Valve springs are probably the most stressed components of an engine.
Yet they are also one of the most oft-overlooked parts in the entire engine.
The fact of the matter is valve springs are among the most critical pieces of an engine assembly, essentially acting as the glue that holds the entire valvetrain together. By choosing the wrong set of valve springs, you can severely limit the power output of your engine and even cause catastrophic damage. That’s why it’s important to select the right valve springs for your application.
Determining the right spring starts with figuring out the correct installed valve spring height.
What is Installed Height?
The installed height of a valve spring is the total height of the spring when the valve is closed. It is measured from where the spring meets the bottom of the retainer to where it rests against the cylinder head. All camshafts come with a recommended valve spring height; it is usually on the spec card included with the cam. If you follow that recommendation, you will have a spring that is not too weak (causing the valves to float at high rpm), or one that has too much spring pressure (which can wipe a lobe off the cam).
Now that we mentioned spring pressure, we’ll quickly discuss the two basic types.
- Seat pressure is the pressure the valve spring places on the valve when it is in closed position.
- Open pressure is the spring pressure that forces the valve to close after the rocker arm pushes it open during the combustion cycle.
As you probably guessed, open pressure is greater than seat pressure. According to the folks at Lunati, seat pressure for non-roller hydraulic cams should be around 300 pounds maximum (350 pounds for non-roller mechanical cams). Seat pressure should be about 110-120 pounds maximum (140 pounds for mechanical cams). Roller cams are another story.
Because it moves the valve at higher velocities, a roller cam requires a spring with more seat pressure to prevent valve float. Also, rollers don’t have the scuffing associated with flat tappet cams and lifters, so springs with higher open pressures (around 500-800 pounds at peak valve lift according to Lunati) can be used to let the engine rev higher.
Checking Installed Height
Most cam grinders offer springs matched to their camshafts. We recommend you follow these manufacturer specifications. But it doesn’t matter if you use springs matched to your cam, are running different springs, or are changing springs—you must check the installed height to make sure it matches what the cam card says. There are two ways to measure installed height:
- Using a good mechanic’s ruler, measure the spring from where it meets the bottom of the retainer to where it sits on the cylinder head. DO NOT INCLUDE THE THICKNESS OF THE SHIMS.
- Use a valve spring height micrometer. Much more precise than a ruler, the micrometer replaces the spring itself. All you have to do is turn the dial until the valve is seated snugly in its seat, then note the measurement. If the spring is not at the recommended installed height, you will either have to shim it until it reaches the proper height, or you can machine the spring pocket in the head deeper to get the installed height you need.
If you go to a spring with a larger diameter than stock (common with bigger cams), you may have to machine the spring seats on your cylinder heads. Also, if you change from a spring with a rotator to one without, you will need to change to the recommended retainer to avoid changing the installed height.
But the power and higher capacity will be well worth the trouble.