is recommended to check valve lash every 5,000 to 6,000 miles with solid lifters, the need to do so is greatly reduced with rocker arms that have poly locks.
The valvesprings sit in recesses, or pockets, machined into the cylinder heads and provide tension upon the valves. Valvesprings force the valves shut against the seats until they’re compressed by the rocker arms, at which point the valves open. Valvesprings attach to the valves with retainers and locks, which center the springs around the valves and keep them in a slightly preloaded state. This load is known as the spring’s seat pressure, and the load the spring provides at maximum valve lift is called open pressure.
The position of the valvesprings in relation to the rest of the valvetrain helps put their importance into perspective. During compression, the springs are responsible for keeping the lifters in contact with the cam lobes so they can precisely follow the contour of the ramps. As the springs rebound after the point of maximum valve lift, their job is to close the valves in a controlled fashion while preventing them from bouncing off their seats. Accomplishing both of these functions requires having just enough spring pressure.
Valvespring pressure is determined by several factors, chief among them are spring rate and load. The rate of a valvespring is simply the force required to compress it 1 inch. For instance, a spring that compress 1 inch under 100 pounds of force has a spring rate of 100 pounds per inch. The spring rate, combined with how much the spring is compressed (load), determines valvespring pressure. For illustrative purposes, let’s take a look at a set of Comp Cams springs designed specifically for LS-series smallblocks, part number 26921. They feature a spring rate of 408 pounds per inch, which, at an installed height of 1.770 inches, yields 135 pounds of seat pressure. Compressed to a height of 1.120 inches, the spring load increases to 400 pounds of open pressure.
Because spring diameter, wire thickness, and the number of active coils affect the spring rate and, therefore, pressure, aftermarket manufacturers are constantly juggling these variables around to establish a wide variety of pressures for a diverse range of applications. Nevertheless, because wire diameter and the number of coils are built into a spring and can’t be changed, the only two factors relevant to engine builders are the diameter of the spring and how much the spring is compressed after installation. That’s because spring compression, or installed height, can be adjusted after the spring has been installed on the cylinder head. Using shims to adjust the installed height serves as a handy fine-tuning tool in achieving target spring pressure. As for spring diameter, the size of the valvespring pocket determines the maximum-diameter spring that can be used. Larger-diameter springs provide more pressure, but they also require the spring pockets to be machined wider, and there’s a physical limit to how much the pockets can be opened up.
With stock Gen III cathedral-port cylinder heads, a 1.250-inch-outsidediameter valvespring is as large as you can go. The valvespring pocket can safely be machined to about 1.450 inches, but anything larger runs the risk of breaking into the ports. Factory rectangle-port castings have more commodious pockets and can be enlarged safely to approximately 1.550 inches. Even so, with the very heavy-duty spring pressures that aggressive camshafts require, sometimes running a larger-diameter valvespring just isn’t enough. Consequently, aftermarket manufacturers offer dual valvesprings that feature a small inner spring that fits inside the primary spring to increase pressure.
Ultimately, optimizing valvespring pressure is a delicate balancing act, as too much pressure will place undue stress on the valvetrain, and not enough pressure will compromise valvetrain stability. Load is just one of the many factors that needs to be addressed when selecting valvesprings, and more isn’t always better. A lower-mass spring with less load often performs far better than a fat spring with more load. The trick is to use a spring that offers just enough pressure to get the job done, and not a pound more. As with camshaft selection, there is no universal rule of thumb to follow when it comes to choosing the right valvesprings. Instead, opting for springs that are proven performers in applications similar to yours will usually suffice. On the other hand, if your combo isn’t exactly mainstream, it’s not a bad idea to seek expert advice to avert potentially catastrophic engine failure. Camshaft manufacturers have thousands of hours invested into developing valvesprings, so it makes sense to tap into their expertise.
Even on a head as mild as a stock LS6 cylinder head, the valvesprings exert 90 pounds of seat pressure. So even at 0 rpm, the valvespring needs to be cinched tightly in place to prevent it from launching off the cylinder head. That’s the job of the retainer, which sits on top of the valvespring and locks into a notch machined into the valvestem with a set of valve locks. Like most production engines, the LS-series small-block utilizes steel retainers that do a fine job in the 6,000 to 7,000 peak RPM that they’re designed for. Beyond that point, however, reducing retainer mass just a few grams can extend an engine’s peak RPM dramatically.
Titanium is the most commonly used material for lightweight retainers, and they weigh roughly 40 percent less than standard steel retainers. This decrease in mass alone is enough to add up to 200 rpm before an engine starts floating the valves. Although that can be the difference between winning and losing in a competitive racing class, the increased cost of titanium retainers makes them cost-prohibitive for most street/strip motors. A set of standard steel retainers can be had for $50, while equivalent titanium units ring up a $250 tab.
