How To Increase Engine RPM - How It Works
Modern race engines turn 9,500-plus rpm with ease, and here’s how they do it
From the November, 2011 issue of Chevy High Performance
By Stephen Kim
It’s all about rpm, baby. In just about every racing class in existence that limits maximum displacement, the quest to turn more rpm than the next guy rules the day. In classes where power adders are prohibited, it’s quite easy to understand why this is the case. Once an engine builder has squeezed every last cfm out of a cylinder head, and torque output plateaus, the only way to increase horsepower is to turn more rpm. For proof, you needn’t look farther than an 11,000-rpm NHRA Pro Stock motor, or the 9,500-rpm mills in NASCAR Sprint Cup. The most extreme example of the importance of rpm is in Formula 1. Not long after the sanctioning body cut down max displacement to 2.4 liters in 2006, engines started spinning up to 20,000 rpm. Consequently, now in F1 there’s a cap on both displacement and maximum rpm. As impressive as those lofty revs may be, the use of pneumatic valvesprings in F1 motors makes them difficult to relate to for 99.9 percent of hot rodders. In some respects, it’s much more difficult to turn half as many rpm with mechanical springs. To learn the intricacies of building an ultrahigh-rpm valvetrain, we contacted some of the best in the business. Our panel of experts includes Judson Massingill of the School of Automotive Machinists, Darin Morgan of Reher-Morrison, Phil Elliot of T&D Machine, and COMP Cams. Follow along as we show you how to give your tach a beat-down.
Judson Massingill: Valvetrain technology has gradually progressed over the years, addressing one weak link after the next. In the late ’80s, we had the ramp technology built into the cam lobes that would have enabled the level of rpm engines are turning today, but we didn’t have the valvesprings to control them. Then by the early ’90s, the valvesprings were much improved, but the lifters started breaking due to all the additional spring pressure. Typically, the axles for the roller wheels or the axle supports were the first area to fail. To address this issue, the aftermarket came out with true race lifters that moved the limit of rpm back to the springs. At this time, the valvespring and lifter technology were adequate for the rpm motors were turning, but racers being racers, they always tried to wring a couple of hundred extra rpm out of their motor. If a company like COMP Cams tested a valvetrain to 9,000 rpm on a Spintron, sure enough, racers would spin their motors to 9,200 rpm. At this point, the weak link became the rocker arms. The stud-mounted rockers of the day just weren’t able to handle the spring loads and rpm that race motors demanded. Once again, the aftermarket responded by developing shaft-mounted rocker arms. Shaft-mount rockers were around long before this time, but they weren’t really necessary because we didn’t have the spring and lifter technology to take advantage of them. With the rocker issue solved, that put the rpm limitation back on the valvesprings. As you can see, it’s not a single component that’s responsible for what has enabled modern race engines to turn more rpm than anyone could have imagined just 5 to 10 years ago. It’s a tapestry of elements that had to come together to make it happen.
Darin Morgan: Valvetrain technology has come a long way in the last 10 years, but like anything else in the development process, you can’t put your finger on one single thing that’s responsible for the forward progress. It’s been a combination of many small advances and failures that got us to the point where we are now. Back 20-25 years ago, we were running stock diameter cams that had lots of resonance. Even if we had the best springs in world, we couldn’t turn more than 9,500 rpm. As soon as engine builders stepped up to 55mm cores, the valvespring technology wasn’t there. Between 1999 and 2003 is when big changes started to happen. By then, we had even larger 60mm cam cores along with the valvespring and cam lobe ramp technology to turn lots of rpm. Before, we used to brutalize the valvetrain, which is the wrong way to do it. Now we finesse the valvetrain to loft it over the nose of the cam. It’s hard to predict where things will go in the future, but the current trend is stepping up to larger core cams and using lifters with bigger wheels to improve valvetrain control at higher rpm. At the Pro Stock level, we start losing control at 10,800 rpm. For engines in the 350 to 380ci range, the ceiling is 11,000 rpm. At Reher-Morrison, we just built a 363ci small-block that makes 1,040 hp at 10,100 rpm naturally aspirated. Numbers like that would have been unheard of just five years ago. The important thing to remember is that every engine is its own animal. You can’t take the valvetrain from one engine and put it in another one and expect it to work perfectly.
