The rocker arms in a pushrod engine serve two purposes. The rockers serve as fulcrums to redirect the upward motion of the lifters and pushrods into downward motion to open the valves. The lift ratio of the rockers also serves to multiply the amount of valve lift generated by the cam lobes.
Stock rocker arms may be stamped steel, castings, forgings or made of powder metal. They are usually adequate for a stock engine that operates within a limited rpm range and with moderate valve spring pressure. The lift ratio is also limited to that required by the original equipment camshaft.
When you’re building a performance engine, you often want the engine to rev higher. More rpms increases airflow, which in turn makes more horsepower up to a point. Achieving this requires stiffer valve springs, stouter pushrods and stronger rocker arms. At high rpm, there can be quite a bit of flex in the valvetrain, especially with stud mounted rockers, so you may have to improve the rigidity of the valvetrain by installing a stud girdle or converting the stud mounted rockers to some type of pedestal mounted rockers or a shaft rocker system.
Changing the lift ratio of the rocker arms is another common modification that can get more lift (and quicker lift) out of a given cam lobe profile. Increasing total valve lift and the velocity at which the valves open and close increases volumetric efficiency for more power. Aftermarket rocker arms are available in a wide range of lift ratios. But the higher the lift ratio of the rocker arms, the more additional modifications you may have to make to prevent coil bind in the valve springs or interference problems between the tops of the valve guides and the valve spring retainers.
Durability can also be a concern when running extremely high valve spring pressures and lift ratios. Lightweight rocker arms are obviously a plus for high rpm applications, but strength is also essential to prevent failure. In recent years, aftermarket steel roller tip rockers have become a popular upgrade for the most demanding racing applications. Some of these steel rockers are nearly as light as aluminum rockers. But their main advantage is that steel has better fatigue strength and stiffness than aluminum, and can absorb more abuse than aluminum over time.
Friction reduction is also essential in a high performance engine. Performance rocker arms typically have needle bearing fulcrums and roller tips. Roller tip rockers are a must with high lift ratios because of the increased arc of travel the tip of the rocker follows as it opens and closes the valve. The roller tip reduces the sideways thrust and scrubbing on the tip of the valve stem.
Given all that we’ve said so far, it should be apparent that building a performance engine requires choosing and matching valvetrain components that can deliver the desired results. The valvetrain requirements of a street performance engine, however, can be quite different from those of a high revving circle track or drag racing motor. Cost can also significantly limit your valvetrain options, particularly with budget-conscious street customers or entry level racers.
To find some answers about how to select the “best” type of rocker arms for a given performance application, we contacted a number of aftermarket rocker arm manufacturers. Here’s what they said.
“The type of rocker arms you can use may be dictated by the rules of a particular racing sanctioning group,” said Chase Knight of Crane Cams. “For some of these applications, we have ductile iron adjustable rockers for Ford FE and Chrysler B, RB and LA engines. The rockers are much better than the stock rockers, are totally adjustable for easy tuning, and require no machining to install. We also have roller tip aluminum rockers with optional lift ratios as well for these engines.”
Knight said for stud rocker applications such as Chevy, Ford, Pontiac and so on, they offer a choice of good, better or best products depending on how much the customer is willing to spend, and what they actually need. For a typical street customer or entry level drag racer, their “good” option would be their Energizer cast aluminum rockers. For more demanding drag racing applications, their “better” recommendation is their Crane Classic extruded aluminum rockers with roller fulcrums and tips. For all-out serious racing applications, the “best” choice would be Crane Gold race rockers.
Cody Mayer at CompCams offers the following recommendations: Use stamped steel rockers when increased performance is not a goal, when other valve train parts are remaining stock, when cost is the driving factor, or when you have to avoid making any cylinder head modification.
Use die cast aluminum rockers when you are looking for a low cost option with big jump in performance over stock, when high pressure springs and/or high rpm are not required, or when you are building a “common” motor to meet certain rules or other limitations.
Use extruded aluminum rockers when you are looking for better performance than die cast, and when you need better durability than die cast (especially with higher spring pressures in a racing application). Since extruded rockers are made from billet, there is no concern about porosity like in a cast rocker. Most extruded aluminum rockers are available in a wide range of lift ratios, which gives you more options with camshaft selection.
Use investment cast steel rockers when the application requires the ultimate in durability (engines with very high spring pressures or heavy valvetrain components). Investment cast steel rockers exceed the stiffness and moment of inertia characteristics of extruded aluminum rockers.
