* Higher pushrod loads
* Higher rocker arm loads
* Less camshaft lobe lift
Increasing the rocker ratio 25 percent increases the maximum velocity of the valve by 25 percent (for the same lobe) as shown in Graph B (page 38).
The net effect of increasing rocker ratio is as follows: As the rocker ratio is increased, the pushrod lever arm that opens the valve is shortened. This increases pushrod loads and may cause problems of galling at the rocker arm end of the pushrod. As a result, stiffer pushrods may be required.
In addition (when using high ratio rocker arms as shown in Graphs A and B), lobe design and cam grinding becomes much more important (critical for power). For a given valve lift, as rocker ratio increases, lobe lift decreases. As a result, any errors in camshaft grinding will be amplified by the higher rocker ratio. Finally, with high pushrod loads, there is a benefit to have the pushrods as straight as possible, thereby reducing bending loads.
Roller-tappet valvetrains do not have the same geometric limits as those using flat-tappets. For example, the cam cannot run off the edge of the roller face as it can with a flat tappet.
Without having to worry about the speed limit at the tappet, the correct approach is to put as much lift at the lobe as possible. In this regard, the diameter of the cam journals then becomes the limiting factor. So, use the largest diameter cam core that the rules will allow. Have the lobe designed specifically for this tappet wheel size and core. Larger base circles also increase the maximum radius of the curvature for the lobe.
As the lifter roller wheel diameter is increased, the lobe designer can design a more aggressive cam.
Roller cams will require significantly higher spring loads than flat-tappet cams, so start your considerations with the valve.
Make the valve as light as the manufacturer recommends for your application. Every gram of mass removed from the valve makes the valvetrain easier to be controlled by the spring. Less force (load) means that the components don't have to be as strong, which therefore means they can be lightened for another cycle of force reductions.
Intake valves with head sizes of 2.200 inches have been successfully run in Cup, using both 7mm (hollow) and 6mm (solid) stems. The large diameter hollow stem provides improved bending stiffness, when compared to the solid stem, but is more complex and can be more expensive. At extreme (high) airflow rates, the smaller diameters can have an incremental improvement in airflow. Be careful not to split hairs on this issue.
Cup engines have run 7mm hollow exhaust valves. And, as you would expect, sodium filling the exhaust valves improves their ability to survive at extremely high temperatures. When leaner air/fuel ratios are run (to reduce fuel consumption), sodium-filled valves are more likely to survive. This is especially (and obviously) true in longer races.
Don't forget the retainers, when looking to improve performance or durability. Depending on your application, appropriate choices include steel, titanium, or even aluminum. Consultation with your chosen retainer manufacturer never hurts. At some point, you can assume they know more about their product than you-although not always. Lightweight metal may need a wear coating to survive, although some coatings can add significantly to retainer mass. Be sure to factor this in when evaluating the overall mass (weight) among different types of retainers.
Today, many Cup teams are using steel rockers because of their improved stiffness. Teams are also removing the adjusters from the rocker for a 10- to 20-percent reduction in MOI. Lash adjustment is accomplished either with a cam at the pivot point or by changing lash-cap thickness.