It’s time that we took a look at a critical valvetrain component – the rocker arm. With today’s “stock” engines capable of supporting so much horsepower, it’s easy to overlook the rocker arms when valve springs and pushrods are such common modifications.
A ‘CAD’ drawing of the Ultra Pro Magnum Rocker
Sometimes it’s easy to forget how much stress the rocker arm is really under. Not only does it have to deal with the stresses of hardened pushrods propelling it, only to be vigorously resisted by the valve spring, but also it must multiply this movement while maintaining proper alignment on the relatively small area of the valve stem. Seems simple enough, right? Well it’s even more sophisticated when you really get into it. For the latest developments, we got in contact with COMP Cams to shed some light on rocker arm choices for the LS family of engines.
Not Bad, for Stock Parts
We can think of a few good reasons to ditch your stock arms, but we asked COMP for their professional insight as to the strengths and weaknesses of the factory equipment. According to Trent Goodwin, “The stock LS rockers are a very good design, but they have some drawbacks. First, the trunions are lift-limited and the bearing packages are not designed for high load use. Also, although very light, the stock rocker body is not very stiff and can give up some valve lift in higher spring rate and/or higher RPM applications.”
Stock LS rockers have some sophisticated design features, but they aren't without their weaknesses.
Not only are you sacrificing some potential performance, but the durability can be called into question. A common catastrophe awaits when the stock needle bearings decide to free themselves from their cage. This can result in dozens of tiny needle bearings floating throughout the engine, making their way into passages, rotating components, and oil galleys.
“This obviously causes much bigger problems than just losing the rocker, as it can severely damage parts all through the motor” states Goodwin. “Typically the lifter has issues first, as it is thrown up in the bore and can impact the cam severely. It the tray or lifter is damaged enough, an oil path can open and oil pressure will be lost. Even under the best situation, it is wise to replace the cam and lifters, and check the spring loads very closely in the event of bearing failure.”
Components of the COMP Trunion Upgrade Kit
A Simple Solution
Now that we have you fully paranoid about your 16 ticking time bombs, we’re here with a solution: COMP Cams’ Rocker Arm Trunion Upgrade Installation Kit for the LS1/2/3/6/7 – Part #13702-KIT. This kit eliminates the factory bearing package and powdered metal trunion, and replaces it with a captive roller bearing design and an 8420 alloy trunion, resulting in less chance of bearing failure and engine damage.
It’s a fairly simple and straightforward installation. COMP recommends the use of a shop press, but it’s possible to do this install with a strong bench vise and some properly sized sockets. The old bearings are pressed out, the new bearings are pressed in with the upgraded trunion, and everything is held in place with a pair of snap rings. The installation kit even includes spacers to aid in the pressing procedure to ensure that everything lines up properly.
For a little how-to video on the installation process, view this Comp Tech Video. The installation instructions can be found here. This system has been performance proven & tested in NASCAR GM LS-spec engine series. Coming in around $140 for the kit, its cheap insurance, and a great value.
The Next Level
At what point should you consider upgrading the entire rocker arm to an aluminum- or steel-bodied unit? “With either the COMP stud mounted or pedestal mounted system, you should feel quite comfortable into the 8000 RPM range if the rest of the valve system is stable, and well past 0.700” lift can be accommodated,” explains Goodwin. “Both of the COMP rockers provide increased stiffness over OEM, and the roller tips are proven to be easier on guides, especially in high spring load applications and at high lift. Roller tip rockers also help to increase the life of the rocker and valve tips in high stress applications.”
COMP Ultra-Gold ARC Series Aluminum Rockers
Two great rocker options from COMP are the Aluminum Ultra Gold ARC Rockers, and Steel Ultra Pro Magnum Rockers. Typically, an aluminum-bodied rocker has less weight, but to compensate for the aluminum’s lower strength, they often end up larger than their steel counterpart to maintain the proper rigidity.
