The Camshaft Conducts The Performance
Whenever any group is brought together to perform a specific function, someone must take charge and direct the efforts of each individual member so the proper result will be achieved. For example, orchestras have conductors, committees have chairmen, companies have presidents, etc. The same is true of the performance of an engine. The conductor of the engines performance is the
camshaft. It directs the timing of all the components of the engine determining the limit of effective RPM range and at what RPM the peak horsepower will be available.
As the camshaft controls the action of all engine components, it can be readily seen that any change in camshaft design can greatly affect the performance of an engine. Just small changes in cam lobe profile can make the difference between an engine being a snarling tiger and a purring pussy cat. Of course, other items should be changed to complement these changes, but the camshaft makes the main difference.
What is a Camshaft?
Camshaft design is an exacting science with a touch of art. Fig. 1 shows a typical cross- section of a lobe with its corresponding nomenclature. The base diameter is the portion of the lobe on which the valve lifter rests while the valve is closed. The nose is the portion where maximum valve lift occurs. There are two flanks, one on either side of the nose, which control the opening and closing velocity and acceleration and deceleration of the valve. The lift of a cam is the difference between the distance from the nose to the heel and the base diameter.
Most camshafts used in today's gasoline engines are ground from hardenable iron castings. The lobe design used differs greatly from one engine to another, depending on the type of performance desired, size of valves, intake and exhaust passages, and carburetor used.
Valve Lifter is Second Link in Chain:
The second link in the engine's chain of command is the valve lifter. Its role is that of following the camshaft lobe and controlling the engine valve operation. The camshaft and valve lifters must be compatible, both in design and material, if the engine is to perform as it was originally designated.
Let's look at the design of valve lifters, discuss the proper installation of camshafts and valve lifters and some of the problems which can cause them to be damaged.
A hydraulic valve lifter is probably the most highly engineered, precision engine component. It is, in effect, a hydraulic cylinder which utilizes engine oil and spring pressure to provide a quiet, constantly snug valve train. A hydraulic valve lifter is made up of a hardenable iron body, a plunger, oil metering disc valve, push rod seat, snap ring, check valve disc, check valve spring, plunger
return spring and cam face (see Fig. 2).
What's the Hydraulic Valve Lifter's Function?
The function of a hydraulic valve lifter is to take up any minor clearances that develop in a valve train. This clearance is called lash and can be caused by wear and the expansion and contraction of the various parts of the valve train due to engine temperature changes (see Fig. 3). The valve lifter performs this function by using engine oil under pressure to fill a high pressure hydraulic
cylinder in the bottom of the lifter. The cylinder is filled through a check valve from a reservoir within the lifter plunger. The check valve closes the high pressure cylinder trapping the oil when the upward thrust of the cam lobe pushes the lifter against the push rod to open the engine valve. When the lifter returns to the cam base circle, the engine valve closes and the spring under the lifter
plunger pushes it up to eliminate any valve train clearance or lash. As this occurs, oil in the reservoir flows through the check valve into the high pressure cylinder and the cycle starts over again (see Fig. 4 & 5).
Precision & Cleanliness a Must:
Correct operation of hydraulic valve lifters is vital to efficient engine performance and precision is the key word in their manufacture. For example, plungers and bodies are electronically classified 33 millionths of an inch and assembled in matched grades to provide only .0002 - .0003 clearance between the body and the plunger. The outside diameter of the lifter body is held to .0005
tolerance to maintain a precise fit in the engine lifter bore. Taper and roundness must be held to very close tolerances. All of this precision is necessary for proper valve lifter operation, but can cause a problem. Dirt, varnish or foreign particles, like metal chips, can cause the plunger to stick body causing the lifters to fail to adjust changes in the valve train. The obvious answer to this is a clean oil system and clean mechanic work habits.
