Camshaft Tips & Definitions

[1Bad62Pro/Street]

Just some helpfull info:


Camshaft Tips & Definitions
Life or Death Cam Knowledge
http://www.hotrod.com/techarticles/c...ons/index.html

What cam grind is right for your street machine? We’ve compiled some basic tips and terms that can prove helpful in the camshaft selection department.

Tip 1: A low compression-ratio powerplant will respond favorably to a camshaft that features relatively short duration figures, wide lobe centers, rapid valve opening rates, and a high lift number (comparatively). On the opposite side of the coin, an engine that features a high static compression ratio easily can use more duration and tighter lobe centers.

Tip 2: Virtually all of the major cam grinders (and more than a few oil companies) discourage the use of synthetic oils during engine break-in--particularly on engines with flat tappet (solid or hydraulic) camshafts. Instead, use a quality grade of naturally formulated oil for the break-in. You can safely use synthetics following the proper break-in period. Besides, synthetics are pricey, and you’d have to drain it out after the first 20 minutes or so anyway.

Tip 3: Valvespring damper failure is more common that we’d like to think (especially on high-lift, radical-profile camshafts). Occasionally, a damper will physically "unwind" and the lower portion of the assembly will work its way between two lower coils of the outer spring. Naturally, this stacks the spring into coil bind. One easy solution to this problem is to inspect the spring, inner spring, and damper carefully before installation. You might find that some valvesprings have added "flashing" on the spring ends (this is quite common on some dampers). If that’s the case with your spring(s), use a small die grinder and very carefully smooth over the burrs. Similarly, some dampers have very sharp edges on the "flats." The life of the damper can be improved by gently deburring and chamfering this section.

Tip 4: Before sliding a set of larger-than-stock ratio rockers inside your Chevy (as an example, adding 1.6:1 rockers in place of 1.5:1 rockers), carefully check for coil bind at the valvespring, especially in applications with stock or stock spec valvesprings. You need at least 0.40-inch clearance between the springs at maximum lift.

Tip 5: When installing an aftermarket, high lift camshaft with stamped-steel, stud-mounted rockers, be sure to check the rocker arm slot that allows the rocker to pivot at maximum lift. There should be approximately 0.60 inch of additional travel left in this slot when the valve is at maximum lift. At the same time, be sure that the rocker arm contacts only the valve tip, not the valvespring or retainer.

Tip 6: In the area of single- and dual-pattern camshafts, controversy seems to reign supreme. Proponents of the dual-pattern grind feel that a standard pushrod engine will breathe better on the intake side than it does on the exhaust side. In this scenario, the exhaust lift and duration figures are greater in order to compensate for the exhaust port’s inability to breathe. The single-pattern group points out that the exhaust is somewhat controlled by cylinder pressure. The piston movement helps to force the exhaust from the combustion chamber and, as a result, the intake port does not have any real advantage. This makes for a single-pattern camshaft that features identical intake and exhaust lobe profiles. Which is the better of the two designs? Both have merit. Your particular combination might respond properly with a dual-pattern cam grind while a slightly different Chevrolet might show promising results with a single-pattern grind.

Tip 7: Piston-to-valve clearance should always be checked when a high lift cam is installed in an engine. It should be pointed out that high-lift figures alone do not always contribute to piston-to-valve clearance problems. Overlap and rod length also can create problems.

Very seldom is a specific valve fully open when the piston reaches top dead center (TDC) in a wide-lobe-center, short-overlap camshaft application. As the lobe-center angle is decreased, the proximity of the valve face to the piston dome is increased. In a narrow-lobe-center, long-overlap application, the valve remains open for a longer period of time as the piston approaches TDC. This problem is magnified when a powerplant is fitted with "long" connecting rods. As the rod ratio number increases (due to the longer-than-stock connecting rod), the piston remains at TDC longer, which can contribute to piston to valve-clearance problems.

Tip 8: If you bend one or more pushrods for no apparent reason, there’s a very good chance the engine is experiencing some form of mechanical interference in the valvetrain. The places to look include the rocker arm-to-stud clearance, valvespring coil bind, interference between the retainer and the valve seal or retainer, and the valve guide. In addition, high engine rpm might be causing the valves to whack the pistons, which in turn bends the pushrods.

Tip 9: Never overlook the fact that pushrods of the wrong length can wreak havoc on the rocker arm geometry, adversely affect the amount of lift, and even contribute to valve guide wear in the engine. A number of factors can influence pushrod length and valvetrain geometry. These include a decked block, installation of a smaller base circle cam, lash caps, non-stock-length lifters, or custom-length valves, and alteration of the valve seat depth. If any of these items have been included in your engine, then the pushrods could be far too short. To measure the correct pushrod length, try a set of adjustable measuring pushrods.

Tip 10: When a stamped-steel rocker arm bites the dust on a Chevy V8, it’s usually an exhaust rocker. Why? Simply because the exhaust side runs hotter. But when it comes time to replace the rockers, swap a good used intake to the exhaust side and install a new rocker on the intake. That way, the new rocker won’t be killed before it’s "seasoned." It’s just cheap insurance.

Definition: Theoretical valve lift is the figure often published in cam manufacturers’ specification charts and is the most common term used to describe the lift of the cam. Theoretical valve lift does not take into account valve lash, valvetrain deflection underload, variables in rocker arm ratio, and so on. Think of theoretical valve lift as maximum valve lift under ideal conditions.

