SPR Rockers allow more lift for LS engines
Words: Brandon Flannery
Limits are being pushed as the LS platform gains popularity in the performance world. One area that’s currently maxed out when increasing power lies in the rocker arm, and COMP Cams has created an affordable solution. Their new SPR Radius Tip Rocker Arms accommodate high-lift camshafts well over .600” (up to .800”) by centering the rocker tip over the valve stem throughout the sweep, reducing stress.
Though a robust valve train system, stock LS rocker arms reach their limit as lift approaches .600”. By design, their geometry moves the rocker tip from one side of the valve stem to the other, and nearly falls off the edge with radical lift.
When the valve is closed, the rear portion of the rocker tip sits .114” from the centerline of the valve stem and then ends up .097” from the centerline at stock lift, and nearly on its nose. This means the OEM’s contact point within the tip travels a total of .211” across the valve surface at the stock lift of .496” (See figure 1.1)
Concernedly, at max stock lift, the rocker tip is on the outermost edge of the valve stem while under full spring load. This increases stress. Given enough abuse, with rpm or increased spring pressure, it will begin to mushroom over and can even crack the tip of the rocker arm. Increasing lift above .600” tilts the OEM arm further downward and off the load-bearing contact point of the rocker tip. Not good.
By modifying the geometry and radiusing the slider tip, COMP was able to create an optimized “foot” that stays closer to the centerline of the valve. In fact, total sweep has been reduced by 400 percent. This creates a solid connection (with excellent surface load distribution) throughout the entire range of the valve motion.
On a closed valve, the tip of the new SPR is only .045” from the stem’s centerline. As the valve reaches the stock lift of .496”, the SPR’s tip is applying forces nearly perpendicular to the center of the valve, with a generous portion squarely on the stem of a scant .015” off center. The result is a stronger, more secure “push” on the valve with increased contact area in the strongest part of the stem and plenty of room for more lift. (see figure 2.1)
In addition to increasing sweep, the SPRs are stiffer than the baseline OEM rocker design. Using the latest engineering software and Finite Element Analysis (FEA), COMP engineers were able to identify and improve stress points and weak areas. Optimizing the design also resulted in a 45 percent increase in stiffness over the OEM and a 7.5 percent increase in Moment of Inertia (MOI), allowing the valve to accelerate off the seat faster.
Design improvements include a radiused and reinforced top area that eliminates a stress point and increases stiffness by 26.3 percent for the LS1 and 49 percent for the LS3. In doing that, they also reduced material near the bridge that contributed to such a slight increase in MOI.
An I-beam design was used for lateral stability and reinforced with two “bar” structures in the webbing. (See Figure 3.1) Instead of a solid cast design, the FEA process allows a tighter design around what basically amounts to a roll cage-type structure that’s fortified with webbed panels. This allows a stronger, yet leaner and lighter design. Each purposely placed raised or recessed area was tested and proven to contribute to overall strength and stability.
For additional robustness, the slider tips are CNC profile-ground and then “nitrided” to reduce friction. This means the new slider tip’s surface is given an extremely high surface finish with a CNC grinder and then pummeled with pins of nitrogen. This strengthens the surface by creating a web of “dimples,” like shot peening but on a much smaller, almost microscopic scale.
The ability to accommodate more lift also comes from increasing the ratio. The OEM lifter and rocker assemblies have a 1.7 ratio, and the SPRs increased to 1.77. This enables the rocker arm to open the valve a little more with the same amount of cam lift.
Adding sweep and alignment through revised geometry keeps the tip centered while the redesigned rear portion eliminates “bend points” and disburses stress into the body.
The colored diagram (Figure 4.1) shows the stress levels on each arm with a 1500-pound load at .300” lift. Dark blue indicates the lowest stress level areas, with yellow and red denoting very high. One can see where the FEA-developed improvements were made. The maximum stress in the rocker arm was also reduced from 141,260 psi on the OEM to 109,120 psi on the SPR.
What does this mean for the racer?
Racers can now push their performance levels by using higher cam lift and stronger springs. Better still, they are a direct bolt-on to OEM trunnion stands for LS1 and LS3 engines and are available with assembled needle bearing or bushed trunnions. LS1 rockers feature a 1.77 straight ratio, while LS3 versions are 1.77 offset and virtually interchangeable with stock ratios, due to their dynamics and less loft.
Source: COMP Cams; compcams.com