Mayer says that in high stress applications, one of the most common failure points for a rocker arm is at the trunion straps or the trunion itself. Steel has a distinct advantage over aluminum because it can handle more stress with less material. With aluminum rockers, there is often not enough clearance to make the trunion straps big enough to provide the same strength as a steel rocker. One way straps are made thicker on aluminum rockers is to run smaller bearings and trunions however this results in a weaker trunion.
If you are looking for a rebuildable option, consider shaft mounted extruded aluminum rockers. These are a good choice if you are looking for all of the advantages of stud mount aluminum rockers but want to increase rocker system stiffness without a moment of inertia penalty. This extra stiffness primarily comes from the fact that even thicker 7/16? rocker studs will bend under high load. This source of deflection is eliminated with a shaft mount setup. Also, without the stud in the way, a shaft mount rocker can be designed to transfer loads more directly, resulting in a stiffer rocker. The only disadvantages are the higher cost of a shaft mounted system, and the possible need to machine the head to accept the shaft system.
With regards to set up, Mayer says the best advice he can offer is to realize how much the rocker ratio can change as a result of spring pressure and pushrod length. “The higher the spring pressure, the less effective ratio you will get out of a rocker arm due to compliance. Even so, it is amazing how much you can change the lift ratio of a rocker by changing the pushrod length. I have seen situations where changing the pushrod length resulted in a .08 point change in ratio.”
Mayer also says there are two different ways to set up the rocker sweep while still keeping the wheel centered on the valve. Since the rocker ratio is variable through the sweep (depending on how you set it up), the maximum ratio could be set at different points on the lift curve. In one setup, the rocker sweeps out during its entire motion, delivering the highest ratio at maximum lift. In the other setup, the rocker sweeps out to about mid-lift and then back in until maximum lift. With this setup, the rocker delivers the most ratio at mid-lift, with decreasing ratio back to maximum lift.
Randy Becker, Jr. says Harland Sharp has had a full line of shaft-mounted rockers for small block Fords for a number of years, as well as shaft rocker systems for all the popular aftermarket heads (AFR, TrickFlow, Brodix, Dart, etc.).
“Our new small block Chevy system is different from many of the other rocker systems that are out there in that is uses a mounting system with machined aluminum base plates. The shaft has no bolt holes passing through it, and is supported by the stand uprights. The shaft is 11/16? diameter 52100 tool steel, with two rockers on each shaft. Available rocker ratios range from 1.5:1 to 1.75:1.”
Becker says his company has also developed a similar setup for big block Chevy applications. The only difference is that each big block rocker is mounted on its own shaft.
Becker says most of the shaft-mounted rocker systems they make are custom built for specific engine applications. “We design each system according to valve lift, spring pressure, and the amount of offset a customer wants.”
Rob Remesi of Jesel says his company has been making shaft rockers for over 30 years, including both aluminum and steel rockers. “A lot of the work we do is custom rockers for professional racers.”
The main advantages of shaft mounted rockers include more power (due to reduced friction and improved valvetrain stability), increased reliability (because the rocker does not side load the valve as much), and easier adjustment.
Lashing the valves with stud rockers that have a stud girdle can be a time-consuming process. With a stud girdle, you first have to loosen the girdle, then set the lash. When the girdle is retightened, the lash may change if any of the studs are out of perfect alignment.
Other features available with shaft rockers include the ability to handle pushrod offsets to clear wider ports, adaptability to a variety of aftermarket cylinder heads, and a side range of lift ratios from 1.4 to as much as 2.25.
Gordon Johnstone of Scorpion Racing Products says Scorpion stud mount rocker arms are applicable to street, marine and 95% of all race engines. “A lot of the choice of which to use boils down to rule restrictions and budget. If there are no limitations then the best selection would be a 7/16? stud mount rocker (for maximum rigidity) from our “Endurance” line to fit the application.”
Johnstone says valvetrain setup is surprisingly the same for all situations. Correct pushrod lengths, physical clearances and proper adjustment are essential. Two areas often overlooked are pushrod rigidity and valve spring loads. “As far as pushrods go, you cannot have a rigid enough pushrod. Even for mild street applications, an .080? wall pushrod should be considered bare minimum. This is important to properly transmit the cam motion into valve motion which helps with power and control at rpm.”
Proper valve spring selection is also important, says Johnstone. “Don’t guess on this one. All flat tappet or all roller cams do not use the same spring pressures. Cam lobe profiles have a lot to do with the required spring loads, so always check the cam spec card. If pressures are too light, the shock loads from valve bounce will limit rpm and power, and may damage parts within the valvetrain. Too much spring pressure in a street engine causes excessive heat within the valvetrain, possibly causing premature wear and or failures.”
