Building a bulletproof racing valvetrain requires attention to many moving parts.

 

When a four-stroke, two-valve-per-cylinder, pushrod engine operates at 6,000 rpm, the intake and exhaust valves are opening and closing 50 times per second. And that's at only six grand, where some engines are just waking up.

Building a valvetrain to handle those kinds of stresses and at the same time produce power under competition conditions requires paying attention to a lot of moving parts.

"It isn't just flow; it isn't just lift. There are a lot of things you have to worry about in the valvetrain as an engine builder," said Claude Holguin of CHE Precision, Newbury Park, California. "All this has to work together."

Holguin was one of several valvetrain experts we spoke to about how to keep a valvetrain from failing under race conditions. In what follows, we'll unpack in greater detail those "lots of things" he referred to.

 

Lobe Design

Camshaft lobe design plays a big role in "being easy on valvetrain parts," said Bobby Biancaniello of Cam Motion, Baton Rouge, Louisiana. "To look at the valvetrain and why valvetrain parts fail, you have to start with the camshaft, especially in hydraulic roller applications. If you're trying to accelerate the valve off the seat extremely quickly, you can piss off a hydraulic lifter pretty easily. And if you have some deflection on opening, you're definitely going to have some seat bounce on the closing. That's hard on valve seats, it's hard on valves, and the valve springs won't last as long."

"If a part deflects—be it a rocker arm, a pushrod, a retainer—kind of bowing a little bit, all of those become springs," said Chris Potter of COMP Cams, Memphis, Tennessee. "They absorb energy, and then they give energy back. The more we understand that and help control that through lobe design, the better the engine is going to perform."

 

Lifers & Pushrods

"The bottom line is, the bigger the lifter, the stronger the lifter," said Guy Aguayo of Crower Cams & Equipment, San Diego, California. "We sell an 0.842-, 0.874-, 0.903-, and a 0.937-inch lifter. On the Fuel end, the big lifter is 1.062-inch. We sell them in needle bearing and bushing styles."

Crower's oiling recommendation for both kinds of lifters is a HIPPO system, which stands for high-pressure pin oiling. "It feeds oil to either the bushing or the needle bearing as the engine has oil pressure," Aguayo said. "In the golden days—or let's call it a few years ago—you would just rely on splash oiling, the splash coming from the rotating of the engine. That doesn't cut it anymore."

Bigger is better thinking holds true for pushrods as well, Aguayo said. "The more spring pressure, the more pushrod you're going to need to keep them from flexing. You don't want it to flex, because you're losing camshaft lift out of the flex of the pushrod."

Pushrod length is important "to get the proper geometry," said Jay Ryan of S.B. International (SBI), Nashville, Tennessee. "Even though the engine might be based on an OEM design, that doesn't necessarily mean that's the geometry you should go after, because there's going to be a lot of changes in that geometry, with rocker arm ratio, lengths of valves, tip lengths of valves—all those things factor into that pushrod length."

He also said to "use hardened chromoly pushrods, especially when you're dealing with high lift cams and higher rpm. This will assist in avoiding deflection or breakage. Pushrods tend to bend a little. So a chromoly hardened rod, with an 0.080-inch wall thickness, is very common."

 

Rocker Arms

Rocker arm material choice is critical for valvetrain life, said our experts.

"In general, a steel rocker arm material will have a higher stiffness ratio than aluminum," said Potter. "It's not going to deflect as easily."

But in the right application, "aluminum rocker arms are fantastic," said Chris Mays, who works at COMP with Potter. "They're lighter weight, so we can get by with a little less spring pressure. But that only goes to a certain power level, a certain rpm level. There are a lot of things that dictate when it's time to move to maybe a chromoly steel or billet rocker."

 

 

Biancaniello called an aftermarket cylinder head with a stock rocker arm and a bronze valve guide "a recipe for disaster. Stock rockers don't have a roller wheel on the tip. When people increase lift and use an aftermarket cylinder head with a bronze guide, they don't have a good range of motion without a roller wheel on the end. It'll kill the valve guide. If you just use a different rocker that has a roller wheel on the end of it, that solves the problem."

Aguayo said Crower is continually developing shaft rocker systems "for the new 'hot' cylinder heads." Each Crower shaft rocker setup is designed to maximize valvetrain rigidity and accuracy. "We utilize the longest arm length possible for each application, resulting in minimal rocker tip travel. This results in less frictional power loss, and it accommodates larger diameter valve springs."

He said Crower shaft rocker setups allow for quick 'at the track' valvetrain modifications. "Any change that requires rocker removal can be performed without concern for absolute accuracy upon assembly."

