AV - Valve Train Geometry

* Date: Fri, 23 Apr 2004 19:23:39

Valve Train Geometry

The basic principle is quite simple: The rocker-arm, which serves as a lever, must act thru an arc. To convert the maximum amount of the arc-motion of the rocker into the maximum amount of linear-motion at the valve, the mid-point of the linear travel must fall exactly upon the tangent of the arc.

You will find the above endlessly repeated in various ways in the hot-rod magazines and that would be jus’ swell... if we could apply the procedure to the Volkswagen. Or Corvair. Or Lycoming, et al. But we can’t, unless we are looking at a bone-stock engine. That’s because the method outlined above addresses only the output side of the geometry equation. The input is not addressed because it doesn’t need to be, so long as we are talking mono-bloc engines, in which the distance & angle between the cam and the axis of the rocker-arm is fixed, or virtually so.

Unfortunately, most flying Volkswagens are big-bore strokers and a properly built stroker is wider than the stock engine. Making the engine wider not only increases the distance between the cam and the rocker-arm axis, it changes the angle between them. To achieve optimum valve-train geometry we must address two arc/lever systems, one for the input of motion to the rocker-arm as well as the output of motion from the rocker-arm to the valve. When dealing with the input side of the equation the same rule for maximum transfer applies, in that the half-point of the push-rod's linear travel must fall exactly upon the tangent of the rocker-arm's arc.

The tricky bit is the fact any change to one side of the equation will be reflected in the other, since the points of maximum transfer of motion must precisely coincide.. Most don't. Indeed, unless you're looking at a professionally built engine it isn't uncommon to see VW valve trains so mal-adjusted as to give away 25% of their potential lift.

The Conventional Wisdom fix to such geometrical disasters is to install larger valves and a cam having more lift. Of course, the larger valves will require heavier springs and the combination of higher lift and greater valve spring compression must be paid for with energy and wear. However, having arrived at this point because the person building the engine doesn’t understand the basic problem, there’s no guarantee they’ll get it right the second time around.

Indeed, across the range of rpm most suitable for slinging a propeller even the largest big-bore stroker has a very modest flow-rate, easily satisfied with single-port heads fitted with stock valves actuated by the stock cam. Assuming of course that the valve train’s geometry is properly set.

In setting-up the valve train's geometry the variables are the length of the push-rod, the length of the valve stem and the height of the pivot-point. Rocker-arm ratio (ie, the length of the input arm to the output arm) has relatively little effect since the length of the output arm remains unchanged and the point of tangency for the input of even the wildest ratio-rockers will still fall within the available limit of vertical travel for the push-rod (ie, in traversing the chord of the arc there is always some component of movement perpendicular to that axis).

Determination of proper valve train geometry begins with the basic blueprinting of the engine, when you measure the actual lift of your particular cam. This data is used in setting up the rocker shaft height relative to the valve stem height and may be done in a simple jig before the heads are installed on the engine.

Another necessary tool is a stock adjusting screw, modified by accurately grinding it to a fine point. The tops of the valve stems are coated with soot, lipstick or Dykem and an optical comparitor is used to determine where the point falls upon the face of the valve stem, the position of which is used to make any required adjustments. The adjustment procedure and a few drawings may be found in the HVX files covering engine assembly but will be included on this blog if time permits.

Push-rod length is best determined for each valve during trial assembly.

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Optimizing your valve train's geometry will improve the engine's volumetric efficiency which translates into more torque at low rpm and reduced fuel consumption for the same power output right across the band. Proper valve train geometry also guarantees the system is absorbing the smallest amount of energy, which translates into reduced wear and better output.

Getting the geometry correct isn't especially difficult but it takes a bit of time, calls for precision instruments such as dial indicators, and a simple jig that allows convenient manipulation of the valves & rocker arm.

Most VW 'experts' lump valve train geometry with dynamic balancing and a host of other 'unimportant' details. Rather than address the basic issue they tend to shovel money at the problem in the form of after-market heads having valves the size of dinner plates, hot-rod cams with Himalayan lifts and valve springs more suitable for a punch-press than a light aircraft engine. The fact their engines run and the plane flies is taken as proof that proper valve train geometry is just another of those 'unimportant' details :-)

It's up to you. You're the Mechanic in Charge.

-R.S.Hoover