Jan. - Feb. 1998 Lane Racing And Rodding Article By Jim Kaekel, Jr.
The camshaft is often referred to as the heart of an engine. It is definitely the heart of the engine's performance capability. That's why it's very important that the camshaft be carefully matched to a particular combination. A properly selected camshaft will allow an engine to perform to its full potential, but even the most carefully assembled engine can be quickly turned into a "stone" by installing the wrong camshaft. A cam which is too large will kill the torque characteristics of the engine and engines which have been "over-cammed" idle miserably in street applications. Circle track cars will be weak coming out of the corner and drag cars will suffer poor 60 ft. times, translating into equally poor e.t.'s. Don't ever make the mistake of purchasing a camshaft because "that Joe guy" won the last couple of features and he uses a GoReelFast brand 258 duration cam with .578" lift. It is very likely that "that Joe guy" has a car that is set up with completely different cylinder heads, carburetion, compression, rear end, etc.
When selecting a camshaft, many factors should be considered. Is the vehicle going to be used for dual purpose (street/strip) or strictly as a drag or circle track car? If it is to be used in a circle track car, what track will the car run on? 1/4, 3/8 or 1/2 mile? Dirt or asphalt? Naturally, a car running on a 1/2 mile track will require a cam with more top end power than one running on a 1/4 mile track. Engine displacement and modifications, vehicle weight, transmission type, rear end ratio, operating RPM range and the given sanctioning body's rules must all be carefully considered. Some circle track and stock drag racing classes even require that the stock valve lift be retained but place no limits on duration.
During the selection process, several cam specifications should be carefully examined. VALVE LIFT is the theoretical distance that the valve opens at its maximum travel off of the valve seat. This distance is theoretical because actual lift depends on the ratio of the rocker arms used. Valve lift is found by multiplying the cam's lobe lift (the amount of lift actually built into the cam lobe) by the rocker arm ratio. For example if you have a camshaft with .310" lobe lift and it is used with a set of 1.5 rocker arms, the valve lift would be .465". If you then change to 1.6 ratio rockers, the lift would then be .496". However, the only way to find the ACTUAL LIFT is to measure it with a dial indicator at the valve retainer on an assembled engine. If you are using hydraulic lifters, a pair of solid lifters must be substituted before checking lift on individual cylinders.
Probably the most important specification of a camshaft is its DURATION; the length of time the valve is open. Duration is measured in crankshaft degrees from the point where the lifter is .050" off of the base circle on the opening side of the ramp to a point .050" off of the base circle of the closing side. ADVERTISED DURATION is also listed in cam manufacturer's catalogs and was used predominantly several years ago by OEM manufacturers, particularly during the muscle car era of the '60's and early '70's. Advertised duration is measured to include the lobe clearance ramps, areas of the lobe when the valve is not actually off of the seat. Therefore, advertised duration is always greater than the duration at .050". For example: Comp Cam's Magnum grind #12-212-2 has 280 degrees of advertised duration, but only 230 degrees when measured at .050" lift. During the selection process, duration at .050", rather than advertised duration,should be focused upon when comparing cam profiles.
All camshafts are classified as either SINGLE PATTERN or DUAL PATTERN designs. A single pattern cam has identical valve lift and duration on both the intake and exhaust lobes. Dual pattern cams have a different duration and valve lift for the intake and exhaust lobes. For example, Comp Cam's Xtreme Energy grind #12-254-3 has 250 degrees of duration (at .050") on the intake side and 256 degrees on the exhaust with .519" lift on the intake and .523" on the exhaust. Longer duration and higher lift on the exhaust side may partially make up for an inefficient set of exhaust ports by improving volumetric efficiency and exhaust flow.
The Lobe Separation Angle, commonly referred to as the LOBE CENTER is the angle measured in camshaft degrees between the maximum lift of the intake lobe versus the maximum lift of the companion exhaust lobe. The Lobe Center is permanent and may only be changed by having the cam re-ground. The LOBE CENTERLINE, often confused with lobe center, is the point of maximum lift on a cam lobe and is measured in crankshaft degrees from TDC to the cam's maximum lift and is used during the process of degreeing a camshaft (see Racing & Rodding/Motor State Newsletter, Jan.-Feb., 1996). Specs for degreeing a camshaft can always be found on the manufacturer's "cam card" packaged with every performance camshaft.
A camshaft's OVERLAP refers to the amount of time the intake and exhaust valves are open at the same time. It is the period at the end of the exhaust gas stroke when the remaining burnt gases are being expelled and the intake stroke is beginning with the opening of the intake valve. Race only cams have lots of overlap and, because of this, they produce very low levels of vacuum at low engine speeds, resulting in rough, lopey engine idles.
Taking care in selecting a camshaft (and then taking the time to degree it) will pay big dividends in the long run, especially when many of your competitors have made the all-too-common mistake of choosing a cam which is too large, or one that "Joe So-and-So" ran after he "just lined up the timing marks."