Introduced as a successor to the venerable small block Chevy, the LS engine family had a good run. Recently replaced by the fancy, direct-injected, LT-series engine, the LS still reigns supreme in terms of market share, and for good reason. The LS has much going for it, offering everything we want in a motivational source, including displacement, compression and head flow. Toss in effective factory intake manifolds, especially on the LS3 versions, and you have almost everything you need to make serious power. We like the LS engine family for a number of big reasons, including how well they respond to cam upgrades and how much boost the stock short-blocks will withstand. We also like them for a number of little, simple reasons, like their use of a factory windage tray and effective, bolt-down, roller rockers. It is this final element that lead to our amazing dyno discovery.
The use of the bolt-down, roller rockers on the LS requires the use of a proper length pushrod. Truth be told, pushrod length is critical even with an adjustable valvetrain, but even more so on the non-adjustable LS. Know that improper length, meaning too long or too short, can cause anything from a drop in power to catastrophic failure of one or more major components. The procedure for checking pushrod length on the LS is a simple one. With the cam on the base circle, simply tighten the rocker bolt until all of the lash has been removed. This occurs when the pushrod has just contacted both the rocker and lifter. Determining this zero-lash point is easy, simply rotate the pushrod between your fingers as you tighten the rocker bolt. When you feel drag on the pushrod, making it more difficult to turn, you have reached the zero-lash point. From here, count the number of turns it takes to seat the rocker into the rocker stand. You are looking for between 1/2 and 1 full turn.
The number of turns required to seat the rocker on the stand past the zero-lash point translates directly to the amount of preload on the hydraulic roller lifter. Our recommendation of between 1/2 and 1 full turn applies to standard-travel, drop-in (OEM-type) hydraulic roller lifters, not the short-travel variety. Short-travel lifters require less preload, often accommodated by custom pushrod lengths to optimize the amount of preload. Standard-travel lifters can usually work with off-the-shelf pushrod lengths that come in 0.050-inch increments. If you tighten the rocker bolt all the way down to the stand and never feel any resistance, the pushrod is too short. This results in what is called valve lash. On a hydraulic roller cam you don’t want any lash, which is why we preload the lifter. The lash created by having too short of a pushrod can cause anything from excess valvetrain noise to component failure of the cam, lifter, or rocker arm. It can also cause damage to the pushrod, valve tip, or even the valveguide.
Just as excessive lash caused by insufficient pushrod length can wreak havoc, the same is true with excessive pushrod length. A sure indication that the pushrod is too long is when it takes more than 1 full turn to seat the rocker past the zero-lash point. Though we have successfully run pushrods with more than 1.5 turns, more than 2 turns can actually hold the valve open once the lifter is pumped up with sufficient oil pressure. Excessive pushrod length can then eliminate the ability of the cylinder to build compression. Naturally, this can cause anything from poor performance to, in severe cases, the engine not to start or even run. Having the valve hang open when it should be closed can also cause piston-to-valve interference, resulting in bent valves. Just like the porridge for Goldilocks, pushrod length needs to be neither too long nor too short, but just right.
Having dialed-in the pushrod length on countless LS combinations, we began to wonder what would happen if we ran one with pushrods that were slightly too long and slightly too short, not to mention a set that was just right? Obviously we erred on the side of safety, meaning we ran the engine neither with a length that created lash nor hung the valves open. After dialing-in the pushrod length on our 383 test engine, we determined that the optimum length was 7.450 inches. To illustrate what happens when they are too long and too short, we also ran sets measuring 7.400 inches and 7.500 inches. Because we suspect the effect might differ on a stock engine with milder cam timing, we built a dedicated 383 test engine.
Does Length Matter? LS Pushrod Test
The fact that the combination was carbureted had no effect on the pushrod test, but we suspect that the use of standard-travel Comp Cams lifter (PN 850-16) certainly did. The tests were run with identical air/fuel and timing values, as well as oil and air temps. Equipped with the ideal pushrod length of 7.450 inches, the 383 stroker produced 524 hp at 6,300 rpm and 492 lb-ft of torque at 4,900 rpm. After swapping in the shorter 7.400-inch pushrods, the power numbers dropped to 515 hp at 6,300 rpm and 490 lb-ft at 4,900 rpm. The final test was with the 7.500-inch pushrods, which produced 529 hp at 6,400 rpm and 490 lb-ft at 5,000 rpm. The longer pushrods enhanced power production above 4,900 rpm, but produce nearly identical peak torque numbers to the other two sets. The change in pushrod length acted much like the effect of valve lash on a solid roller (or flat tappet) on a conventional small-block. Decreasing lash effectively adds both lift and (to a lesser extent) duration. This tends to improve peak power while (often) decreasing low-speed torque.
We began throwing around theories about what might cause these results. At first, we suspected that the change in hydraulic plunger depth (of the lifter) created by the increased pushrod length may have altered the internal oil flow by covering or uncovering the metering orifice. We could find no better way to verify our theory than to call the resident expert over at Comp Cams, Billy Godbold. According to Billy, the change in plunger depth decreased the available volume of aerated oil that resides under the plunger. The air in the oil acted like lash on a solid cam so we effectively reduced the amount of lash inherent in the system. Basically, the longer pushrod made the standard-travel lifter act more like a limited-travel lifter, thereby making our cam profile slightly wilder. Think of the lifter as a shock absorber. With a short pushrod, there is a larger reservoir and therefore more deflection in our shock absorber. With a longer pushrod, there is less reservoir, meaning the shock is set on stiff, allowing less deflection.
Photos by Richard Holdener