The name sounds like some experimental jet fighter — XD-A. But what those letters actually refer to is COMP Cams’ new shim-adjustable, two-piece pushrod for LS engines. Right off the bat, there may be a few readers already thinking — “A two-piece pushrod can’t work.” But it does — if you design it correctly.
The idea was born out of the need to reduce mass on both sides of the rocker arm. If there were a way to retain the lightweight stock LS rocker but still give the engine builder a way to adjust the lifter preload, it would offer measurable benefits. The overlapping two-piece design is also significantly stronger and less prone to bending in high-RPM applications.
COMP’s Head of Valvetrain Development, Billy Godbold, told us COMP’s testing reveals the XD-A pushrod is 90-percent stronger than the same length 5/16-inch, 0.105-inch wall thickness pushrod – which is saying something. Because the 7/16-inch diameter section of the pushrod overlaps with around 2 inches of the 5/16-inch-diameter portion of the XD-A, it makes this area nearly solid, creating a very stable base for the upper 5/16-inch portion of the pushrod.
From Concept to Product
Like all good ideas, the concept is as simple as it is elegant. While pushrods appear to live an uncomplicated life, the reality they face is harsh in an 8,500-rpm race engine. When pushing to open a valve at speed, while valvespring load is multiplied by the rocker ratio back into the pushrod, Godbold tells us the real load values occur in two places on the valve lift curve.
The first point is when the lifter begins to accelerate the rocker arm, valve, spring, retainer, and keepers off the seat. The second high-load situation occurs on the closing side as the valve approaches the seat.
As RPM increases, so does the load. Godbold reveals that, at 5,000 rpm on an LS engine with 5/16-inch pushrods, the strain gauge shows these instantaneous loads can be 1,500 pounds. At 7,000 rpm they double to 3,000 pounds.
Think about a 5/16-inch diameter pushrod attempting to support the weight of a small car. This is why building stiffness into a pushrod is always a good idea. Godbold says he’s seen over 10,000 pounds of load in a Pro Stock application, even with all their lightweight components.
In the drag racing world, it is standard fare to see big displacement drag race engines using 1/2- inch, and even 9/16-inch diameter pushrods with robust wall thicknesses. To circle this back around to the LS engine family, the pushrods are significantly shorter than older pushrod engines, which helps, but at the same time, the LS pushrod is limited to its 5/16-inch diameter.
Executing the Impossible
The XD-A plan starts with a larger 7/16-inch base section which is roughly three-inches long. The inside diameter is sized to accept a smaller, 5/16-inch diameter section that varies in length depending upon the overall pushrod length required.
The third component in this design triumvirate is 16 adjustment shims, ranging in thickness from 0.060 to 0.116-inch. This allows the engine builder to create a specific pushrod length, accurate to 0.004-inch. And even this 0.004-inch can be reduced to 0.001-inch if you’re willing to do a little extra-fine sanding work.
Roughly a third of the length of the smaller pushrod slides inside of the larger 7/16-inch pushrod with approximately two-inches of overlap. This makes the pushrod incredibly strong near the center, where it would tend to bend under high loads. You can think of the overlapped section as being near solid, except for the oil passage.
One of the XD-A’s goals was to give an LS engine builder the freedom to assemble a very accurate pushrod length well-within the 0.050-inch or 0.025-inch off-the-shelf lengths, while also avoiding expensive custom-length pushrods that may also require weeks to deliver.
The advantage of this for a hydraulic-roller-cammed LS engine is the ability to set the lifter preload. LS engines all use a net-lash system, which does not allow for adjusting the position of the rocker arm. This means that when the rocker is torqued in place, pushrod length determines the amount of lifter preload.
Setting Preload on the LS
The easiest way to check the amount of preload is first to put the lifter on its base circle. Place the pushrod and rocker in place and gently tighten the rocker bolt until there is zero free play. Now count the number of turns required to tighten the bolt.
The thread count on the rocker bolt is the metric equivalent to 20 threads-per-inch (TPI). This makes one turn of the bolt move the piston inside the hydraulic lifter 0.050-inch (1/20 = 0.050-inch). Establishing a custom pushrod length that will depress the lifter piston between half of a turn (0.025-inch) and one full turn (0.050-inch) is the ultimate goal.
The XD-A’s shim feature is doubly helpful here because the engine builder can experiment with different pushrod lengths for his specific application to determine the ideal lifter preload. This is easier because the pushrods can be quickly disassembled, and internal shims changed to alter the length.
Previous dyno tests by our colleague Richard Holdener have shown an advantage to increasing the amount of preload in the hydraulic lifter on any engine. His testing was performed on a 383ci LS engine that uncovered a power gain of 14 horsepower, going from 515 to 529 hp.
