In the pre-LS era, a typical small-block-powered street car would rev willingly to around 5,000 rpm. You could assemble a cam package for it—even using heavy components—and you probably wouldn't have any problems. With LS engines, however, the redline is in the mid-6,000-rpm range, and the mighty LS7 revs to 7,000 rpm, with some owners pushing things even higher. On top of that, valve lift is typically far greater than anything an old-school small-block would see, with upwards of 0.650-inch lift from a street-friendly hydraulic roller cam.
With engine speeds so much higher than they used to be, valvetrain stability has become a more important issue. It's all too easy to slap together an unproven package that generates instability beyond the 6,000-rpm mark, beating up valvetrain components and adversely affecting performance.
"Some people think it's all good, as long as it's not floating the valves on the chassis dyno,"says Katech's Jason Harding. "Valve float kills the power, which is pretty much a [known] fact, but it's the valve bounce that accompanies an instable setup that does the most harm. It's kind of like that adage, 'It's not the fall the kills you, it's the sudden stop at the bottom.'"
When that valve slams shut and then bounces open, sometimes numerous times, it beats up the seats and can break valves or springs.
"Some believe having a double spring makes them safer, because if the main spring breaks, the inner spring will hold the valve from dropping into the cylinder,"says Harding. "Well, how about just making a combination that doesn't break springs? That's our philosophy with the beehive springs that are carefully tested and combined with our packages.”
The theory and practice of valvetrain stability are particularly relevant when it comes to the LS7, because one of the urban legends about it—that it has a tendency to drop or break the exhaust valves—has some merit. There are plenty of documented cases of the lightweight, two-piece, sodium-filled parts breaking. Typically, the valve's head separates from the stem, causing catastrophic damage.
This is an issue experienced mostly by racers and those with more-modified engines—particularly when employing a larger cam—but owners of stock, street-driven Z06s have also encountered it.
Katech engineers tell us their research has found extremely high valve-guide wear in LS7 and LS9 engines—so much so, that in some cases a valve can "wiggle"around in the guide. That, they say, is the culprit behind failures of both intake and exhaust valves, not the valves themselves. For the purposes of this article, we'll be focusing on valvetrain stability rather than valve-guide wear, which is another story entirely.
Implications of a Valve Change
Of course, an enthusiast who's going to the trouble of upgrading or rebuilding an LS7 will want to know whether replacing the stock valves is prudent. At a glance, swapping the hollow-stem, two-piece sodium valves for conventional single-piece valves seems like a no-brainer, but there are critical implications on valvetrain stability to consider, regardless of the camshaft or valve springs used.
"Since Newton's Law of Motion tells us force equals mass times acceleration, if we change either the mass of the valvetrain components, or the acceleration rates of the camshaft, we change the forces applied to the valvetrain components."says Harding. "Changing the lightweight valvesprings with heavier, solid-stem one-piece valves is going to fundamentally affect the durability of the valvetrain and, ultimately, the engine's performance."
Think of it this way: The mass of the valve and all the reciprocating components of the valvetrain affect the forces in the system. The acceleration and force of the system are dictated by the camshaft lobe profile—or the rate at which the lobe moves the hydraulic lifter—so a careful balance of component mass, stiffness, natural frequency, cam-lobe acceleration, and forces experienced in the system is critical to high-rpm valvetrain stability.
That's why simply replacing the exhaust valves with those with a different mass has significant implications on performance: It upsets the balance that was built into the system in the first place.
We recently attended a test session at Katech's Detroit-area headquarters, in which company engineers tried to definitively determine the effect on LS7 valvetrain stability that comes with changing to heavier solid-stem, stainless-steel exhaust valves.
Note that while there is much more to valvetrain stability than simply the valves—the rocker arms, springs, pushrods, and camshaft all play critical roles—for this experiment, we're focused solely on how heavier valves affect stability.
"You can test combinations endlessly,"says Harding. "Typically, taking weight out of the system can improve stability. You can try a lighter lifter—if one exists—a lighter pushrod, different spring, lighter retainer, lighter valve, and so on. You want the lightest-possible spring that can control the valve. Some try to 'over-spring' the system, thinking a stronger spring will close the valve better, but sometimes slamming it shut hard can cause it to bounce. That's why the lighter beehive springs are usually better.”