As rpm and valvespring pressure increases, the studs on a pedestal-mount rocker arm’s setup will flex and eventually break. Mounting the rocker arms on a shaft instead of a stud increases rigidity tremendously, and therefore shaft-mount rockers are a must in high-rpm race motors. That said, rigidity is just part of the equation. Shaft-mount rockers also make it much easier to achieve proper valvetrain geometry. With a stud-mounted rocker system, the only way you can adjust the geometry is with different length pushrods. On the other hand, with shaft-mounted rockers the centerline of the pivot point of the rockers can be positioned perfectly in relation to the tip of the valve, since the rockers are mounted on a stand. All you have to do then is measure for the correct length pushrods afterward. It is this combination of improved geometry and rigidity with shaft-mount rockers that enable turning more rpm.
Phil Elliot: Valvetrain stability is what everyone is after, and shaft-mount rockers are a great way to accomplish this. Years ago, people put plexiglass on valve covers, and recorded the valvetrain movement with a high-speed camera. They were scared by how much things moved around even with stud girdles. This reinforced what racers had known all along, which is that stud-mounted rockers didn’t provide enough stability in race motors. The notion that you need to step up to shaft-mount rockers at a certain rpm is a bit misleading. The stress imparted on a rocker arm is a product of both spring pressure and rpm. Spring pressure is actually what tries to rip the stud out of head. Fortunately, race engine builders don’t have to deal with all the design parameters that the OEMs have to. In racing we don’t need to worry about whether or not the valve covers will fit under an A/C compressor. We just build a new valve cover that will fit around the cylinder head and rocker arms.
Darin Morgan: At the Pro Stock and Comp Eliminator level, the increases in horsepower we’re seeing today are directly related to the loft curve built into the cam. Do it right, and it’s like having a variable-duration camshaft. At 8,000 rpm is where the loft curve comes into play. Generally, a smooth loft curve will give an additional 0.008 inch of lobe lift by 8,000 rpm. That figure increases with rpm, so by 10,000 rpm you end up with an extra 10-15 degrees of duration. However, accomplishing this is much easier said than done. You want as much valve speed as you can get, but you have to balance it with the proper rate of valve acceleration to maintain valvetrain control. The rate of the springs, and the weight of the coils, retainers, and locks must all be optimized. With larger base circle cams we can now get up to 0.600-inch lobe lift. This allows us to use lower ratio rockers so the initial valve acceleration isn’t as quick, which helps stabilize the valvetrain.
Getting the valvesprings to live isn’t difficult if you have a limited amount of valve lift. However, cylinder head technology has improved dramatically in the last 10-15 years, so now we’re picking up the valves much more than ever before. That puts much more stress on the valvesprings. For a while, the focus was on the metallurgy, wire thickness, and alloy of the springs, but several years ago manufacturers realized that impurities in the spring wire were causing them to break. As a result, using clean wire in the springs is a top priority these days. Furthermore, what we’ve learned in the last four to five years is that you don’t need enormous spring diameters anymore. Not too long ago, we used to have double- and triple-duty springs with 1.600-inch diameters. What happened was that the springs were getting so big and heavy that you needed higher spring rates just to control the weight of the spring. With the better, cleaner metal we have these days, engine builders are using smaller-diameter springs. Smaller springs also let you use smaller retainers, which further reduces valvetrain mass. A great example of this is a beehive valvespring. By reducing the diameter of the top of the spring, it cuts down on mass as well. Our ’99 Camaro drag car has an LS motor that turns 9,600 rpm. The exhaust valvesprings are just 1.550 inches, but they have 1,000 pounds of open pressure.