Lifters Not Need Be Look-Alikes:
Here is a very important thing to remember. Valve lifters designed for the same engine, depending on the manufacturer, can have different overall body lengths and push rod seat design. This has no effect on performance. If you have the right valve lifter, the distance from the cam face to the push rod seat will be correct. This is the important dimension (see Fig. 6).
Camshaft & Valve Lifter Relationship Critical:
Now that we have discussed the individual design of the camshafts and hydraulic valve lifters, let;s look at how they must work together. The relationship of the camshaft lobe to the lifter face is very critical. From all appearances, the cam lobes look straight across, but actually on all but a very few engines, passenger car cam lobes are tapered. A lobe can taper from .0007-.002 across the face (see Fig. 7). Lifter cam faces are also shaped differently than they appear. They are ground spherically with a .002 crown in the
center (see Fig. 8).
When the camshaft is in place and the lifters are installed, the lifters are offset from the cam lobe (see Fig. 9). (The illustrations and figures have been greatly exaggerated to better show the relationship between the cam lobe and valve lifter face.) This offset between the cam lobe and valve lifters allows the valve
lifter to rotate, which distributes the valve spring load over a much wider area of the cam lobe and lifter cam face. Figure 10 shows a normal valve lifter contact pattern on the cam lobe. Distributing this load over a wide area is very important as the loading on the cam lobe and valve lifter cam face easily reaches 100,000 psi in normal engine operation.
The most important aspect of this offset design is preventing cam lobe edge loading - the greatest cause of cam lobe damage. If this extremely high loading occurs at the outside edges of the cam lobe, they will first chip and gall and soon completely wipe out (see Fig. 11). This destruction occurs very rapidly. When cam lobes wear in this manner, a cupped valve lifter cam face is usually found such as shown in Fig. 12. When a cam lobe and valve lifter like those shown in Figs. 11 & 12 are found, the first reaction is to say that the cam lobe was too soft, but cast iron camshafts are hardened as a unit so one lobe can't be softer than another. Valve lifters and camshafts wearing in this manner quickly result in rough engine performance. This is usually detected before the entire camshaft is wiped out, sometimes leading the mechanic to believe one cam lobe was at fault when in reality had the camshaft been left in the engine a short time longer, it would have been entirely wiped out.
When Should Camshafts & Valve Lifters be Replaced?
Obviously, if a cam lobe or valve lifter cam face are worn as we have shown above, they should be replaced. If either the camshaft or valve lifter are found to be worn at all, both should be replaced. Do not install worn valve lifters with a new camshaft. Even if the lifters look okay, the spherical shape may be worn flat. If so, the lifter face will contact the edges of the cam causing the condition
we discussed earlier - edge loading - and will result in early destruction of the cam lobe. Do not install new valve lifters on a worn camshaft. Improper contact will again result due to the taper being worn from the cam lobe. This can cause edge contact, or can prevent valve lifter rotation, thus concentrating the entire load on a very small area of the lifter cam face, again resulting in early
destruction. Also, when valve lifters are badly varnished or filled with sludge (see Fig. 13), they're worn and should be replaced.
Lubrication & Break-In Important:
When installing a new camshaft and valve lifters, the most important thing to remember is that the first few minutes of engine operation are the most critical in the life of a camshaft and valve lifters. Both must be properly lubricated or the initial stages of cam lobe destruction will begin in these first few minutes of operation and no corrective action will prevent the process from continuing until
one or more lobes are damaged to such a great extent that the engine will fail to operate properly. Prelube the valve lifters, the lifter bores in the engine and the camshaft generously with an engine oil of at least an MS classification. The lubricant used must have an EP (extreme pressure) additive that increases the film strength of the oil to reduce the possibility of metal to metal contact between the lobes and the lifters. Most engine oils of classification ML and MM lack sufficient additives to prevent this. Some camshafts are supplied with recommended lubricant included - use it.
