On our test subject, a 2001 Camaro SS, these upgrades were realized during an engine build that was completed in 2007. At the time, the budget allowed for a set of PRC Stage 2.5 5.3L heads, while a TRex cam (242/248 duration, .608/.612-inch lift, 110 LSA) punished the valvetrain in daily driver duty. The PRC dual springs were more than up for the task but the factory rockers weren't. As time went on and the engine continued to see high rpm, the monster cam damaged the valveguides and reduced power. A smaller custom cam from Pete Incaudo at V Max Motorsports was installed (232/244 duration, .595/.602-inch lift, 114+4 LSA) to help combat the issue, but the damage was already done. The new cam brought the power down to a more conservative engine speed than the TRex, and picked up a lot of low-end power to make it more streetable.
According to Trick Flow, the factory rocker arms from GM were never designed to be used with a higher-lift camshaft. GM rates the OE rockers for .550-inch lift but this hasn't prevented the majority of LS-engine owners from reusing their factory rockers. The reason why it gets ignored is because it's easier and cheaper to just keep using them and appears to work at first. The potential long-term effects are worn valveguides like we have experienced, or even valvespring and stem failures.
Learning from our mistake, we decided to step up in the cylinder head department instead of simply replacing the worn valveguides. Trick Flow Specialties GenX 215cc CNC heads were chosen for their insurmountable reputation, and compatibility with our setup. The 215cc intake runner was more conventional for our forged 347-cubic-inch LS-mill, so we were confident there would be a healthy power additive. Most importantly, to prevent this from happening again in the future, we wanted to install a set of roller rockers. Crane Cams supplied us with their new Gold Race extruded aluminum roller rocker arms. They came as a kit with Crane 7.250-inch hardened pushrods and were ready to bolt in. The roller tip doesn't cause valve deflection, which is when the rocker will actually push the valve stem to one side and cause it to wear faster. This should help our valvetrain to stay alive longer in addition to being quieter than the factory rockers.
GM LS Cylinder Head Valvetrain Geometery With Stock Rocker Arms And Larger Than Factory Camshafts
The GM LS-series engines are by far the best small-blocks GM has ever produced. They represent over 50 years of performance refinement and manufacturing technology. One of the key components to any efficient engine is the cylinder head and induction system. GM enthusiasts are blessed with a variety of OE and aftermarket LS cylinder head castings that are capable of producing good power and torque, the heart and soul of any Bowtie gearhead. Some GM LS enthusiasts find it tempting to change out the camshaft and/or cylinder heads for increased performance and thrill factor. GM LS engines come with roller fulcrum rocker arms from the factory, with finger follower-type tips that actuate the intake and exhaust valves via pushrod and camshaft. The finger follower design of the rocker tip is designed to roll and slide across the tip of the valve, much like arched runners on a rocking chair. Anything mechanical has a limit. The OE 1.7 ratio LS rocker arm is designed to be used with moderate camshafts less than .550-inch lift and valvesprings designed for 100,000-mile drivetrain warranties.
Running High-Lift Camshafts With OE LS Rocker Arms
OK, so what happens when you use these OE rockers arms with camshafts greater than .550-inch lift, increase the duration of the camshaft, and add valvesprings that could be used for struts on a Cobalt?
> Pitted, galled, and/or destroyed valve tips
> Blued rocker tips
> Worn or destroyed fulcrum bearings
> Valvespring failures
> Valve stem failures
> Worn-out valveguide inserts
Rocker Arm Types
Most roller tip rockers are considered to be full arc motion rocker arms, whereas the roller tip is to be centered on the tip of the valve as much as possible throughout its lift motion. Keeping the roller centered on the valve tip reduces the amount of forward and back load on the valveguide.
The GM LS 1.7 ratio rocker arms are considered a partial arc finger follower because of the way they contact the valve tip when actuated by the pushrod and camshaft. The arc that is on the tip of the LS rocker is a cam itself. When the rocker arm actuates the valve, the arc on the tip acts like a wedge to push the valve down. In contrast to the full arc roller tip rocker arm, the GM LS 1.7 ratio rocker arm is designed to start at the inboard edge of the valve tip then rolls and slides across the valve tip during lift to the outer edge of the valve tip. This type of rocker arm/valve tip motion tends to load the valveguide to one side throughout the lift cycle up to about .550-inch lift.
The GM LS 1.7 ratio rocker arms at approximately .550-inch lift, run out arc on the tip, causing the tip of the rocker arm to "jamb" or "nose dive" into the tip of the valve. It is this "jambing" or "nose diving" that causes most problems and component failures using these rocker arms. In some cases, people have tried to shim the rocker arms up to make it function more like a full arc rocker arm. Shimming tends to limit your lift more and increases the contact stress to the valve tip and tip of the rocker arm.
Most OE engines built within the last 10 years use powder metal technology for making valveguides. Powder metal valveguides are typically made up of iron (FE), carbon (C), copper (Cu), (NI),(Mo),(Mn), (Sn), (Cr), (Co), (P), and can be impregnated with oil and/or dry lubricants. So basically, the lubricating elements that have been taken out of engine oil are manufactured into the component when it was produced. These semi-self lubricating powder metal valveguides will outlast bronze alloy type valveguide inserts in most applications. Bronze alloy valveguide inserts are intended to be used in cylinder heads that are serviced regularly, for racing applications, not 100,000-mile drivetrain warranties. In GM LS valvetrains, most bronze alloy valveguides do not have enough strength to support the loads that the OE LS stock rocker arm puts on the valveguide.
In general, when higher rate valvesprings are installed onto cylinder heads they increase the amount of combined stresses in the valvetrain system. Below is a FEA model of the LS rocker arm with a static 450lb force applied to the tip, simulating a performance valvespring with the valve open. Higher spring rates also increase the amount of contact stress between the rocker tip and valve tip. Four hundred-pound open force springs are the safe limit for the OE LS rocker arm body, according to the FEA model.
So what does all this mean? Basically if performance cylinder heads or camshafts are installed on these engines, it is good practice to upgrade the supporting components so they work together effectively and reliably. Most OE components are designed to only support the loads and stresses within a given range.
> If an LS cylinder head has bronze alloy valveguides, roller tip rocker arms should be used to reduce the amount of forward and back load on the valveguide.
> If bronze alloy valveguides are used in conjunction with the OE LS 1.7 ratio rocker arms, they will eventually fail, causing the engine to use oil.
> Powder metal valveguides should be used if it is required to use OE LS 1.7 ratio rocker arms, and limit cam lift under .550-inch. The mass of the other valvetrain components should also be kept to a minimum, but not sacrifice reliability.
BY: Trick Flow Specialties