When we asked Holguin if there were engine families that had particular issues with rocker arms, he brought up earlier generations of the Hemi.

"One of the toughest [valvetrains] to get right is the old-style Hemi," he said. "You can't have a rocker arm that goes across the street. There's too much breeze in there." The newer generation Hemis "are much better," he acknowledged. "They fixed a lot of problems."

Some late-model engines, "like LS and LT engines, the Gen III Hemis, and the new Ford Godzilla, don't have an adjustable rocker arm," Biancaniello pointed out. "You can't put a solid roller cam in it unless you change the rocker arms. You'd have no way to lash the valves."

 

Valve Springs

"Race engines are race engines. They're going to kill valve springs," Biancaniello said. "You crank up the lift, and you spin these things to 8,500–10,500 rpm, you're asking a lot of the springs. They become a regular maintenance item." He acknowledged that valve spring technology has improved over the years, and racers don't have to change springs as often.

Technicians at Howard's Cams in Oshkosh, Wisconsin, agreed that valve springs are consumables. The harder you run the car, they advised, the more often the springs should be checked to ensure they're within correct parameters. Otherwise, the lifters could take a beating, and as the Howard's team said, "lifters are only good for a couple of bounces, and then really bad things happen."

"The best thing you can do with a valve spring is to keep it from having any harmonics," Aguayo said. "You want to utilize the full spring, but you want to keep it between 50- and 70-thousandths before coil bind. Any more than that and it'll cause the spring to harmonic. It will want to bounce around."

 

 

Valve spring material is "extremely important in assisting with valve closing, maintaining a good seal, and minimizing valve floating," said Ryan. SBI manufactures its valve springs from several high-tensile strength alloys, including Silicon-Chrome-Vanadium and Silicon-Chrome-Vanadium+Nickel. These types of materials have a greater resistance to fatigue and relaxation than carbon steels as they are exposed to elevated temperatures. "There are also processes the springs go through to give them a longer life, like a nitriding process. Some race engines might have valve springs with an isotropic type of finish, which has a chrome look to it. With the combo of high-tensile materials and additional processes, you increase fatigue life and wear resistance, which is very important in performance applications."

On that point, Potter noted that COMP uses "super-fine micro-polish finishes to get the loads, the rates, and the longevity we need to be able to control these systems." He warned engine builders not to touch those valve springs with their bare hands. "The combination of oils and salt on your skin will leave a fingerprint rust mark on a spring, and that is an immediate stress riser. That can lead to failure, especially on our race springs."

When choosing a spring, Ryan pointed out that "you can go too heavy on seat pressures, which can cause some damage to the cam lobes."

Mays said COMP is tending to "back off of valve spring pressures, diameters, and weights, because we've got so much control with the cam lobe design and how we're setting up these systems. We don't need as much valve spring as we did 25 years ago. We have engine builders to this day who still think the complete opposite of that. They want to run everything at 300 pounds of seat pressure and 900 pounds of open pressure and keep it as far away from coil bind as they possibly can. That's the opposite of what we're doing in today's valvetrain systems."

"As we get away from being over-sprung, it frees us up to do a whole lot more," Potter added. "We know we're controlling energy in the system a lot better. We know we're transferring more lift and more area to the valve through less deflection. We're taking advantage of everything we can offer to make that engine happy. We have drag racing customers with the older lobe designs and heavier springs who are changing springs 15 or 20 runs in. When we make some modern lobe changes and tweaks to their package, we have springs that will last three times as long."

Much of what's been said in this story pertains to severe-duty applications like drag racing. Other race disciplines need other approaches. In road racing, for example, "you're going to use a different cam. You'll be much more conservative on valve lift," Biancaniello said. "Instead of an inch or inch-plus of lift, you may be in the high six-hundredths or mid seven-hundredths. That will allow you to run a valve spring that can hold up a lot longer, something that can be raced for two hours straight, put away, and then pulled back out for the next weekend. The difference between 0.750 lift and 0.850 lift is huge in terms of making parts stay alive."

 

Valves

"Most of the time, valve failure is not the valve's fault," Ryan pointed out. "Normally the person who picked the valve chose the wrong valve dimensions or materials for the application. If you're trying to increase horsepower by whatever means, you might consider something other than a stainless intake valve. Depending on weight requirements and the engine environments and dynamics, some might run a titanium intake valve and an Inconel exhaust valve to handle the heat and the cylinder pressure."

"Buy the best American-made quality valve you can," Aguayo said. "You have three choices: stainless steel, titanium, and Inconel. You're limited to about 7,500 rpm with a stainless valve. Titanium valves can go to higher rpm, and they are lighter. An Inconel valve works well as an exhaust valve under pressure and temperature."

