Shop VALVE SPRINGS
Say NO to valve float...
When it comes to upgrades, the valve train is one thing that is seriously overlooked in the 951 engine. The effect of boost on the valve train is very important to consider, the first part of this discussion will focus on the valve spring. First off you must consider the parameters that this engine was designed around. From the factory, this engine's red line was 6400 rpm and was limited to 11.8 pounds of boost by the computer via the cycling valve. Porsche used a valve spring in this engine that was designed to operate within these limits, but they cannot handle a lot more. Read on to further understand just why that is.
When you start increasing the boost pressure, you also increase the force that is trying to prevent the valve from closing. The stock intake valve is 45 mm in diameter and has a 9mm-stem diameter. The factory valve spring has a seat pressure of 130 pounds and an open pressure of 230 pounds. When you calculate the surface area of the valve, it comes out to be just over 2.3 square inches. This means that when the valve has 11.8 pounds of boost pushing against the backside of the valve, it takes approximately 27 pounds of spring pressure to counter the effect of the boost pressure.
When you increase the boost pressure, for example, by increasing the boost from 11.8 pounds to 18 pounds. Now when you multiply the 18 pounds times the surface area of the valve and you now need over 41 pounds of spring pressure to control the valve, that is an increase of 14.2 pounds. Larger intake valves or higher boost pressures will make this problem even worse.
If that is not enough to convince you, consider this: The valve spring actually does two things, not only does it control the valve, but it also has to be able to control the weight of the hydraulic lifter, which happens to be over 100 grams each. Porsche designed their spring rate to handle this very heavy hydraulic lifter at the factory red line of 6400 RPM. With the aftermarket chips today, a red line of over 7000 RPM is commonplace. The inertia of the heavy lifter at that RPM adds significant stress to the valve spring.
All of this discussion so far has revolved around the stock camshaft. The stock cam has an intake lift of .472 and a duration of 223 degrees. Imagine what happens to the spring and lifter when you increase both of these values. The most popular aftermarket camshaft is the Web #274 grind. This cam has a lift of .480" and a duration of 234 degrees on both the intake and exhaust. The more aggressive duration and the ramp speed of the lifter with this duration adds even more stress on the lifter as well as the valve spring. These faster ramp speeds require higher open (nose) pressures to keep the lifter in contact with the cam. If the nose pressure is not high enough, the lifter can actually be thrown off the lobe of the cam (referred to as lofting). Depending on the severity of this, the lifter can now be slammed back against the cam with a hammering effect. This can be serious enough to actually fracture the surface of the cam lobe. Once this begins, it can quickly lead to the lobe being worn down and it can even pound down the face of the lifter and to the extreme, wear holes completely through the face of the lifter. See the photos showing this extreme situation.
When you combine the effect the additional boost pressure has against the intake valve with the weight of the lifter operating at 7000 rpm and the added stresses from a more radical cam, it doesn't take long to see that it takes more spring to properly control both of these critical functions.
Lindsey Racing is now offering a high quality spring, which has a seat pressure of 146 pounds at the manufacturer's recommended installed height. This is enough spring pressure to handle 20 plus pounds of boost pressure and can handle valve lift of over .500". If you are going to run higher boost pressures than that, we offer an upgrade to the inner spring that increases the seat pressure even more. Our racing springs are heat-treated, shot peened, polished, and then a final step that most spring manufacturers don't do is they are low temperature stress relieved.
These springs are set up at a different installed height than the factory spring and require the use of our custom spring cups and moly or titanium retainers to properly establish this critical height. This spring package is listed on this web page.
There are some additional components available that will help the problems I have listed here: