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
7311 Madison Ave., Unit A
Paramount, CA 90723
A 13mm socket removes the rocker arms, which can be set aside. Be careful that the studs d
Start by rotating the engine clockwise so that piston Number One is at the top of the comp
Fit on the spring compressor tool over the rocker arm studs and tighten it down. The fork
Remove the two keeper halves with a magnet and set them aside. Pull off the retainer and t
When re-installing the keepers, a little bit of grease will keep them in their three-groov
After the springs for the Number One piston have been replaced, rotate the crank pulley cl
When you are finished, replace the rocker arms and torque them to 18ft./lbs.. And that's i
Some of the tools you will need are a spring compression tool, an air nozzle attachment to
By comparison, the right photo shows a seven-degree, single-groove Chevy keeper and stem.