Most rocker manufacturers now offer steel body rockers as an alternative to aluminum.
In reality, the purpose of valvetrain development is to make more power and accomplish this reliably. As we discussed in last month's series segment, improvements in gas exchange (cylinder filling or volumetric efficiency) is at the heart of power production. One way to improve power with the valvetrain is to open and close the valves in such a way to maximize volumetric efficiency and airflow, as rpm increases (particularly in the higher ranges).
Many Cup teams have taken a "high stiffness" approach to valvetrain development. The trend has been to increase the stiffness of the pushrod and rocker arm while using very aggressive lobe profiles to improve gas exchange and overall perform-ance.
For example, a current trend with Cup engines is to change from aluminum body rocker arms to steel body rockers, principally to improve stiffness. Most rocker manufacturers now offer steel body rockers as an alternative to aluminum.
However, stiff parts tend to have a bit more mass. As a consequence, more aggressive cams and increased mass of the stiffer rockers and pushrods require increased spring force (pressure) to keep everything under control. Generally speaking, increased weight leads to more "stored" energy at high rpm and greater difficulty in maintaining a given valvetrain's design parameters. While low mass (weight) and stiffness may seem diametrically opposed, you may want to err on the side of stiffness, especially when higher rpm comes into play.
Some Cup engine departments have been inspired by Formula One engine developments, focusing on power gains through valvetrain development. Instead of maximizing stiffness, teams maximize "specific" stiffness, or the stiffness-to-mass ratio. This tends to result in lighter components and the requirement for somewhat less spring force. Accordingly, a spring force reduction can result in lower frictional losses and net gains in power. Don't overlook this aspect in your Saturday night engine package.
On the subject of cam followers, flat tappets and roller tappets often pose different challenges which can result in the necessity for different valvetrain components. Here are some examples:
A flat-tappet valvetrain with a non-mushroom tappet and a tappet bore of fixed diameter has a geometric limit to how fast the cam (lobe) can move across the face of the tappet. If the velocity is too high, the cam lobe can run off the edge of the tappet.
For flat tappet engines, always use the largest tappet bore that packaging or regulations allow. Have the cam designed specifically for the tappet diameter you're using.
The equation for the maximum velocity of the lobe for a given tappet diameter is:
Vmax (inches/degree) = diameter (inches)/114.43
For a Cup tappet, NASCAR allows a maximum diameter of 0.875 inches. The maximum velocity is:
Vmax = 0.007654 inches/degree
Improving Maximum Valve Velocity with Flat-Tappet Valvetrains Traditional NSC valvetrains use the deflection of the system as a spring to store energy which is then used to accelerate the movement of the valve. (Think of this deflection motion as what you would see in a flexible "pole vaulting" pole, as viewed at a track meet. In the case of an engine, energy is stored in the overall valvetrain, not just the pushrod.) As the overall stiffness of the valvetrain is increased, other methods must be used to increase valve velocity and acceleration to achieve equivalent engine performance.
As an example, increasing rocker ratio is one way to increase valve velocity without breaking the speed limit of the flat-tappet. Some Cup engines have used rocker ratios as high as 2.5:1. Most teams are running ratios in the 2.1:1 to 2.3:1 range.
Increased Rocker Ratio has the following effects:
* Increase valve velocity and acceleration
* More aggressive valve motion
* Lower rocker Moment of Inertia (MOI)