Owner, B³ Racing Engines
Welcome back, everyone! In the last segment we discussed the different fulcrum lengths and the effects on roller placement on the tip of the valve. I also threw in a teaser with the corrected Proform rocker arm, which is widely regarded as one of the worst rockers for Mopar engines available. Even the Proform can have acceptable geometry, and in a very mild, low rpm street application should work well for a long time. Although the rocker is very short, when the rocker shaft is in the right location, there is ample room for a 1.550” diameter spring.
In this segment, I want to address the notion that the rocker should reach its perpendicular plane (90 degrees) to the valve at 2/3 of the valve lift instead of 1/2 of the valve lift. This concept was introduced in the early 1900s to minimize scrubbing across the valve tip at higher lifts where the spring pressures were greater. The focus at the time was strictly on longevity and wear, not performance. This was well before the first automotive roller rocker was invented by Harland Sharp in 1958. With the “shoe” type rocker, the geometric pivot point was at the contact area of the rocker and the valve tip, meaning the rocker was scrubbing across the tip of the valve throughout the lift cycle. As the shoe contact point changed during the lift cycle, moving inside to outside, the effective fulcrum length was also increased which actually increased the rocker ratio. To address this problem, engine builders were advised to start setting up the valvetrain for 2/3 geometry which reduced the side loading at high lift with the compromise being more sweep in the lower lifts where spring pressure was also lower. This method of setting geometry has been abandoned by automobile manufacturers and elite race teams beginning in the late 80s and early 90s.
Enter the roller rocker arm. A roller tip was added to the nose of the rocker and everything changed. No longer was the geometric point of rotation at the contact area of the roller and the valve tip. It is now at the center of the roller axle and maintains that point throughout the whole lift range, which means the ratio stays constant as well. This is also why a roller rocker fulcrum (shaft) needs to be raised from the factory location, instead of lowered. This is entirely different than the “shoe” type rocker which, as previously mentioned, has its geometric point of rotation at the contact area of the valve stem and rocker. While many may think that the purpose of the roller tip is to reduce friction by rolling across the tip of the valve, it is not. Under spring pressure in a running engine, it has more of a tendency to skip across the tip of the valve than to roll. It, like a roller lifter, is meant to create a constant, unchanging point of rotation through its range of motion. One only need think about the way a flat tappet follows the cam lobe to see that it does not contact the lobe at the same point on the tappet face through the lift cycle. Like the shoe type rocker, the result is a scrubbing action across the face of the tappet which limits the velocity of the lobe to what the tappet diameter can handle. This is where the biggest advantage is to having the .904” lifter. Ever heard of a cam with 904 lobes? But hey, I thought we were talking about rocker arms.
Let’s talk about what happens as a rocker arm goes through its arc during the valve opening and closing events. Think of it in the same terms as the rotating assembly in the engine. Many are aware that piston speed is the highest when it is in the middle of its stroke in the bore. What some may not be aware of is that the crankpin (rod journal) is at a 90 degree angle to the cylinder bore. The geometric line is from the rotational center (main journal) to the center of the rod journal. As the piston approaches top dead center, TDC, it decelerates and comes to a brief, but complete stop before accelerating back to the middle of the stroke. The same deceleration occurs as the piston approaches bottom dead center, BDC, and the cycle starts all over again. Imagine that the piston speed is the highest at TDC and there is no deceleration or dwell time. This of course is not possible, because the crankshaft makes a full rotation and does not reverse direction. Regardless, if it was possible, the piston pin would be ripped out of the piston as it tries to change directions instantaneously. This is akin to what is happening to Mopar valvetrains every day. Much like the crankshaft acts upon the piston and connecting rod, the rocker arm acts upon the valve. Unfortunately, this scenario is possible in the valvetrain because the rocker arm does not make a full rotation, and instead reverses direction. If the perpendicular line is at or near full lift, the valve will have no time to decelerate and the spring will lose control of the valve. This is why having the highest velocity at the middle of the lift cycle is so important. The seat position should be thought of as being BDC, and the full lift position as TDC, with the highest velocity point in the middle. Think about how much additional stress the valvetrain parts are exposed to if this setup isn’t right, and how many broken parts you have seen or heard about over the years. It’s a sobering thought, isn’t it?
Simple logic would tell you it is foolish to use the 2/3 lift method because the valve has no ability to decelerate completely as it approaches full lift and therefore, will require more spring pressure to control the valve than necessary. Ever heard of a Mopar needing more spring pressure? Yeah, I thought you did, but we will get into spring pressures in another segment.
There is still more information that has not been covered, but I am going to let this soak in a while. Stay tuned for Part 3 of our geometry tech series where I will get into the performance losses from incorrect geometry. Until then, Enjoy!