Unfortunately, the aircooled VW motor has a few inherent problems when it comes to modifications that double or triple its intended output. It is safe to say the Reimspiess or Volkswagen (perhaps not Porsche however) never envisioned their engines to achieve the horsepower numbers that are being produced today. The stock case, crank and heads (and others) just don't have the strength built into them for that kind of power. One part in particular, one that is often over looked, is valve train components, valve springs specifically.
Adding a turbo system, nitrous or anything that can increase the rpms of your engine over the stock "limits," the pushrods have a tendency to lose contact with the cam lobes (via the rocker arms) because the valve springs are not strong enough to overcome the momentum of the engine on the valve train components. This is commonly referred to as valve float, when the valves fail to completely close during the compression stroke, and/or can sometimes bounce off of the valve seat as it tries to follow the profile of the cam. An engine suffering from valve float has a characteristic sound; the engine will begin to cut out, making a rumbling noise. Valve float causes loss of power and usually leads to engine damage.
One way to combat valve float at high rpms is to add stronger springs, which will overcome the force of the valves being slammed shut as well as it bouncing off of the seat. It is an easy process to change the springs, closely related to a simple valve adjustment. Some specialized tools will be needed, such as the fork-type spring compressor (different from the C-type, which is used when the heads are off of the engine) and the threaded air pressure nozzle that fits into the spark plug hole. Both of these tools can be purchased at most advertisers in this issue, but specifically at Old Speed in Paramount, Calif.
Keepers and Failure
Because the triple-groove design of the keeper/valve stem is notorious for failure, performance options can be found in other car makers. While most enthusiasts assume that more is better, the opposite is true for keepers. The fact that the keeper has three small retaining slots and the valve three receiver grooves does not mean that there is extra holding power on the valve tip. These grooves are just what a crack needs, a stress riser. Stress risers are simply surface irregularities such as scratches and nicks (including machined grooves) that will eventually lead to cracks and component failure. Remember that it is harder to break a smooth surface than one with nicks and surface imperfections.
Though the number of grooves weakens the valve tip, a worse culprit is the method of retention. Take a look at the lowest groove on a stock valve/keeper combo. The keeper does not extend down past the groove, but rather ends right at the parting line of the groove. This means that the vast majority of the spring load is placed at this point, and it may be fine for stock springs/cams but add to this the increased pressure of a modified engine, and you may be looking for new valve stems sooner than later. Several advertisers offer a single groove keeper from a Chevy that extends well past the groove so that it applies pressure to the solid stem and not the groove edge.--RLP
SOURCE
Old Speed
7311 Madison Ave., Unit A
Paramount, CA 90723
(562) 531-4190
 A 13mm socket removes the...  A 13mm socket removes the rocker arms, which can be set aside. Be careful that the studs don't come out with the rocker arms. |
 Start by rotating the engine...  Start by rotating the engine clockwise so that piston Number One is at the top of the compression stroke, or TDC. Verify this by checking the location of the distributor. Remove the spark plug and fit a threaded attachment that will allow you to pressurize the piston to roughly 90psi. This will ensure that the valve will remain closed and so it doesn't push into the piston causing damage. |
 Fit on the spring compressor...  Fit on the spring compressor tool over the rocker arm studs and tighten it down. The fork presses onto the spring retainer, pushing the spring down and exposing the small keepers (two halves per valve). The spring may not immediately separate from the keeper, so a gentle rap with a rubber hammer will convince it, along with the back pressure from the cylinder side. |
 Remove the two keeper halves...  Remove the two keeper halves with a magnet and set them aside. Pull off the retainer and the old spring. Once the old spring is out of the way, the heavy duty spring can be installed in the reverse steps. |
 When re-installing the keepers,...  When re-installing the keepers, a little bit of grease will keep them in their three-grooved home, while allowing the first one to slip around to the bottom, making room for the top one. Note that the three grooves on the keeper match the three grooves on the valve stem. |
 After the springs for the...  After the springs for the Number One piston have been replaced, rotate the crank pulley clockwise to line up the next piston at TDC (verify the position with the distributor). Attach the air nozzle to that piston's spark plug and follow the above steps for each piston. |
 When you are finished, replace...  When you are finished, replace the rocker arms and torque them to 18ft./lbs.. And that's it. Easy, right? |
 Some of the tools you will...  Some of the tools you will need are a spring compression tool, an air nozzle attachment to your spark plug hole, a small magnet and a 13mm socket and torque wrench. |
 By comparison, the right photo...  By comparison, the right photo shows a seven-degree, single-groove Chevy keeper and stem. As well, a 10-degree keeper/valve is also available. Note that the new keepers extend past the groove onto the solid section of the stem. |
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