The camshaft designs the airflow, but it does nothing on its own. Between the cam lobe and the valve sits a chain of components that all have to work together: lifter, pushrod, rocker arm and valve spring. If a single link in that chain is weak, it makes no difference how perfect the cam profile is.
The chain in an OHV valvetrain
In a classic OHV engine (overhead valve) with the camshaft in the block, the chain looks like this:
- The cam lobe rotates and pushes the lifter upward. The profile's duration, lift and lobe separation define the valve events.
- The lifter follows the cam lobe. Flat tappet or roller, hydraulic or mechanical.
- The pushrod carries the motion from the lifter in the block up to the rocker arm on the head. A simple part, but a critical one.
- The rocker arm acts as a lever that inverts and multiplies the motion. The lifter pushes up and the rocker pushes the valve down.
- The valve opens and closes the port in the head, guided in its linear travel by the valve guide.
- The valve spring closes the valve and keeps the whole chain in contact with the cam lobe.
OHC engines: a shorter chain
In an OHC engine (overhead cam) the camshaft sits in the head, directly above the valves. That eliminates the pushrods and, in DOHC engines with bucket followers, the rocker arms as well. The cam lobe presses directly on a bucket sitting on top of the valve stem.
CatCams, who design cams for European OHC engines, have developed their own slipper followers to replace the OEM roller finger followers. A shorter chain means less flex, lower weight and a better ability to follow aggressive cam profiles at high rpm.
The pushrod: underrated but decisive
In an OHV engine the pushrod is the weakest link in terms of stiffness. OnAllCylinders and modern engine builders point out that development has moved from standard 5/16" pushrods to thicker, tapered or elliptical designs in chrome moly steel.
Why does it matter? A pushrod that flexes under load absorbs part of the cam lobe's motion. The lifter gets pushed up, but the rocker arm never receives the full movement. That costs effective lift and distorts the valve timing profile. The stronger the springs and the more aggressive the cam profile, the stiffer the pushrod needs to be. Column strength, the rod's resistance to buckling, is the measurement that matters.
Rocker arm multiplication: rocker ratio
The rocker arm works as a lever with different arm lengths on each side of its pivot. The relationship between those lengths is called the rocker ratio, and it multiplies the lift of the cam lobe.
Typical rocker ratios:
- Chevrolet small block (older): 1.5:1, so a cam lobe with 7.6 mm of lift gives 11.4 mm of valve lift
- GM LS series: 1.7:1, where the same lobe gives 12.9 mm
- Ford Windsor: 1.6:1
Kent Cams specifies rocker ratios for all their engine applications (from 1.46:1 to 1.69:1 depending on the engine) and stresses that it is valve lift, not cam lift, that decides the flow through the head.
A higher rocker ratio increases lift without a cam swap, but it also raises the acceleration in the valvetrain and puts higher demands on the springs and pushrods. It is not free power. It is redistributed load.
Valve float: when the chain breaks down
Valve float occurs when the valve spring can no longer keep the lifter in contact with the cam lobe. The valve floats, following its own path instead of the lobe profile. COMP Cams and MooreGoodInk describe three related phenomena:
- Float means the spring cannot accelerate the valve back toward its seat fast enough. Most common at high rpm.
- Bounce means the valve rebounds off its seat on closing. The spring does close the valve, but with too much speed, so the valve strikes the seat and bounces open again.
- Surge means the spring vibrates at its resonant frequency and loses its control force. Beehive springs and dual springs damp this.
All three mean the valvetrain has lost control, and in an engine with tight valve-to-piston clearance the outcome can be catastrophic.
Matching the components
The valvetrain is a system, not a list of separate parts. COMP Cams stresses that the cam profile, the lifter type, pushrod stiffness, rocker ratio and spring capacity all have to be sized as a whole:
- A more aggressive cam calls for stiffer pushrods, stronger springs and possibly roller rockers
- A higher rocker ratio increases the load on the pushrod and the spring
- A hydraulic lifter caps the usable rpm no matter how strong the springs are
- The wrong pushrod length changes the rocker geometry and ruins the contact point on the valve stem
Summary
A camshaft delivers nothing without a valvetrain that can follow the profile all the way. Every component, from the lifters to the springs, has to be sized for the cam profile and the rpm you plan to run. The right cam choice starts with understanding the whole chain.
Contact Meksta and we will make sure every part of your valvetrain works together.