Air-Cooled 911 Valve Spring
Valve springs control the valvetrain. It is valve spring pressure that forces the valves to stay in contact with the rocker arm. Without the proper spring pressure, the valvetrain becomes uncontrollable. Not enough pressure and you can get valve float when the valvetrain gear loses contact with the camshaft lobe. Too much pressure and you can get premature valvetrain wear. Causing excess wear on the rocker arm, camshaft, and the valve face and seat.
Testing a Valve Spring
The 911 valve spring is a duel spring. There is an innerspring and outer spring. They are also directional on the outer spring, meaning there is a top and a bottom to the spring. This is a performance set up because dual springs allow for high load pressures. Dual springs also allow for frequency dampening, which means the engine can rev faster with fewer concerns about bounce.
You can see that the best spring pressure is the spring pressure the factory designed. Over time the material in the spring loses its temper. Heat is the major cause of temper loss. The 911 cylinder head runs hot by nature and the spring life is finite. This means that every spring should be tested. And unless you know the complete history of the head, things could be out of specification. What’s been done to the head? How many miles are on the spring set? Most of the time it it best just to replace the valve springs.
If you want to know the spring pressure of your spring sets you need to test the springs. When you test the valve spring assembly, you must set them up correctly. Assemble the inner and outer spring, the valve spring seat, and the valve spring retainer is also in place.
The assembly should be tested at Seat Pressure, which is the valve closed height. And also tested at Nose Pressure, which is the valve completely open. The springs used in the intake and exhaust valves are all the same part number. The intake and exhaust springs are just set at different pressures. Springs should be within +/- 5% of the specification.
Valve Spring Pressures
Intake Seat Pressure 44 PSI @ 42 mm +/- 5%
Intake Full Load Pressure 177.6 PSI @ 30.5 mm +/- 5%
Exhaust Seat pressure 44 PSI @ 42 mm +/- 5%
Exhaust Full load pressure 165.3 PSI @ 31.5 mm +/- 5%
Tools you may need
The spring compression tester that we use is old, but it does the job. There are many choices out there. We test new springs as well as old. Testing springs makes you informed, if you don’t know your spring pressure, you don’t know.
How to rebuild a 911 Cylinder Head. Lesson 4, Valve Guides
In the next lesson, we cover the Valve Guides. How to inspect them, getting precise measurements, and how to replace them.