Weight doesn’t directly correlate to the effectiveness of a rocker, however. The moment of inertia plays a bigger role. Moment of inertia can be defined as the measurement of an object’s resistance to changes to its rotation. “Rocker arms with a higher moment of inertia can reduce power at higher RPM by inducing valvetrain instability, resulting in valve float,” says Goodwin. Unfortunately moment of inertia is not a simple calculation that you can figure out on your smartphone’s calculator, but it is part of the overall design considerations when creating a new rocker arm.
Goodwin explains that this creates a common point of confusion. “The most frequent question we get is whether to run our aluminum Ultra Gold Arc Rockers or our steel Ultra Pro Mag rockers. Both have their advantages, but for customers looking for a bolt-on upgrade without needing to buy a pair of aftermarket valve covers, I recommend the Ultra Pro Mag Rockers. Being that steel is much stronger than aluminum, we are able to make the Ultra Pro Mag rockers not only stronger, but also more compact than their aluminum counterparts thus giving them more clearance to the valve covers.”
Just Talkin’ ‘Bout Shaft
Shaft mounted rockers can improve system stiffness, but the investment might be better spent on airflow improvements – Trent Goodwin, COMP
At what point should you begin to justify the expense of a shaft-mounted system? Goodwin says, “It is often a financial as well as a performance decision. Shaft mounted rockers can improve system stiffness, but the investment might be better spent on airflow improvements. Also, many of the best flowing heads do require shaft-mounted rockers because of port relocation. That being said, unless the rules require stud mounted rocker arms, shaft systems certainly provide a more rigid base for the rocker body than even our new ½-inch studs or stud girdles can provide in a traditional system.” Shaft mounted systems utilize a one piece steel mount per cylinder head, and rocker bodies made from 2024 aluminum. This combination provides the ultimate in valvetrain stability, but with a significantly increased cost.
COMP Shaft Mounted Rocker Arm System
Luckily for you, these rocker arms require no special tools as long as you aren’t replacing the valve springs (which would require a valve spring compressor, and a fitting to fill the cylinder with compressed air to keep the valves in place during the process). For those of you contemplating going to a solid roller lifter setup, COMP has you covered. “The Ultra Pro Mag rockers come in either pedestal mount (which are non-adjustable) or stud mount which are adjustable for solid lifters. The Ultra Gold Arc rockers are all pedestal mount, but we make a series that have adjusters for solid lifters.”
The Long Arm of the Rocker
COMP’s ARC rockers are available in stock 1.72 ratio, or 1.82 as shown here for added lift
Now we must determine what ratio would be best for the specific engine combination. The ratio of the rocker is a correlation between camshaft lobe movement, and valve movement. For example, on a 1.7:1 ratio, you would take the camshaft lobe lift and multiply it by 1.7 to determine the valve lift. By increasing the rocker arm ratio, you are effectively increasing valve lift, which results in increased airflow. But here’s the catch – you can’t live by the “bigger is better” mantra and buy the largest ratio rocker you can find, especially if you already have an aftermarket camshaft with more lift than stock. Doing so risks interference between the valves and pistons, with disastrous results. So how do we know if higher ratio arms will work with a stock camshaft and pistons?
Goodwin explains how this works: “At peak lift, the piston is several inches away from the valve. Hence, max lift has virtually nothing to do with piston to valve clearance. However, the increased ratio will increase the lift at overlap by the ratio change as well as the max lift. The tappet lift at closest piston to valve clearance is typically in the 0.080” to 0.150” range. Just assuming 0.100” tappet lift at closest approach, the resulting valve lift would grow from 0.170” to 0.180” with 1.8:1 vs 1.7:1 ratio. So, you should expect to lose about .008” to 0.016” piston to valve clearance with a ratio change. The cause for concern with such a change is certainly dependent on the camshaft in question as well as the clearance of the build, but most applications have enough extra room for this not to be an issue. If you are close enough that 0.015” would be a big deal, the PTV clearance needs to be measured closely anyway.”
For more helpful information visit COMP Cams’ tech support forum or give them a call at 800-999-0853.