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Comp Cams' Billy Godbold Talks Cam Theory And Valvetrain Tech

Stephen Kim Dec 1, 2007 0 Comment(s)
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Simply tweaking an engine's cam specs can transform it from a bring-on-those-3.08:1-gears torque beast to a give-me-rpm-or-give-me-death screamer. In other words, a cam has the power to impart bipolar disorder. Although cylinder heads may more profoundly influence how much power a motor produces, no other component determines how that power is produced and defines the overall character of a motor as significantly as the camshaft. With the thousands of camshaft options available for your Mouse or Rat, it pays to study up.

There's far more to understanding bumpsticks than duration, lift, and lobe separation angle (LSA). Furthermore, optimum camshaft performance is impossible without the valvetrain hardware to back it all up. Recent breakthroughs in valvetrain technology have allowed engineers to push the envelope of camshaft design. Some trends offer genuine performance benefits, while others don't quite live up to the hype. Fortunately, we've compiled a cheat sheet for you, courtesy of Billy Godbold of Comp Cams, to distinguish fact from fiction.

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"We have more than 8,000 lobe designs at Comp. Many of these have similar duration and lift figures, but their acceleration curves are different. On the R&D side, we spend an extensive amount of time trying to tweak the acceleration-curve shape of lobes to maximize engine speed for a given area, duration, and lift than we do trying to pick the right overall duration, lift, and lobe separation angle. An analogy using cylinder heads would be runner cc and valve size. There are hundreds of different ways to make a 180cc small-block head with 2.02-inch intake valves. However, the real trick is in the port shape. In cam design, it's important to understand where you can really push the tappet and valve hard without upsetting the spring, and know where to take it easy. This is why we have such a large R&D staff, two Spintrons, and our own data analysis software. We also have invested a substantial amount in valvetrain modeling software. Anyone can look at our Web site and copy lift, duration, and lobe separation angle. The question is, can you get as much air in the engine and keep it all together dependably at the required engine speed?"

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Duration VS. Lift
Increasing either duration or lift can improve power, so how do you know which is more critical when selecting a camshaft? "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," explains Billy. "However, they are typically closely related in flat-tappet profiles due to the velocity limit imposed by the tappet, and there is only so much lobe lift you can get for a given amount of duration. Choose the duration based mostly on the engine's target operating rpm and the lift based mostly on cylinder head flow." In addition to that, other engine factors to consider when selecting lobes include valvespring selection and valvetrain durability.

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High-Lift Lobes
A growing trend is packing a whole lot of lift into relatively short-duration camshafts. This has led some enthusiasts to postulate that lift takes precedence over duration when spec'ing out a cam, but that isn't necessarily the case. "Now that improvements in valvetrain technology allow increasing lift with short-duration cams through steeper lobes, we tend to do that more often," says Billy. Modern cylinder heads flow very well in the 0.500- to 0.650-inch range, but that wasn't always true a few years ago. "It used to be the case that we needed large-duration cams to get into that lift range. Now we can run shorter-duration cams and still lift the valve into the range where the heads flow best. This gives both the drivability missing in large cams and the power we could not previously achieve with short-lift cams."

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Installed Height
"Many years ago, springs were not designed around the solid stress state, when the coils are pressed together. That has since changed, and the damping effect of springs is used to reduce any surge waves resulting from a rapid valve opening as the spring approaches a solid state. If you do not check the solid height of the spring and the installed height, then you can't control how close you get to a solid state. Likewise, if you don't check installed height, you can only guess how much load you have on the seat or anywhere else to control the valve. Obviously, if you try to lift the valve past solid height, then something has to fail. However, now we know that if you are too far away from solid, you will not be able to use the coil interaction over the nose to reduce surge. Depending on the application, it's ideal to have the spring come to within 0.060-0.120 inch of solid over the nose."

Some industry insiders tout lofting as a hot new fad in engine building, but not everyone is convinced. Lofting is a method of increasing lift where the lifter momentarily loses some contact with the nose of the cam lobe. "We have yet to find an application in which you wouldn't be better off with the increased lift designed right into the cam or rocker arm instead to allow controlled valvetrain operation," Billy opines. "Lofting does not increase duration, and in fact, the increased loading associated with it often decreases duration at high speed. Hence, it is not a poor man's variable valve timing system as thought some 10 years ago in NASCAR."