The second extremely important point in the life of a camshaft and valve lifters is break-in procedure. When a new camshaft and valve lifters are installed and the engine is started initially, it should be run at 1000 - 1500 RPM for the first fifteen minutes. You should not attempt to rev the engine higher or allow it to idle. Both could cause extreme loading of the cam lobes and lifter cam faces with early destruction resulting. The characteristics of the material used in both camshafts and valve lifter bodies allows the surfaces to wear in to each other after a few minutes operation. Do not attempt to adjust valve lash or make any other engine adjustments until after this break-in period has been completed.
The fact that camshaft lobes and valve lifter cam faces wear themselves in to each other brings up one other important point. When lifters are removed from an engine they should be placed in a lifter holder to insure that all components are reassembled in the engine in the same position. This insures that those cam lobes and valve lifter cam faces that have been worn in to each other are
kept together. Mixing them up can cause early destruction.
Push rods should also be removed, placed in a holder and returned to the same position. Many of the engines used in today's passenger cars (note: this is circa 1978) do not have adjustable valve trains. To compensate for this, push rods are available both undersize and oversize and can be used in the same engine when warranted. For this reason, it is possible to collapse a lifter and hold a valve open by using a push rod that is too long or have too great an amount of
valve lash by using too short a push rod.
Adjustment Sometimes Required:
While most mechanics are familiar with adjustment procedures on mechanical lifters, the procedures used on hydraulic valve lifters varies from engine to engine. As we have said, some engine equipped with hydraulic valve lifters have no adjustment, others do, and when repair or installation of new parts causes major changes in the valve train, an adjustment may have to be made by the mechanic. In this way, the mechanic can reduce the major changes to make them minor and within the control range of the hydraulic lifter.
Two simple checks will show if a valve train equipped with hydraulic lifters is adjusted within it's take-up range. First, check to see if the engine valves are being held open by a collapsed lifter. This can be done by first warming up the engine and letting it idle with the valve covers removed. Apply a force on the push rod side of the rocker arm as shown in Fig. 14. You will shortly produce a clicking noise between the rocker arm and the valve tip. The absence of the
noise tells you that the lifter is operating in a collapsed position and is holding the valve open. Secondly, check to see if the valve train has more clearance than the hydraulic lifter can take up. This can be done simply by making certain that the push rod is held firmly in place when the lifter is on the base circle of the
camshaft. The valve train should be snug enough to press the plunger away from the retainer as shown in Fig. 15.
Weak Springs Cause Pump Up :
Another condition which can occur with hydraulic valve lifters is lifter pump up. As we have said before, a hydraulic valve lifter's function is to maintain a quiet, snug valve train. Sticky valves, or weak valve springs which allow valve train separation, will cause hydraulic valve lifters to pump-up or extend to eliminate that separation. They have no way of knowing what has caused the separation, only that it exists and must be eliminated. This pump-up is all too often incorrectly blamed on the hydraulic valve lifter when it is really caused by a weakness in the valve train. If allowed to continue, this condition can cause valve to piston interference, resulting in extensive engine damage.
Re-Install Valve Lifters With Care:
If an inspection reveals that both the camshaft and valve lifters in an engine are in good condition, a few precautions should be taken before re-installing them in the engine. Purge the engine oil system by turning the engine over a few times with the starter. Then, thoroughly clean the lifter bores with a clean rag saturated with engine oil. Prelube hydraulic valve lifters, lifter bores and camshafts the same as when installing the camshaft and valve lifters. Install a new oil filter and for added insurance, drain the crankcase and refill with a lubricating oil meeting classification MS or above.
Proper Installation Prolongs Life:
As we have said before, the most important time in the life of a camshaft and valve lifter is the first few minutes. Initial lubrication and break-in procedures must be followed or early destruction is likely. If properly broken-in and lubricated, a camshaft will be able to stand up under the extreme load imposed
upon it and give many miles of service.
The AERA Technical Committee
September 1978 - TB 177R & TB 98 |
