Valve Train Total System Stiffness

This article and information was originated by Billy Godbold at Comp Cams and is being reproduced with their permission.

Whenever you are looking at any component between the camshaft and the valve, there are only three system attributes that effect engine performance as follows:

  1. Stiffness
  2. Mass
  3. Durability

If possible, we would want an infinitely stiff valve train system with zero mass that would last forever.  Over the past few years, I believe most engine builders have really began concentrating on maximizing stiffness while still minimizing mass at the component level.  The next step is to understand how all of the components work together to maximize the ratio of SYSTEM stiffness to SYSTEM effective mass.  The best place to start is to look closely at the system stiffness.  The static system stiffness can be calculated by measuring lift with a checking versus real spring and zero lash as follows:

System Stiffness (Lbs/Inch) = (Difference in Load)/(Difference in Lift )

The differences are between loaded load (LBS Spring Force of Real – Checking) divided by the change in lift with the checking spring and real spring.  If you had 500# open load on the real spring and virtually zero on the checking spring and then measured .020” lift difference the stiffness would be 500/.02 = 25,000 lbs/inch.

Typically, going to a larger cam journal or a slightly thicker wall pushrod can move that up by a couple hundred pounds per inch.  Increasing the pushrod diameter or going to a stiffer rocker and rocker stand would be the path to make larger changes.  However, with each change, you can measure the stiffness and find what components have the largest effect.

The mass side is a bit more difficult to calculate.  The valve side components are simply their mass as measured on a gram scale.  The pushrod side can be calculated by measuring the mass then dividing that by the rocker ratio.  The rocker is a bit too difficult to get into details here, but you generally start with something like a three wire pendulum to measure the moment of inertia (you can likely find details on these online).  Just always remember that the rocker mass is not important (unless someone is throwing the rocker at you), but the moment of inertia is paramount.  Mass toward the center of the trunion is moving slowly, but mass towards the ends moves quickly as the rocker oscillates and the valve opens and closes.

The current state-of-the-art is to use these measurement techniques to ensure stiffness is maximized while total mass stays the same or increases significantly less than the stiffness increases (percentage wise) as you improve the system.

For more information like this in a hands-on instructor led environment, check out our Competition Engine Development (CED) course at where we explore in depth topics to maximize your engine’s power output.