Valve springs have one job: close the valve and keep it closed. As cam profiles get more aggressive and engine speeds climb, the demands on that job rise dramatically. The wrong spring can cause valve float, spring surge, bent valves or premature wear on the cam and lifters. The right spring protects the entire valvetrain and lets the cam profile do its work in full.
Why spring choice matters
The camshaft pushes the valve open and the spring pushes it back. As long as the spring can keep up with the lobe profile, the valve closes properly on every cycle. If the spring is too weak, or cannot follow the ramp speed of the lobe, the lifter loses contact with the cam. That is valve float, and it quickly damages the lifters and the camshaft, and can end with a valve meeting a piston.
A spring that is too strong, on the other hand, adds friction and wear at the cam lobe, especially on flat tappet cams where the contact surface is already hard pressed. Kelford Cams recommends never running a flat tappet cam with more open pressure than the manufacturer specifies, precisely to protect the lobe surface.
Seat pressure and open pressure
Spring pressure is measured at two points: at installed height (seat pressure) and at maximum lift (open pressure).
- Seat pressure is the spring force with the valve closed. You need enough of it to keep the valve sealed against its seat and to stop the valve bouncing on closing. A mild street engine typically runs seat pressures of 40 to 60 kg, while racing applications can call for 70 to 90 kg or more.
- Open pressure is the spring force at maximum valve lift. This is the force working against the ramp speed of the lobe. The more aggressive the cam profile and the higher the lift, the more open pressure it takes for the lifter to follow the lobe all the way.
PF Racing, for example, states that their mildest turbo cams (PF-1T, PF-2T) run on stock Volvo springs, while the PF-3T and up require uprated single springs, simply because lift climbs to 13 mm and beyond and puts higher demands on open pressure.
Installed height
The installed height of the spring determines seat pressure at assembly. If the cylinder head is decked, the valves sit deeper, the spring compresses further and seat pressure rises. The height also changes if you fit valves of a different length or switch to different retainers.
Always check installed height against the spring manufacturer's specification. Valve spring shims (thin washers) can be used to correct the height if it is off.
Spring types
Single springs
The most common type. One cylindrical spring per valve. Suits most street engines and mild performance builds. The standard springs on most engines are singles.
Dual springs
An inner and an outer spring working together. They deliver higher total spring pressure and better resonance control, and they are used in more demanding applications and in racing. When breaking in a flat tappet cam, the inner spring can be removed temporarily to reduce the load during the critical first 20 minutes.
Beehive springs
Conical in shape, wider at the base and narrower at the top. The reduced mass at the top lets the spring follow more aggressive lobe profiles at high rpm without surging. Beehive springs are a popular modern upgrade that outperforms conventional cylindrical springs without raising seat pressure unnecessarily.
Coil bind: a critical limit
Coil bind happens when the spring is compressed until the coils stack solid against each other. If that occurs at maximum valve lift, the result can be a bent valve, a broken spring or a damaged camshaft.
There must always be margin between maximum lift and coil bind, normally at least 1.5 mm of spring travel in reserve. Check it by measuring the spring's free length, working out installed height minus maximum lift, and comparing the result with the spring's solid height (coil bind height).
Spring surge and resonance
Valve springs have a natural resonant frequency. At certain engine speeds the spring can start vibrating in time with the impulses from the lobe, which is known as surge. The symptoms are unstable valve motion and increased wear. Dual springs damp this effectively because the two springs have different resonant frequencies, and the conical shape of a beehive spring provides natural damping as well.
Signs of tired springs
- Reduced seat pressure (measure it with a spring tester)
- Free length shorter than specification
- Rough idle or valvetrain clatter at high rpm
- Visible clues such as discoloration, surface cracking or uneven wear marks
Valve springs have a limited service life, especially in engines that get worked hard. Check the springs at every head job and replace any that fall outside tolerance.
Matching the spring to the cam
Always start from the cam manufacturer's recommendation. The spring must:
- Have enough seat pressure to keep the valve closed at your target rpm
- Have enough open pressure to follow the ramp speed of the lobe
- Have margin against coil bind at maximum lift
- Suit the installed height of your particular engine and head
A well-chosen valve spring protects the lifters, the camshaft and the valves. A poorly matched spring can destroy a new camshaft in short order.
Need help matching springs to your cam profile? Contact Meksta and we will sort it out with you.