To bridge that gap, Comp Cams has recently introduced a new line of lightweight tool steel retainers that offer the best of both worlds. These retainers tap into the company’s NASCAR connections, and according to Comp, each one weighs just 2 to 4 grams more than a titanium retainer. Furthermore, extensive testing has revealed that they are just as durable. The best news is the price, as Comp’s tool steel retainers cost $150 for a set of 16.
A rocker arm is responsible for converting the upward motion of the pushrod into the downward motion that pushes the valve open. To accomplish this, a rocker pivots like a see-saw on a trunion, which acts as a fulcrum. The stiff valvespring pressure and high RPM associated with aggressive camshaft grinds place tremendous loads on the rocker arms. The upward force of the lifters and pushrods works against the pressure of the valvesprings, and steeper cam lobe profiles and stiffer springs only compound the situation.
Fortunately, the factory GM rocker arms are excellent pieces of hardware. All LS-series small-blocks come equipped with 1.7:1 roller rocker arms from the factory. The only exception is the LS7, which uses 1.8:1 roller rockers. Compared to those of the Gen I small-block Chevys, most of which were equipped with stamped 1.5:1 rockers, Gen III/IV rocker arms are more like race hardware than typical factory equipment.
Stock LS rockers perform extremely well at engine speeds up to 7,000 rpm. For many stroker combinations, it’s not even necessary to replace the stock rockers with aftermarket units. They do have their limits, however, and with elevated valvespring pressures and sustained 7,000-plus-rpm operation, aftermarket rocker arms are a wise investment. Another drawback of stock rocker arms is that they are not adjustable. That’s great for reducing manufacturing costs in a high-production, volume environment, but in performance applications where the valvetrain geometry has been altered, the only way to adjust the stock rockers is by changing pushrod length and the rocker stand height.
The three primary advantages that aftermarket rockers offer over their stock counterparts are reduced deflection, lower mass, and a higher multiplication ratio. Deflection can be reduced using several different methods. Aftermarket rocker arms are built using stronger alloys than those in production units, which reduces flex. Also, production and entry-level aftermarket rocker arms are pedestal-mount designs that attach to the cylinder heads using bolts or studs. In this type of arrangement, the stud is often the area of the valvetrain that’s most prone to flex. The aftermarket offers several solutions, with companies, such as Comp Cams and ARP, offering stiffer 3/8-inch rocker studs that replace the factory bolts.
A decades-old technique is to bolt a girdle on top of the rocker arm assembly. This dramatically reduces deflection and stabilizes the valvetrain. With quality aftermarket rocker arms, studs, and a girdle, a pedestal-mount rocker system can operate safely at 8,000 rpm. Even so, such an arrangement is fairly uncommon in a typical LS stroker build, because, in recent years, more costeffective shaft-mount rocker arm systems have entered the market.
Shaft-mount rockers pivot on a centrally mounted shaft that’s bolted to the cylinder head. The body of the rockers actually slides around the shaft, decreasing friction and increasing mounting stiffness. Although shaftmount rockers don’t make horsepower in and of themselves, they allow an engine builder to make more power by providing a stable, high-RPM valvetrain platform to work with. An entry-level shaft-mount rocker system costs about $800 to $1,000, which is only marginally more than the combined cost of pedestal-mount rockers, aftermarket studs, and a girdle. Furthermore, stud girdles also require using a valve cover spacer, which is another strike against a pedestal-mount rocker system in a high-RPM application.
Because valve lift is simply lobe lift multiplied by the rocker arm ratio, there are several lobe-and-rocker ratio combinations that can be used to achieve a target valve lift figure. Some engine combinations utilize lots of lobe lift with a relatively conservative rocker arm ratio, and others feature conservative lobe lift and a very aggressive rocker arm ratio. One combo isn’t necessarily better than another, and there is a time and place for each. Adding the acceleration speed with the rocker is easier on harmonics and valvetrain stability in relation to RPM. With a higher ratio, the rocker is responsible for valvetrain acceleration, which allows for a gentler cam lobe ramp design. Generally, high ratios can be used to open the valve off the seat more quickly, and lower ratios can be used to stabilize a valvetrain that is out of control. Using a lower rocker ratio reduces the load on the pushrod and thereby helps increase stiffness. Ultimately, cam manufacturers design their lobe profiles around specific rocker ratios in mind, so for the average hot rodder, the issue isn’t worth splitting hairs over.