Phil Elliott of T&D Machine says he does not recommend his products for street performance applications. “Our parts are designed for serious racing only. We make shaft mounted rocker systems for professional racers, and have systems to fit most aftermarket performance heads. The biggest benefit of the shaft mount rockers is stability, especially at high rpm.”
Elliott says in recent years there has been a strong exodus from aluminum to steel billet rocker body construction. “The reasons are quite simple: a rocker arm must last through the high rpm, fully orchestrated chaos that is now the NASCAR norm. NASCAR has for sometime been the high water mark. If a component can be built to stand up to its rigors, it will more-than-likely be over engineered for anything else. The top NASCAR teams continue to experiment to achieve ever-loftier power numbers in hopes of gaining slight advantages over their competition. If they ask for a lighter rocker arm with a little more ratio, we try to give it to them. While extra stiffness and less mass are both desired features, longevity is a key factor. The rocker must last through all the laps of tuning, practice, and the race itself. Teams that are running our steel rockers are getting three to four times the longevity they were getting with aluminum rockers.”
Elliott says his rockers are rebuildable, but within certain limits. If the trunion bearing bore is out of round by more than .0005?, the rocker needs to be replaced. “Half-a-thousandth seems like a very small amount, but we have found that when aluminum moves at all, it is ready to fracture. It is far cheaper to replace the rocker than to replace the entire engine due to a rocker body failure later.”
As for rocker materials, Elliott offers the following observations:
Stamped steel rockers should only be used on engines with less than .600? of valve lift and at engine speeds below 6,500 rpm. Higher lifts or engine speeds require upgrading to some type of performance rocker (aluminum or steel).
Aluminum rockers are lightweight, easy to manufacture, and have a dampening effect on the valvetrain. The downside is their mortality rate, since all aluminum rockers have a cycle life.
The pros of using steel rockers are that they are more durable (extended cycle life), and stiffer with a slight deflection improvement over aluminum. The downside with steel rockers is that they are not as easy to manufacture, they can be heavier and harder on other valvetrain parts.
The lighter the rocker, the less mass the valve spring and pushrod have to accelerate, stop and accelerate in the opposite direction. But each application dictates how light of a rocker can be used. Removing weight from the roller tip always pays the biggest dividends, as long as the rocker is capable of handling the loads for that application.
Roger Vinci at Yella Terra High Performance Engine Parts agrees with the other rocker suppliers that more and more racers are choosing steel rocker arms and shaft systems for high rpm applications. “Keeping the valvetrain as stiff as possible while maintaining the lightest weight over the nose of the rocker is our goal. We have one of the smallest nose wheels in the industry. The lightweight technology in our Ultralite LS rocker designs allows the use of lighter spring weights.”
Vinci says strength is of utmost importance in a rocker arm (far more important than its appearance or packaging!). “We use a 2024 T3500 alloy in all of our higher end rockers. Some manufacturers start with an inferior alloy and anodize it to make it look good. But anodizing an alloy can reduce its strength up to 50%. That’s why we do not anodize our rocker arms.”
Vinci says creating as much lift as possible via the rocker arms is paramount to stabilizing the valvetrain. “We know that utilizing the leverage of the rocker shortens lifter travel, which lowers friction. It also lowers lifter and pushrod inertia and can greatly increase the rate of lift of the valve as comes off the seat to initiate and sustain airflow. This can allow the engine builder to use lighter pressure valve springs, which we believe is always more favorable when possible.”
Vinci says his unique shaft mount design offers a much stronger rocker than a stud mount, while maintaining a true bolt-on capability. “Our 11/16? shafts are among the largest and strongest in the industry. The rockers utilize large diameter Torrington bearings for greater load capacity and longer service life.”
Yella Terra has three basic rocker designs: LS Ultralite Series rockers for spring loads on street cams up to 500 lbs. open pressure, the Yella Terra Series rockers, for spring loads up to about 650 lbs. open pressure, and for serious competition the Platinum Series rockers for spring loads up to 1,000 lbs. open pressure. All three rocker lines require little or no machine work and are a true bolt-on product. “For our LS1 and LS3/L92 kits we just introduced a kit that upgrades the small 8mm center bolt to a much larger and more stable 10mm bolt, a simple way to add strength and reliability to the valvetrain,” said Vinci.
Latest posts by Larry Carley (see all)
- Piston and Sleeve Finishing and Break-In Tips - Mar 19, 2015
- The Challenges and Opportunities of Variable Valve Timing - Mar 6, 2015
- New Engines Force New Pistons - Feb 27, 2015