When it comes to preserving valves, "never overheat them or let them go lean," Aguayo said. "You run them lean, you're going to burn them and hurt them."

"Weight is everything when it comes to racing, period," Ryan said. "The more you lighten the engine up, the more of an advantage you could gain. That's why titanium valves are widely used in some of these higher performance applications. But in a circle track car, a weekend racer application, it's not uncommon to see the use of a one-piece stainless-steel valve."

"It depends on the application, but we like to keep valve weights down as much as we can," Mays said, "whether it be with a titanium valve or a hollow stem stainless or a smaller diameter: 8 millimeter versus 11/32s, 11/32s versus 3/8s. It means a lot to the system's overall weight and what we're trying to control with the cam lobe design and the valve spring."

 

Valve Guides

"Stem to valve guide clearance is so critical," Ryan said. "The guide-to-seat concentricity is very important, especially in these types of engines. If you're not concentric between the valve guide and valve seat, then the valve is going to basically start impacting and flexing as it contacts the seat, and it can and will break the head of the valve off very fast."

"Valve guides made out of brass are worn out before you get started," Holguin said. CHE Precision developed a blended bronze alloy "that is very strong, has very good thermal conductivity, and very good wear resistance, as well as being slick and smooth so it doesn't hang up the valve in any way." The company makes other guide variations as well, he pointed out, "because the rougher things get—the more nitrous they put in, the more fuel they put in—there's some applications where nothing lives. We make parts for that as well that are stronger and will take more punishment.

 

"A lot has to do with the guy who's putting it together," Holguin added. "If he doesn't know how to get the geometry right, you could put unobtanium in there and it wouldn't live."

 

Forced Induction

Introducing a power adder "alters the equation big-time on the exhaust side because you're trying to open that exhaust valve against a bunch of cylinder pressure," Biancaniello said. A lightweight aluminum rocker arm may be fine on the intake side, "but an aluminum exhaust rocker won't hold up because you're trying to wrench that exhaust valve off the seat against all that cylinder pressure. A lot of guys will run a steel rocker on the exhaust. If you get to a high enough level and you have enough valve spring pressure on the intake too, you're going to run a steel rocker on the intake as well."

Holguin agreed. "Once you put a turbo or nitrous in there, things change," he added. "You're going to put a different valve seat in there, something that will take a lot more temperature. And it wouldn't be smart to put a skinny valve in there. You get that hot and it gets plastic."

"The higher the boost in the engine, the more critical any of the valvetrain components become," Ryan said, "especially the spring and the force there. And it's very, very critical that the pushrod be a hardened, robust rod. The rocker components also must be extremely robust. Choosing the right mating components for the environment will directly impact how well you perform."

"Billet rockers work well on supercharger, turbocharger, and ProCharger applications," Aguayo said, "because the boost puts a lot of pressure on the exhaust, and the billet rocker will withstand the load."

COMP's valvetrain philosophy, not just for engines with forced induction but with valvetrain systems in general, is to build an engineered system of components designed to work together. "Especially on the race side, it's so hard to piecemeal parts together into something that's going to be successful and competitive," Potter said.

This approach becomes even more important in a forced induction scenario where "you've increased the power level typically by twos and threes," Mays said. "Now that engineered package has to be spot on. You're now asking more out of all of the rest of the components of the engineered system.

"If you're increasing the size of the power adder," he continued, "we would start looking at the valve spring and the pressures that are going to be required to get us into that area that we like to see for the engineered package. If I can get a 7/16, straight-wall pushrod in there, I'll do it every time. It eliminates the pole-vaulting effect. The pushrod is not bending and flexing as we go through revolutions of the engine."

 

 

Failure Analysis

In racing, building the engine is just one step to success. What's also important, said Ryan, is to observe what went on during a race. "What did we not do this time? What could we change for the next run that's going to improve that result? All people who build race engines have to have a level of failure analysis capability to look at and observe what this engine is going through. That's how we get better at what we do. Unfortunately, in the racing world, you learn a lot by trial and error. It's not a matter of if you're going to have a failed component on a run, it's when."

Or, as Holguin put it, "You learn because of your errors. That doesn't come in school, or an engineering degree. You have to have busted knuckles to prove it."

 

Sources

Cam Motion
cammotion.com

CHE Precision
cheprecision.com

COMP Cams
compcams.com

Crower Cams & Equipment Co.
crower.com

Howard's Cams
howardscams.com

S.B. International
sbintl.com

 

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