That’s impressive and was using COMP’s standard off-the-shelf 0.080-inch wall 5/16-inch-diameter pushrods. While not all engines will experience this same level of improvement, it does reveal the importance of accurately setting the lifter preload.
More Than Just Rocker Preload
But the benefits of the XD-A doesn’t end with hydraulic-roller-cammed engines. Consider the benefits for a solid-roller-cammed LS engine. Since this type of valvetrain will be intended to operate at much higher engine speeds, lightening the components is a good idea.
For example, eliminating the adjuster on the pushrod end of the rocker arm is roughly equal to 20 grams. This is approximately the weight difference (depending upon valve size) between a steel and titanium intake valve.
Granted, the 20-gram reduction is on the pushrod side and, therefore, not multiplied by the higher acceleration forces of the rocker ratio. But, it is still a reduction in mass, making it beneficial because, the mass would otherwise be located at the end of the rocker arm, subject to acceleration forces at 8,000 rpm. Every little bit counts.
As mentioned before, a 2,000-rpm increase in engine speed can double the inertia forces, so it behooves the engine builder to minimize valvetrain mass — especially with the rocker arm. One reason the factory LS rockers work so well is that they are a really efficient design that isn’t burdened by the weight of an adjuster on the pushrod side of the rocker.
The XD-A can be used to establish very tight pushrod-length tolerances, which can be used to create the necessary valve lash on a mechanical-roller cam for the LS engine. First, a COMP adjustable pushrod can be used to determine the near-net length for a mechanical-roller lash application.
Shims of varying thicknesses can then be substituted in the XD-A pushrod to establish the actual lash necessary. Once again, by eliminating the adjuster on the end of the rocker arm, rocker mass is reduced and lowers the moment-of-inertia power required to accelerate the additional mass.
Godbold told us that establishing the lash for a solid roller LS combination requires a little bit more effort than usual. To accurately set the lash, you use a thicker than normal shim and then carefully sand the shim down on a 400-grit piece of sandpaper taped to a granite base.
He said it takes about a minute to sand roughly 0.001-inch off one of these shims, so you can expect to spend perhaps four hours setting lash on 16 valves. But in the end, this procedure will make the system rock-solid. Godbold says the only time he’s seen the lash change is when valves in new seats settle by roughly 0.001 to 0.002-inch.
XD-A in the Real World
While all of its benefits sound good on paper, it doesn’t mean much if the parts can’t survive in the real world. One of the best ways to really test components like these is on the Spintron. COMP has done just that by pushing the equivalent of an LS7 XD-A pushrod system, in conjunction with one of COMP’s shaft rocker BSR kits, using a stock LS7 intake valve, 1.8:1 rocker ratio, controlled by COMP’s latest conical valvespring (P/N: 7230) with nearly 500 pounds of open load. They didn’t pussy-foot around with this test, ultimately spinning the test module repeatedly to 10,000 rpm.
In tests like these, it’s important to note that the valvetrain needs to be viewed as a complete system. In other words, the XD-A pushrod by itself, while offering some impressive test data numbers, is just one component in this system. With that in mind, we’ve reproduced a couple of the Spintron tests.
The blue line in each test is the baseline, which is a static trace of the lift profile. The red line is what the Spintron recorded at that RPM. For the 6,000-rpm test, note there is a small increase in lift. Until recently, this has generally been considered valve loft, but Godbold tells us there is always deflection in a pushrod system. With weak components and high valvespring loads, this can be 0.020-inch or more.
Godbold says the red trace at 6,000 rpm reveals the valvespring open-load deflection value (that 0.020-inch value) that would show up if there were a third line in this curve showing the valve-lift curve with no spring pressure applied to the system.
“Subtracting” that difference from the original static-load blue line, due to inertial loads trying to change direction at peak valve lift, is what is delivered in the red trace at 6,000. The shape of the red curve at different engine speeds reflects how the inertial loads change with varying engine speeds. This is a very common occurrence.
The second trace is at 9,000 and 10,000 rpm. This trace looks very similar to the 7,000 rpm trace, except with just the beginnings of valve bounce on the closing side. Note how the loft over the peak lobe lift is still very much controlled. There was one point at around 9,000 rpm where the system was fussy and exhibited additional loft and some valve bounce on the seat, but by 9,400 rpm this “fussiness” disappeared.
All of this reinforces how stiff and resistant-to-deflection the XD-A pushrods are. A different valvespring or a cam swap could change these results, but the point is that it appears the XD-A pushrod concept has some serious merit. Combined with COMP’s BSR shaft rocker system, which uses a single shaft to tie all the rockers together, this could add significant valvetrain stability to a serious LS engine package.