Definition: Net valve lift is the real-world number that your particular powerplant experiences with all variables taken into account. Items such as valve lash, valvetrain deflection, and rocker arm ratio variation are considered when this particular measurement is determined. It should be pointed out that it is virtually impossible for any camshaft grinder to print a true "net" figure on its specification card. The only viable method of determining actual net valve lift is by setting up a dial indicator on the nose of the rocker arm and measuring the lift while the powerplant is in an "operational" mode (but, of course, not running).

Definition: Lobe lift or camshaft lift is the term that describes the amount of lift the camshaft provides without any benefit of rocker arm ratio multiplication factors. For example, a small-block Chevrolet roller camshaft may have an advertised lobe lift of 0.350. If the engine makes use of a standard 1.50:1 ratio rocker arm, the final theoretical valve lift works out to 0.525 (0.350 X 1.5). In the event that the powerplant uses a 1.60:1 ratio rocker arm, the theoretical valve lift works out to 0.560-inch. Think of cam lift as the true amount the camshaft lifts the valve lifter in the respective lifter bore.

Definition: Advertised duration is the most common form of listing specific duration figures. It is measured in crankshaft degrees and basically expresses the length of time that a given valve is open. Unfortunately, this particular figure can be more optimistic than theoretical valve lift. A good portion of the discrepancy regarding advertised duration figures is due to the fact that many manufacturers tend to include the camshaft lobe clearance ramp in their duration figures. For example, a Chevrolet ZL1 camshaft has an advertised duration figure of 359 degrees on the exhaust side. Obviously, this is an extremely stout number and, when you give it some consideration, you can clearly see that the powerplant could hardly run with such a radical camshaft. This is where 0.050-inch duration figures come into play. Measured with the 0.050-inch method, the very same ZL1 grind features a duration figure of 273 degrees. That’s a significant difference.

Definition: The 0.050-inch duration figure is determined when the valve lifter has risen 0.050-inch off the camshaft base circle (opening side) and closed to within 0.050-inch (closing side) on the ramp. This particular duration figure is quite accurate for comparison purposes and in most areas is much closer to true duration than the advertised number. Airflow in the intake or exhaust port is minimal at low lift figures, especially when the clearance ramp numbers are included in the discussion. Since this airflow is almost nil in most situations, the camshaft manufacturers began using the 0.050-inch method to determine a universal number that could be used for camshaft comparison. The newer 0.050-inch number is more accurate and can, at least, be compared without worrying about variables such as ramp clearance.

Definition: Several camshaft grinders have begun to use the 0.020-inch duration figure in addition to the previously mentioned 0.050-inch number. It is primarily a seat duration figure and most often is used to determine valve timing at the seat. This figure is helpful when plotting a specific cam profile on graph paper and also can be of some assistance when comparing various camshaft profiles.

Definition: Overlap was once a very common figure but, much like advertised duration, it has fallen from favor as a comparison tool among many cam grinders. Basically, overlap is the number of degrees that the camshaft features with both the intake and exhaust valves open simultaneously. Overlap takes place at the beginning of the intake stroke and also at the end of the exhaust stroke.

Definition: Lobe center is the amount of camshaft degrees between the point of maximum lift on the intake lobe and the point of maximum lift on the exhaust side of a given pair of cam lobes. As an example, lobe center is calculated for cylinder number one only and does not deviate between number one and the other cylinders in the powerplant. To make a representative sketch of lobe center, draw a line between the very center of a camshaft intake lobe and then draw a similar line through the respective exhaust lobe for the same cylinder. The number of camshaft degrees between these two imaginary lines is the camshaft lobe center.

Also called lobe displacement angle (LDA), the lobe center has a definite bearing on how a particular camshaft will operate in a specific powerplant. If the lobe center of the camshaft is increased, the valve overlap will be decreased. The overlap decrease is created because the exhaust timing will occur earlier and the intake timing event will occur later in relation to crankshaft position. Conversely, if the lobe center or displacement angle is decreased, overlap increases. Note that lobe center cannot be changed once a camshaft has been ground.

Definition: Lobe centerline should not be confused with lobe center or lobe displacement angle. The term "lobe centerline" refers to an imaginary line drawn through each respective lobe but does not combine the separation angle between the intake and exhaust lobes in a given pairing. Lobe centerline can be altered by advancing or retarding a camshaft, while lobe center is a figure that cannot be altered since it is incorporated into the camshaft when it is designed and manufactured.

Definition: A symmetrical camshaft makes use of the same profile on the opening and closing side of a specific lobe. In other words, the opening side of an intake lobe features a shape that is exactly the same on the closing side of the lobe. This term should not be confused with single-pattern or dual-pattern camshafts (see below).

Definition: An asymmetrical camshaft features a lobe shape or profile that is different on the opening side than the closing side of the same lobe. For example, a camshaft could feature a very rapid valve opening profile, but when the valve is closing on the same lobe, the shape could be extremely smooth and gentle.

Definition: Single-pattern camshaft grinds feature identical intake and exhaust lobe configurations. This actually means that the valve timing is the same for both the intake lobe and the exhaust lobe. It should be pointed out that a camshaft can be asymmetrical in design yet still be a single-pattern grind. On the other hand, a camshaft can be symmetrical yet also be a single-pattern configuration.

Definition: Dual-pattern camshaft grinds make use of different profiles on the intake and exhaust lobes. This means that the exhaust lobe and the intake lobe are not identical in shape. A cam profile of this type could feature an asymmetrical lobe on the intake and a symmetrical lobe on the exhaust. It also could be symmetrical in both, asymmetrical in both, or any combination these types of profiles.