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Valve Spring Rate and How to Use It

  
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Valve Spring Rate and How to Use It

 
Pontiacman8 Pontiacman8
Moderator | Posts: 5777 | Joined: 02/08
Posted: 12/17/13
07:16 AM

The rate of a spring is the force necessary to compress (or deflect) the spring a specified distance. For example, if we say that a spring has a rate of 250 lbs. per inch (250#/in.), it will take 250 pounds of force to compress the spring 1 inch. Fortunately, valve springs are coil springs, and coil springs are easy to understand because they have an almost linear spring rate. In other words, if it takes 400 lbs. to compress a spring 1 inch, it only takes 100 lbs. to compress the spring .250 in., 200 lbs. to compress it .500 in., and 300 lbs. to compress it .750 in. Some people refer to spring rate as "stiffness", and it is the understanding of this spring characteristic that is most important in selecting and setting up springs on an automotive cylinder head.

Frequently a taller, softer spring is a better choice for a performance application than a short, stiff spring.

Consider the following possibility:
A vehicle owner wants to use a .520" valve lift camshaft in an application and is considering different valve springs.


Spring A has an installed pressure of 125# at 1.750" installed height and has a rate of 280#/in.


Spring B has an installed pressure of 115# at 1.750" installed height with a rate of 410#/in.


At .520" lift, Spring A has an open pressure of 271# (this is 125# of seat pressure plus [.520" x 280#/in] = 146# from spring compression). At .520" lift, Spring B has an open pressure of 328# (this is 115# of seat pressure plus [.520" x 410#/in] = 213# from spring compression). Both of these springs would work on a street performance application requiring good performance and reliability.
However, Spring A with a lower open pressure of 271# could probably be used on a cylinder head with pressed in rocker studs; while Spring B would definitely require screw in studs for adequate reliability. Spring B would probably provide better performance above 6000 RPM (especially with relatively heavy valves) because of its higher open pressure of 328#. Spring A would probably idle a little smoother with higher vacuum, especially if a high pressure oil pump or thicker oil is used. This is a result of Spring A's higher seat pressure of 125#.

As you can see from the example above, there are often different springs that can offer different benefits on the same cam profile. Spring A offers good performance over a wide RPM range at a lower total valve train cost (this assumes that the heads were not machined for screw in studs). Spring B offers the possibility of somewhat improved performance beyond 6000 RPM. The vehicle owner needs to decide what he wants from his vehicle and what he wants to spend.

In all-out racing, we frequently see the need for different springs on the same lobe profile depending on the anticipated RPM range. Frequently, circle track racers will run two different tracks with the same engine but with different rear end gearing. Often there can be as much as 500-700 RPM difference in the top end engine speed between the two tracks. It is not uncommon to find that the car runs better on the track with the lower peak RPM using a spring with a lower seat pressure and softer rate. At the track where the engine runs to the higher speed, the engine needs more seat pressure and a "stiffer" spring rate. Every combination of engine, chassis, and track is different.

Significant performance improvements can often be achieved by experimenting with valve springs. If you aren't paying attention to your springs, the guy winning most of the races probably is!  
Engine builder,self taught auto body guy.
Horsepower sells engines and torque wins races

Pontiacman8
Pontiacman8
Pontiacman8