COMP Cams: In recent years, a lot of progress has been made with regard to valvesprings. One of the newer trends is that we are now designing springs for specific applications. In the past, we would try to find a spring that we thought would suit a specific engine combo. In a lot of cases now, we will create a clean-sheet design to get the spring ideally matched to the rest of the system. The biggest advantage to the newer springs is the reduction of mass. I’ll also say we have only started to scratch the surface with regard to spring design and materials. The metallurgy, spring design, and overall size of the spring all contribute to the performance of a spring.
You can spend all kinds of time and money designing the best cam in the world, but unless you can get the valve to properly follow the lobe profile, all that R&D work is worthless. Any flex in the valvetrain means that the valves are not doing what the cam wants them to do. The goal is to have extremely stiff parts, and this is particularly true with the pushrods. Up until the early ’90s, engine builders thought that as long as the pushrod didn’t bend, everything was OK. Now we’ve learned that even if a pushrod doesn’t bend, it can still flex tremendously. To combat this, the trend these days is to use giant diameter pushrods. In racing classes that allow it, using a shorter deck height block is also common. This allows using shorter pushrods, which reduces both pushrod flex and mass. In fact, GM Performance Parts sells low deck height small-block Chevy blocks that have an 8.325-inch-tall deck opposed to a standard 9.025-inch tall deck.
Darin Morgan: The Spintron is a great tool that helps simulate valvetrain dynamics, but it by no means has the final answer for everything. Interestingly, a loft curve that looks great on the Spintron does not necessarily correlate to good numbers on the dyno and at the track numbers. That’s because a Spintron can’t simulate the crankshaft pulsations that are transmitted into the cam belt and valvetrain. It’s just another example of why there is no substitute for real-world testing. Only after dyno testing can you begin fine-tuning the loft curve.
Judson Massingill: Camshafts with larger journal diameters definitely decrease how much the cam flexes, but one of the biggest advantages of larger journals is much simpler to understand. When you’re sliding a cam into a block, the amount of lift you can pack into the lobes is limited by the size of the cam bores. If you make the lobes too big, the cam won’t physically fit inside the block. That’s where big journal cams come into play. Many aftermarket blocks are available with larger diameter cam bores. This allows installing a cam with larger, more aggressive lobes. To achieve any given amount of valve lift, you generally want the most lobe lift as possible with the lowest rocker arm ratio possible to help stabilize the valvetrain. The reason why NASCAR Sprint Cup teams use 2.4:1 rockers is because they have to run flat-tappet cams that can’t accelerate the lifters as fast as a roller lifter motor. Since they can’t run as much lobe lift as they’d like to, they have to make up for it with higher-ratio rockers. If you’re racing in a class that allows it, using a larger journal cam with bigger lobes and a lower rocker ratio is a better way of achieving high valve lifts. With a 50mm cam, about .440-inch lobe lift is the limit and with a 60mm cam you can get about 0.590-inch lobe lift.
COMP Cams: The barrel diameter of a camshaft plays a big role in the overall stiffness of the valvetrain. In high-rpm applications, it’s best to opt for the largest journal diameter you can get for a specific engine. The base circle of a cam lobe is determined by journal diameter and lobe lift. When starting from a standard journal, whether small- or big-block, going to a larger journal will increase your barrel diameter and base circle size. There are blocks available now that feature raised cam locations. Furthermore, I would advise anyone not to let the stroke or rods dictate your base circle size. When designing the 400 small-block, there is a reason why Chevrolet made changes to the connecting rod in order to clear the cam instead of using a smaller base circle cam. Doing so would have been cheaper than designing a new connecting rod, but bigger is always better with regard to the size of the base circle.