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Valve Events
Of the different valve events-intake valve opening, intake valve closing, exhaust valve opening, and exhaust valve closing-intake valve closing is the most important in terms of the rpm at which the engine will make peak power. "When you close the intake valve, air stops entering the cylinder," says Billy. "If you close the intake valve early, you will make more low-speed torque because less air escapes back into the manifold at low rpm, since the air has a slower intake-charge velocity and more time per degree of engine rotation to fill the cylinders. At high rpm, you need more time to fill the cylinder, so a later intake valve closing helps tune the engine more at high rpm, where peak power is made. The cam phasers found on L92 GM applications are great because they let you move the cam around to better tune at each rpm range."

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Lobe Separation Angle
One of the most misunderstood aspects of camshafts is how LSA affects the power curve. Typically, wider-LSA cams have a wider powerband, reduced maximum cylinder pressure, decreased dynamic compression, and better idle quality and vacuum. On the other hand, tighter-LSA cams have a narrower powerband, increased cylinder pressure and dynamic compression, and reduced idle quality and vacuum. However, there are some exceptions to the rule. "As we found when testing our Thumpr cams, we can widen the power curve on a tight-LSA cam by increasing the exhaust duration," explains Billy. "The downside is that it will greatly decrease vacuum and is not a good path for EFI applications. Taking all these factors into account, it's not surprising that some cams in production motors today are on 120-degree-or-wider LSA."

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Spring Pressure
Like choosing a camshaft, going with valvesprings that have produced good results in applications similar to yours will often 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. "I know people want some sort of rule of thumb when selecting a set of springs, but unfortunately, it's not nearly that simple," says Billy. Load is just one of the factors that need to be considered when selecting valvesprings. "A lighter (mass) spring with less load is often far better than a fat spring with more load. There is also a tremendous consequence of the spring's natural frequency, the speed it vibrates at when struck by an outside force. This is another place where you should really trust the cam company to tell you what spring a certain lobe profile likes."

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Beehive Springs
Simple tweaks to a venerable design can yield big dividends in performance, and beehive springs are a perfect example. Elementary physics is why they work so well. "The difference in the active mass of a beehive spring compared to that of a conventional dual spring for the same application is more than the difference between a steel and a titanium valve in the same application," explains Billy. "Beehive springs take more mass off the top of the spring, which is the area that moves the fastest and the longest distance. Beehives are also naturally progressive in rate, therefore each coil vibrates at a slightly different frequency. This keeps the spring from going into resonance like a conventional spring and results in more of a self-damping effect."

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When To Go Custom
According to Billy, cam manufacturers have done most of the homework for you. He recommends off-the-shelf camshafts whenever the application has something available for it that was designed with a similar application in mind. "These cams have a great deal of testing and are going to be better all-around performers than a custom cam 99 percent of the time," he states. "If we could make a better cam for a typical 383ci small-block Chevy, a ZZ4 crate motor, a 5.7L LS1 with aftermarket heads, or even late-model dirt-track and Super Comp-style NHRA applications, you can bet we would already have that part tested, verified, and included in our next catalog. However, chances are, if you're building a turbocharged NASCAR-style SB2 Chevy for a Suburban that you want to take on a cruise or bracket race, we are going to need to grind something custom, since you're doing something very unique. Likewise, we do custom grinds on many land speed and 24-hour endurance racing camshafts." Talking to someone at a cam company you trust is the best way to figure out whether or not you need a custom cam.

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Solid-Roller Reliability
"Originally, solid roller lifters were designed for high-rpm race use. The only source for oiling the needle bearings was from oil thrown up by the crank. That works great at speed, but not so well near idle. The same lifters that lasted for several seasons in bracket and circle-track race cars were failing more quickly just going to the local cruise night and back. The other factor was spring load. Most of the older roller cams were designed for use with high spring loads. This helps keep everything under control at high speed with an aggressive cam, but greatly increases the load at low speed. The solution was twofold. First, we totally redesigned all of our roller lifters, both street and race, so that any lifter we sell with an oil band has a small hole feeding oil down to the needles. At the same time, we greatly improved the steel used for the axle while providing stricter control of the needle sizes to better distribute the load. The second part of the solution was to develop new profiles and springs for street roller use. These new profiles work great in the 2,000-7,000 rpm band while requiring far less spring pressure than the older race profiles. Together, these changes make it so the only maintenance consideration in buying a street roller from Comp Cams is whether or not you want to have to set and check lash. I recommend going through and checking the valves along with your oil changes. If you run Poly-Locks, this is probably overkill. Once you know what the engine sounds like, you will most likely be able to distinguish the tick of loose lash."

