Muscle Mustangs & Fast FordsHow To Chassis Suspension
Crower Shaft Mount Rockers Add Power & Durability
In racing, the valvetrain is one of the most highly stressed engine systems. High valve lift, high engine speed, and high spring pressures exponentially increase their demands on the lifters, pushrods, rockers, and springs. At 7,200 rpm, each valve opens and closes sixty times a second! The forces at play in the valvetrain are mind-boggling.
The rocker arms are a critical link in the valvetrain, as they transfer the pushrod's upward movement to push open the valves. Many early overhead valve engines used shaft rockers, but manufacturers soon found out that stud-mount rockers were cheaper to produce. These have their place, but when it comes to high performance applications, it's hard to beat a premium shaft-mount rocker setup.
Because of the geometry of today's high-performance engines, the studs can flex quite a bit. High-angle valve heads and higher spring pressures can cause the studs to flex before the valve even opens. If you already invested in stud-mount rockers, a stud girdle is an effective band-aid to spread the loads across all the studs—but if you're starting from scratch, a high-quality shaft-mount rocker setup is the way to go.
When building a race engine, a shaft-mount rocker system increases power and durability. The shaft system adds a stable foundation for the rockers, which greatly reduces flex. This means less of the camshaft lobe's lift is wasted moving parts around, and more lift opens the valves.
Shaft rockers also reduce friction by reducing flex and increasing rocker arm radius. When the rocker stud flexes, the rocker arm tip moves a longer distance across the valve tip. Each thousandth of an inch that the rocker tip drags (or "scrubs") across the valve is wasted movement that isn't opening the valve.
To further reduce rocker tip scrub, shaft rockers usually relocate the pivot point toward the pushrod, which increases the radius of the rocker's arc and reduces the distance the tip drags across the valve. Reducing rocker tip scrub cuts friction, increases power, and extends valve guide life.
For an upcoming build, we chose Crower's shaft-mount rockers. While slightly more costly than a stud-mount setup and a girdle, we knew the benefits were worth the extra cash. Crower offers shaft-mount rocker systems in stainless steel for maximum durability and we opted for needle bearing tips to further reduce friction.
Because the shaft keeps the rocker arms aligned, pushrod guide plates (another source of friction) are eliminated.
When our race engine is buzzing through the speed traps, this Crower shaft-mounted rocker system is one less thing we will worry about.
Photos by Henry De Los Santos