Selecting valvesprings is obviously a critical decision point in the development of an engine package. The valvespring is the Witney Carson to the cam profile’s Alfonso Ribeiro – it has to move just as the cam profile directs, but with the disadvantage of being a comparatively low frequency component – that means it tends to jiggle a bit 😉. As I’ve heard Billy Godbold say, “…the valvespring has to do everything backwards and in high heels.” Billy has a gift for making the technical immediately understandable!
The valvespring has two jobs to do. It must::
- Control the mass of the valvetrain
- Control its own mass
On the first point, controlling valvetrain mass is a function of the mass itself, the engine speed, and the cam profile. What I’ve observed others in the business of selling valvesprings do, is ask the customer questions on valvetrain component weights, intended engine speed, and cam lift and duration. The last item functions as a stand-in for actual cam profile data, which most if not all valvespring suppliers are not proficient in generating. Generating cam profile data also takes time, and requires a skilled individual to do.
Using lift and duration to gauge valvespring load requirements is like playing a game of odds. The odds often fall in the customer’s favor, but there are some situations where you would go bust. I’ll toss out some amplified examples that show the risks:
Example 1: Highly Asymmetrical Profiles
Asymmetrical profiles have been around for a long while. Harold Brookshire built a company based on them (UltraDyne). Asymmetrical profiles can drive unique requirements for valvesprings.
Both of these profiles have near identical duration at 0.050″, and identical valve lift. That’s where the similarities end.
What may not be obvious is that both of these profiles are asymmetrical, in that they both have an opening ramp that is different to the closing ramp. One of the profiles contains a high degree of asymmetry in the negative acceleration region – this is where the valvespring load is especially critical. The profile with large asymmetry would lose control of the valvetrain at an earlier speed than the profile with less asymmetry.
In the examples below, the valvetrain equivalent mass is 424g, the engine speed is 6500rpm, and spring is a PAC 1343 installed @ 1.860″ with 240lb on the seat and 566lb over the nose.
Since both of these profiles have identical max lift and 0.050” duration, using these characteristics, along with mass and engine speed would result in the same spring recommendation. The reality is these profiles require two different springs. This is evident when the blue “Spring Acceleration” curve is above the “Cam Profile Acceleration” curve.
Example 2: “Cheater” Cams
“Cheater” cams are used in situations where sanctioning bodies implement lift rules. They seek to maximize flow area under the lift curve while respecting a max valve lift restriction. They aren’t always labelled as “cheater” cams, so you may not know you have one. They are known for having poor dynamics, and part of the reason is shown below:
As you can see, despite these profiles having the same lift and duration, the valvespring requirements are very different. If the wrong valvespring is chosen, damage can result.
Choosing the correct valvespring for the application is a job best given to someone who has the ability to reverse engineer and analyze the cam profile. If you’re unsure about your valvespring match, we’re happy to help you.
In the next installment, we’ll discuss the second job of the valvespring: controlling its own mass. The cam profile plays a key role in this as well.
