What Does It All Mean?
How often have you called a performance parts shop for advice on which parts would work for you and received a lot of information that you can't understand? Don't feel bad it happens all the time. Many professionals (including myself) are so accustomed to dealing with specs that we forget the fact that our customer probably doesn't have a clue what we are talking about.
Of all the products out in the marketplace the camshaft probably has the most mysterious terms and numbers. Knowing the basics of cam language is the first step to understanding how a cam works.
Let's start with the types of cams and lifters that are available:
Flat Tappet Cams are cams designed for use with either hydraulic or solid lifters (but not both) with a bottom surface which is nearly flat. I say nearly flat because these lifters are actually slightly convex in shape. When the convex surface of the lifter matches with the slightly angled surface of the cam lobe (the portion of the camshaft that creates valve train movement) the lifter will rotate in its bore. If the lifter doesn't rotate for any reason the cam and lifter will wear out very quickly.
Hydraulic lifters use the engine's oiling system to
automatially adjust the valve lash (clearance) to zero. They are
the most common type of flat tappet cams and lifters for street use.
Hydraulic cams can use any one of the three types of these oil
Mechanical or Solid lifter cams use a solid or oil control flat tappet (lifter) which requires regular valve adjustment. Some performance shops prefer solid lifters, even for street use, because they can adjust the way a cam will perform to a limited extent by changing the amount of lash (clearance) in the valve train. Decreasing the lash increases the duration and lift, increasing the lash decreases the duration and lift. For race use solid lifters will perform up to about 8500 rpm.
Mushroom lifter cams use solid lifters that look similar to an upside-down mushroom. The base of the lifter (where it contacts the cam lobe) is wider than the body of the lifter. These are used mostly on oval tracks when roller lifters aren't allowed. Block machining at the bottom of the lifter bores is required.
Roller lifter cams, as the name implies, use a lifter with a roller as the surface that follows the cam lobe. The roller surface allows very precise valve movement, as well as reduced friction, permitting a much wider operating range than is available with a flat tappet cam. Hydraulic roller lifters are common in late model engines and a variety of performance cams are available. They are as quiet and maintainance free as flat hydraulic lifters. Solid roller lifters are used mostly in highly modified race engines although they are available for 'pro street' applications. For maximum rpm use a solid roller design is definitely the best choice.
Now that you know the advantages and disadvantages of the various types of cams we can look at what all of the specs mean.
Duration is the lenth of time that the valve is held open by the cam. This is measured by the degrees that the crankshaft rotates. More degrees of duration will make the engine operate in a higher rpm range. There are two ways of rating duration.
Advertised duration was originally the S.A.E. (Society of Automotive Engineers) standard as measured from .006 of valve lift. Over the years this has been altered by most performance cam makers to make their cams look hotter, or different, than the specs of their competitors. Valve lift points as low as .002 are sometimes used and this can add up to thirty degrees to the advertised figure. Even when the cams being compared are all measured the same way the figures can still be misleading if you don't know what the cams were designed for. Cams designed for quiet street operation will show higher .006 duration numbers than performance cams of the same rpm range.
Duration measured from .050 of cam lift is the best for comparison of specs because most of the variations in cam design are reduced and the valves are open enough to start getting some flow past them. Most cam makers give accurate .050 ratings and good comparisons are possible between cams of the same type (hydraulic or solid or roller).
Lift is usually measured as gross (total) valve lift. This works for hydraulic lifter cams but is misleading for solids and rollers because you must subtract the valve clearance to get the net (real) valve lift. Cam lift is sometimes given and is just the lift of the cam only before the rocker ratio is figured in.
Lobe Area is obtained by measuring the lift at each degree of rotation and adding them all together. This will tell you very quickly how much difference (if any) there is between two cams with the same lift and duration. This is rarely supplied by cam makers.
Lobe Center is the degrees the crankshaft turns from top dead center to the center of the top of the cam lobe. If you add the centers of both cam lobes together and divide by two you will have the lobe center separation.
Lobe Center Separation is the degrees the cam turns from the center of the exhaust lobe to the center of the intake lobe on the same cylinder. Wide lobe center separations (114) give minimal valve overlap on street cams under 220 degrees at .050. This produces high manifold vacuum for street engines. Closer lobe separations of 108 degrees willnot allow computer engine controls to function properly but will give better mid to high rpm performance when used in carbureted engines.
Valve Lash is the amount of clearance required at the valve tip with solid and roller lifters.
Valve Timing is the opening and closing points of the valves measured in relation to the degrees of crankshaft rotation. These specs are often given by both the advertised and the .050 methods. These points can be advanced or retarded (as a group) after installation with a multi-keyway crank gear, offset keys, or special bushings.
That covers most of the terms that you are likely to hear when you are shopping for a new camshaft. It's good to know what all of the information that is available means but the specs and technical descriptions are only part of the story. Some cam manufacturers will also include comments about the intended use and rpm range of each cam in their catalogue. Often these comments give information that wouldn't otherwise be apparent by checking the specs only.