In some respects, pushrods represent the inherent inefficiency of mounting a camshaft in the middle of the block and having it transfer the reciprocating motion of the cam lobes and lifters all the way to the top of the cylinder heads. Breakthroughs in valvetrain technology have helped overcome this setback, and improvements to the pushrod itself are part of that equation. As with the rest of the valvetrain, pushrods must be stiff to resist deflection and accurately translate cam motion to the valves, but they also need to be lightweight to reduce inertia.
To accomplish this, pushrods come in a dizzying array of steel alloys, such as 1010, 4130, and 4340. The other two ingredients to pushrod stiffness are diameter and wall thickness. All stock LS-series small-blocks come equipped with 5/16-inch pushrods, except for the LS7, which has 3/8-inch pushrods. Although factory pushrods work fine under normal operating conditions, sustained highRPM operation and a few missed shifts can bend them up rather quicker. Consequently, aftermarket pushrods from companies, such as Comp Cams, Manley, Trick Flow, and Isky, are highly recommended in all stroker motor combinations. In addition to using superior alloys, aftermarket pushrods are offered in both 5/16- and 3/8-inch diameters with wall thicknesses ranging from .080 to .125 inch.
Because the length of the pushrod determines the rocker-arm-to-valve-tip geometry, a pushrod’s length is just as important as its stiffness and mass. Block deck height, cylinder head deck thickness, rocker arm design, camshaft base circle size, lifter design, and valve stem length all affect the correct pushrod length of an engine combo. Optimizing pushrod length allows the rocker arm tip to press on the center of the valve stem, and aftermarket manufacturers sell tools that make it easy to measure for the proper length. Tools to check pushrod length are basically threaded rod assemblies that can be varied in height until the correct pushrod length has been determined. Most of the time, the correct-length pushrods are offered as off-the-shelf items, but when they aren’t, manufacturers offer custom-length units at a very reasonable price.
In order for an engine to operate properly, the camshaft’s rotation must be perfectly synchronized with the crankshaft. It’s up to the timing set to get the job done, but the stiffer valvesprings used in high-performance engine combinations increase load and, therefore, the potential for deflection. The good news is that there are dozens of factory and aftermarket timing set options that can easily handle the most demanding of engine combinations.
On the factory front, GM redesigned the timing chain assembly with the introduction of the LS2 in 2005. The singleroller LS2 unit features a stronger alloy steel and thicker sideplates for significant improvements in strength over the Gen III design. This chain has proven reliable in applications having slightly more than 400 pounds of valvespring open pressure.
At anything beyond 400 pounds, an aftermarket timing set buys cheap insurance. They’re offered from companies, such as Comp Cams, Trick Flow, SLP, Manley, and Katech, in both single- and double-roller applications. Generally, double-roller sets are more durable, but some single-roller timing sets are also extremely durable. Aftermarket timing sets have adjustable cam and/or crank gears, making it possible to advance or retard the cam manually.
In addition to offering them as a traditional timing chain setup, aftermarket companies also offer timing sets as belt drives and gear drives. Belt drives are usually mounted outside of the timing cover, which allows for quick and easy timing changes without removing the timing cover or water pump. Also, this feature greatly simplifies the cam swap process. Additionally, belt drives reduce frictional losses and oil windage while helping to dampen engine harmonics. Their biggest downside is that they cost two to three times as much as a chaindrive setup. Gear drive timing sets use a series of gears between the cam and crank sprockets to synchronize them. They offer the ultimate in durability and timing precision, but they also generate lots of noise.
From the factory, LS-series smallblocks come with four different types of timing covers. The Gen III timing cover, also known as the LS1/LS6 cover, is the most basic and has no provisions for a cam sensor. The standard Gen IV timing cover, which is used on most non-VVT Gen IV engines, is similar to the Gen III unit but with an integrated cam sensor. The LS7’s dry sump oiling system uses a larger pump assembly, and it requires its own timing cover for extra clearance. The VVT-equipped Gen IV timing cover has an electric solenoid assembly built in that actuates the oil control valve in the camshaft.
All factory timing covers are durable cast-aluminum units, and there’s no real performance advantage to upgrading them. However, with some aftermarket double-roller timing sets, it’s necessary to grind down the factory timing cover for additional clearance. Two-piece aftermarket timing covers don’t offer much in the way of performance gains, but they do make swapping out cams much easier.
Although it’s possible to calculate spring pressure by hand using simple arithmetic, it’s far more practical to simply look up a manufacturer’s published valvesprings specs or to measure spring pressure in a testing tool.
Written by Barry Kluczyk and Posted with Permission of CarTechBooks