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Measuring Duration
Cam manufacturers publish both advertised duration and duration at 0.050 inch of lift, which can be a bit confusing, but the difference between the two figures is simple. Advertised duration can be given at any lift point. Cam companies pick different points at which to measure, making advertised duration figures relatively useless. "A simple measurement was needed to keep everybody honest about how big their cams really were, and that's how duration at 0.050 came about," explains Billy. "Low-lift numbers between 0.001 and 0.020 inch tell an engine builder a great deal about vacuum and responsiveness, while high-lift numbers greater than 0.200 inch tell an engine builder more about power potential. The 0.050-inch number is relatively easy to measure with a dial indicator and a degree wheel. It does the best job of predicting the operating range of a given lobe in a specified application." Note that this is tappet lift and not valve lift, so those numbers can tell a very different story, depending on rocker-arm ratio.

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Shaft-Mount Rockers
With shaft-mounted rockers becoming more popular and affordable, are they a worthwhile investment for a typical street/ strip motor? Stud-mounted rockers inherently lack the stiffness of a tied-together shaft system. Some racing classes and budgets require stud-mounted rockers, and some of these applications can operate in the 8,000-rpm range even without a stud girdle. "Typically, we recommend either shaft-mount rockers or a girdle above 7,000 rpm, but our Pro Magnum rockers are very stiff even compared with a good shaft rocker setup," explains Billy. "With 71/416-inch studs and a girdle, you'd be surprised how close you can come to the same performance as with a shaft-mount system. It often becomes a question of where to spend your performance dollar. Shaft rockers become more and more attractive when all the other bases of an engine buildup have already been covered."

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Retainer Weight
In an effort to promote valvetrain stability, many weekend warriors pay a hefty premium for titanium retainers, but innovations currently in the works promise to give racers the benefits of titanium at a fraction of the cost. "We could go racing in every series without titanium retainers if necessary, and lightweight steel retainers are already very far down the production chain and should be available soon," says Billy. "Some steel retainers are already in use in NASCAR, but these are very thin and made out of special alloy that costs considerably more than titanium. Our lightweight steel retainers should fall somewhere in between standard steel and titanium retainers in terms of cost while weighing within a gram or two of a titanium retainer. And the durability will be much better than with a titanium retainer."

Solid VS. Hydraulic Lifters
Thanks to recent innovations in reducing valvetrain mass, hydraulic roller cams are revving higher than ever. However, for sheer performance, solid lifters still reign supreme for one simple reason. "Stiffer valvetrain components provide better control as engine speed increases, and this increases the rpm limit of the valvetrain, since deflection is decreased," says Billy. Deflection robs the engine of the valve-open duration it needs as engine speed increases. Bigger cams are conducive to high-rpm performance, but as rpm increases, the loads on the lifters, pushrods, and rockers increase as well. Due to deflection, this means the engine sees a smaller cam at high rpm. "Up to about 6,000 rpm, the inherent deflection of hydraulic lifters isn't an issue, and many are used satisfactorily to about 7,000-7,500 rpm. However, even if valvetrain control is maintained to 7,000 rpm, horsepower improvements are still common when switching to a solid lifter."

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Rocker Ratio"For a while, everyone was in love with big rocker ratios because they helped so much in NASCAR applications. Now we tend to use as little rocker ratio as required to get the motion we want in most applications. This reduces the load on the pushrod, thereby helping increase stiffness. Not wanting to sacrifice stiffness is what kept Pro Stock engine builders from going up to the 2.5:1 rocker ratios seen occasionally in NASCAR Cup applications; however, those ratios are still in the 1.7:1-2.0:1 range, depending on the cam journal's size. Note that up to 70mm (2.756-inch)-diameter camshaft journals are used in NHRA Pro Stock. The diameter limit in NASCAR is 60mm (2.3622-inch), a stock LS1 has 55mm journals (2.165-inch), a stock big-block Chevy is 1.949-inch, and a small-block Chevy uses a stock 1.868-inch journal diameter. The bigger journals allow roller lobes with more tappet acceleration and more lift. If we need more lift and acceleration at the valve and can't increase what we are doing to the tappet, then the rocker ratio must be increased. Again, we are dealing with a balancing act, and there are no perfect answers."


Comp Cams
Memphis, TN 38118


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Comp Cams' Billy Godbold Talks Cam Theory And Valvetrain Tech
Stephen Kim Dec 1, 2007


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