Video: Dyno Testing Pushrod Length’s Effect On Horsepower

Over the years, there have been a lot of little tricks developed to pick up additional horsepower. And has time marches on, we all find ourselves asking the same questions in search of the same little tricks on our more modern platforms, and don’t even know it. Anyone who has put together an LS engine knows how dead-simple rocker arm installation is on the platform.

However, the tradeoff for the simple installation is the need to measure and set lifter preload. We’ve published an in-depth article on properly setting an LS lifter’s preload, and if you read that, you’ll know the preload is set through pushrod length. There are specs for where “proper” preload on a stock or stock-type lifter should be, but that begs the question, “What happens if we alter those numbers?”

Luckily, Richard Holdener — the reigning king of dyno-testing on YouTube — had the same question and decided to try it out on the dyno. For the test mule, Holdener used a 383 LS made up of a factory 5.3-liter block punched out to 3.905 inches with a 4.00-inch-stroke crankshaft. For the sake of thoroughness, the same test was performed on a stock bore and stroke 5.3 dyno mule, and the results tracked with those achieved with the 383.

“If the pushrod length is too long, when you bolt the rocker arm down, you push the hydraulic lifter down,” Holdener explains. “The problem with that is that when you start the engine and provide pressure to the lifter, as you accelerate the engine the lifter can pump up and hold the valve open.” A hanging valve can have results ranging from an annoying loss of power, all the way to introducing the valve to the piston at a very high speed.

COMP’s standard-travel hydraulic lifters were used for the test. While the pushrod lengths seem like the star of the show, this test is really showing the effects of different amounts of lifter preload, as that is what pushrod length affects in the LS engine.

“Now, if the pushrod length is too short, you can have alot of lash where the pushrod isn’t in contact with the lifter and the rocker arm at the same time,” says Holdener. Besides being bad for power, since your camshaft isn’t effectively controlling your valve, that excess movement can also beat up the other components in the valvetrain. “There’s a sweet spot between being too long and too short, that is the optimum pushrod length.”

That Goldilocks zone (yay for repurposing astronomy terms!) of pushrod lengths is what Holdener will be testing. The OEM pushrod length of a 5.3-liter LS is 7.400 inches, so that will be the base length of Holdener’s testing. The valvetrain for the test is comprised of a Crane camshaft with .624-inch lift, a duration split of 232/242 degrees at .050-inch with a 112-degree lobe separation angle. A COMP Cams standard-travel hydraulic-roller lifter was used along with hardened pushrods in all the tests.

The baseline run was made with 7.400-inch, .080-inch wall pushrods, and the 383 made 515 horsepower and 490 lb-ft of torque. Jumping up to the 7.450-inch pushrods, the dyno graph shows an increase on the top end of the graph, with a slight loss compared to the 7.400-inch pushrods below 5,800 rpm.

After installing the 7.500-inch pushrod, the graph showed distinct gains over both the 7.400- and 7.450-inch pushrods, starting at about 4,900 rpm with similar dips below that RPM as the 7.450-inch pushrods. “I even went as short as a 7.350-inch pushrod, and we saw a little bit of a loss up top, although it did pick up down low. It kind of “rocked” the curve like a rocker ratio or even a mild cam change would,” says Holdener.

Looking at the graph, the blue trace is the stock-length 7.400-inch hardened pushrods, red is 7.450-inches, and green is 7.500 inches. As you can see, there is a slight dip in power below 4,800 rpm with the longer pushrods, with a significant gain above, much like what you’d see tightening up the lash in a solid roller application.

Then, just for giggles, Holdener split the intake and exhaust pushrod lengths, running the 7.500-inch pushrod in just the exhaust side, and as you might expect, it split the difference on the graph of the 7.400 and 7.500 only graphs. With the results in hand, the question is now “Why?” Holdener alluded to it when he said the results were similar to those of a lash change or a rocker ratio change.

Essentially, by increasing pushrod length, you are more heavily preloading the lifter. According to Holdener (who consulted with our favorite Nuclear Engineer, Billy Godbold), that additional preload forces the standard-travel lifter to act as a short-travel lifter. That reduction in lifter motion reduces the aeration of the oil in the lifter, which means more of the camshaft’s movements are translated to the valve.

That explains why the results between 7.400-inch and 7.500-inch pushrods appear to have the same effect on the power production as tightening up the lash on a solid roller or adding extra rocker ratio.

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About the author

Greg Acosta

Greg has spent fifteen years and counting in automotive publishing, with most of his work having a very technical focus. Always interested in how things work, he enjoys sharing his passion for automotive technology with the reader.
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