Darin Morgan: Valvetrain technology has come a long way in the last 10 years, but like anything else in the development process, you can’t put your finger on one single thing that’s responsible for the forward progress. It’s been a combination of many small advances and failures that got us to the point where we are now. Back 20-25 years ago, we were running stock diameter cams that had lots of resonance. Even if we had the best springs in world, we couldn’t turn more than 9,500 rpm. As soon as engine builders stepped up to 55mm cores, the valvespring technology wasn’t there. Between 1999 and 2003 is when big changes started to happen. By then, we had even larger 60mm cam cores along with the valvespring and cam lobe ramp technology to turn lots of rpm. Before, we used to brutalize the valvetrain, which is the wrong way to do it. Now we finesse the valvetrain to loft it over the nose of the cam. It’s hard to predict where things will go in the future, but the current trend is stepping up to larger core cams and using lifters with bigger wheels to improve valvetrain control at higher rpm. At the Pro Stock level, we start losing control at 10,800 rpm. For engines in the 350 to 380ci range, the ceiling is 11,000 rpm. At Reher-Morrison, we just built a 363ci small-block that makes 1,040 hp at 10,100 rpm naturally aspirated. Numbers like that would have been unheard of just five years ago. The important thing to remember is that every engine is its own animal. You can’t take the valvetrain from one engine and put it in another one and expect it to work perfectly.
Judson Massingill: As rpm and valvespring pressure increases, the studs on a pedestal-mount rocker arm’s setup will flex and eventually break. Mounting the rocker arms on a shaft instead of a stud increases rigidity tremendously, and therefore shaft-mount rockers are a must in high-rpm race motors. That said, rigidity is just part of the equation. Shaft-mount rockers also make it much easier to achieve proper valvetrain geometry. With a stud-mounted rocker system, the only way you can adjust the geometry is with different length pushrods. On the other hand, with shaft-mounted rockers the centerline of the pivot point of the rockers can be positioned perfectly in relation to the tip of the valve, since the rockers are mounted on a stand. All you have to do then is measure for the correct length pushrods afterward. It is this combination of improved geometry and rigidity with shaft-mount rockers that enable turning more rpm.
Phil Elliot: Valvetrain stability is what everyone is after, and shaft-mount rockers are a great way to accomplish this. Years ago, people put plexiglass on valve covers, and recorded the valvetrain movement with a high-speed camera. They were scared by how much things moved around even with stud girdles. This reinforced what racers had known all along, which is that stud-mounted rockers didn’t provide enough stability in race motors. The notion that you need to step up to shaft-mount rockers at a certain rpm is a bit misleading. The stress imparted on a rocker arm is a product of both spring pressure and rpm. Spring pressure is actually what tries to rip the stud out of head. Fortunately, race engine builders don’t have to deal with all the design parameters that the OEMs have to. In racing we don’t need to worry about whether or not the valve covers will fit under an A/C compressor. We just build a new valve cover that will fit around the cylinder head and rocker arms.