Every component in the valvetrain has a natural resonant frequency, so you have to design a motor to avoid those points. Sometimes, the only way to do that is through trial and error. One example that comes to mind is a particular valvespring we used in one of our crate motors that worked great in drag cars. When those same motors were used in boats, however, the springs started breaking. What we discovered was that the springs had a natural resonance at 7,400 rpm, and if held there long enough, they would get excited and eventually break. That wasn’t an issue in a drag application, but became a problem in boat motors that ran at sustained engine speeds. The adverse effects of resonance are also why it’s so important to use the stiffest pushrods possible. When a pushrod flexes, it stores energy and then releases it later in the lift curve, causing resonance. Reducing weight isn’t as important on the pushrod side of a rocker as on the valve side, so we now use large 9/16- and 3/4-inch diameter pushrods on high-rpm race motors.
COMP Cams: Reducing mass, or weight, is more critical on the valve side of the rocker arm than on the pushrod side. From the rocker arm to the lifter, increasing stiffness will be more advantageous than reducing mass every time. The goal here isn’t to go after the lightest weight parts when considering a lifter or pushrod. The top priority is increasing stiffness and reducing flex. With regards to pushrods, I would recommend going with the largest diameter, thickest-wall pushrods you can fit in an engine. On the valve side of the rocker arm, weight is much more important. Here, it’s critical to get the locks, retainers, and springs as light as possible to reduce inertia.
Judson Massingill: After a valve is accelerated to maximum lift, it comes to a stop and then completely reverses direction as it closes. This makes it difficult to stabilize the valvetrain and keep it out of float since it is constantly fighting this inertia. That’s why reducing valvetrain mass is so important. Titanium engine parts have been around since the ’80s, but now they’re more readily available. To shave every last gram possible, modern race engines have titanium valves, retainers, and locks. Engineers are literally looking everywhere to reduce mass. It wasn’t enough to just make a valve out of titanium. Valve manufacturers started reducing the diameter of the stem, down the 7mm in some cases, and now they’re hollowing out the stems as well. That might seem extreme, but a motor doesn’t know what cam it has in it. All it knows is valve movement, and reducing mass and inertia is critical to achieving valvetrain stability. To put things into perspective, there’s a story of a motor Richard Childress Racing built for its NASCAR Sprint Cup cars several years ago. It had a cam that was worth 8-10 hp more than the grinds they were using before, but the motor would only last 300 miles before the valvetrain broke. By simply removing three grams off the valve side of the rockers, the motors lasted the full 500-mile race distance.
Phil Elliot: Every time a rocker arm moves, it has to start, stop, and then change directions. Naturally, at T&D we’re always trying to reduce mass as much as possible to cut down on inertia. When you remove mass, it’s easier to keep the valvetrain in control. In addition to working closely with race teams, we perform extensive stress tests to see how much we can get away with. We put our rockers through fracture tests that bend the rockers until they break. Likewise, we put our parts in Sprintrons and really try to wreck them. An analogy we like to use is that if a 2x6 is too big, then we use a 2x4 instead. Even so, you can’t take things too far and compromise durability. Lighter is better to point, but parts can’t get too light.
Judson Massingill: Oftentimes it’s a combination of lots of little tricks that help extend the rpm potential of an engine. For example, longer valves enable using taller valvesprings. Likewise, these days it’s common to run cupped pushrods in race motors. Instead of having a cupped section in the rocker arms, with cupped pushrods the ball portion is on the rocker arm and the end of the pushrod is cupped. This allows running much higher rocker arm ratios and spring pressures before everything binds up. Circle track racers were the first to experiment with cupped pushrods, and now they’re trickling over to drag motors as well. Interestingly, some factory FE Ford and Chrysler motors used cupped pushrods. Another trick we’ve learned from the NASCAR guys is building aluminum tubes with oiling jets into the valve covers that direct oil directly onto the valvesprings. This helps keep them cool and extends durability. Another benefit of this arrangement is that it allows running less oil to the top end of the motor. While we’re on the topic of springs, it’s worth mentioning that we’re no longer setting them up like we used to. In the past, we used to think that running them close to coil bind was a bad thing. Now we’ve learned that in motors that turn 9,300 rpm or more, regardless of what the spring pressure is, we set them up 0.060 inch from coil bind. This helps kill harmful valvetrain harmonics. CHP