Here are a few more terms that, while you will not need to know them for selecting a cam, you might come across during a 'bench racing' session.
Asymetrical Cam Lobes are designed with the closing side of the lobe different in shape than the opening side. This difference is only visible in some overhead cams and full race roller cams. When both sides are the same they are Symetrical.
Base Circle, or the heel, is the round portion of the cam lobe. This is where the lifter rides while the valve is closed. A high spot in this area is called Base Circle Runout. If the runout is more than .001 on hydraulic lifter cams the valve will be off of its seat while the lifter is on the runout area. Poor performance and burnt valves will result from this. Small Base Circle Cams have the lobes ground down to the core diameter to give extra clearance for connecting rods used on stroker cranks. Higher lift cams also have smaller base circle diameters than stock lift cams.
Billets and Cores are the blank shafts that the camshafts are made from. Cast Cores and Proferal Iron Billets are used for most flat tappet camshafts. Steel Billets are used for roller tappet camshafts.
Cam Lobes are the parts of the camshaft that create the valve movement.
Cam Profile or Cam Grind is the actual shape of the cam lobe.
Chilled Iron Lifters are heat treated by pouring the molten alloy into a mold that has a chilled steel bottom plate. They are compatible with steel and hardface cams only.
Clearance Ramps are the portion of the cam lobe between the base circle and where the valve starts to open. They slowly take up any slack in the valve train and reduce the shock created by the sudden movement of the lifter.
Core Diameter is the diameter of the camshaft measured between the cam lobes.
Dual Pattern Camshafts have different intake and exhaust lobes. There are various opinions on whether or not there is an increase in performance over a single pattern camshaft. Unfortunately there is no fair way to compare the two styles. Both types work quite well and there is no benefit to turning down one style of camshaft in favor of the other on this basis alone.
Flanks are the sides of the cam lobe that cause the movement that raises and lowers the valve. They are also called the Opening and Closing Ramps.
Hardenable Iron Lifters are high quality lifters compatible with cast and proferal billet cams.
Hardening is achieved by heating the cam and quenching it in oil to give durability. Flame Hardening and Induction Hardening are two methods used. Typical hardness for flat tappet cams and lifters is Rockwell 32C.
Hardface Overlay is used in highly modified race engines when a very hard solid lifter cam is required. Chilled iron lifters must be used on hardface cams.
Nose of the cam lobe is the portion of the lobe with the highest lift. The nose of the cam should be .010 shorter than the bearing surface on a V8 or V6 camshaft except on small base circle cams.
Parkerizing is the application of a special high quality oil-absorptive coating to the surface of the camshaft. This protects the cam lobes during break-in.
Preload is the type of adjustment for hydraulic lifters. When the clearance is removed from the valve train the rocker arms, or adjustable pushrods, are tightened an additional turn to preload the hydraulic lifter.
Pump-up happens in stock hydraulic lifters at high rpm. They simply can't handle the volume of oil and the extra operating speeds so they expand, or pump-up, causing the valves to stay off their seats slightly even while the lifter is on the base circle of the cam. Anti-pump-up lifters reduce this problem and Rhoads Variable Lifters eliminate it altogether.
Rate of Lift refers to the speed that the valve opens and closes. Cams with a higher rate of lift have more lobe area to provide performance gains. Cams with an extremely high rate of lift require mushroom lifters.
Refinishing refers to restoring the cam lobe to its original shape (except slightly smaller) when there is only minimal wear.
Regrinding is the work of restoring a cam with alot of wear or altering a stock cam to performance specs. Both refinishing and regrinding require precision equipment and master lobes.
Split Overlap is the term used when the piston is at top dead center and both the intake and exhaust valves are off their seats the same amount. With a single pattern cam this would mean that the camshaft was timed straight up. Advancing or retarding the camshaft will open one of the valves more at top dead center and reduce the valve to piston clearance.
Valve Float happens when the speed of the engine is too great for the valve springs to handle. The valves will stay open and/or bounce on their seats. The clearance in the valve train created by valve float will also cause hydraulic lifters to pumpup as they try to eliminate the valve clearance.
Valve Lash is the amount of clearance, measured at the valve, in the valve train when using a solid or roller camshaft.
Valve Train is the 'train' of parts leading from the cam lobe to the valve.
As I said at the start, the camshaft probably has more mysterious terms and numbers than any other performance part. If this helps you to learn even a small amount you will be on your way to understanding the basics of how a cam works.
Last updated August 2012.
(C) Copyright 1996-2012 by Doug Friesen, email@example.com. All rights reserved.
The Accelerated Motion name and logo is a registered trademark of Doug Friesen. All other marks, names and part numbers are the property of their respective owners and are used for the sole purpose of promoting sales and proper use of their products. The information contained at this site is accurate to the best of my abilities and is subject to change without notice.