Camshaft Selection Small-Block Chevy
Before computer-controlled engines, the 'brain' of the engine was the camshaft. Vehicle weight, engine compression, cylinder heads, valve size, transmission type, and other factors must be considered when choosing the right cam for your small-block Chevy.
Thoughtful selection and attention to detail are needed to build a strong, reliable small-block Chevy engine. If valves open and close at the appropriate times, maximum engine efficiency and performance will be achieved. Camshaft selection will dictate which components will match and work well together.
Camshaft basics start with the relationship between the rotation of the camshaft and the rotation of the crankshaft. This, along with the cam profile, determines the flow of the air/fuel mixture ratio. Lobes on the camshaft push against the pushrods and rocker arms to open the valves as the camshaft rotates. The valve springs push them closed on their return. In proper sequence, these series of events produce an efficient and strong-running engine.
To a point, the further the valve rises from its seat the more airflow can be released. At very high RPM, valve bounce may result. This happens when valve spring tension is insufficient to keep the valve following the cam at its apex. Too high a lift may also cause interference with the valve head and the top of the piston.
Duration is the number of crankshaft degrees of engine rotation during which the valve is off it's seat. As you increase the duration of the cam, you increase the peak horsepower, but at the expense of low end torque. Do not to overdo the duration when selecting a camshaft. Over-camming is a common mistake when building a motor.
If you can't decide between two cams, choose the milder one.
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Because different measuring points are used for advertised duration, the industry established .050" of tappet lift as a common measuring point, allowing cams from different companies could be accurately compared.
Camshafts were originally designed with the same duration and lift on both the intake and exhaust lobes. These are referred to as single-pattern cams. Through the years, cam manufacturers have found that many engines prefer more duration on the exhaust lobe than on the intake lobe. These are called dual-pattern cams.
On first generation small-block Chevy engines, lifters were either hydraulic flat-tappet or solid flat-tappet, with hydraulic roller and solid roller designs available in the aftermarket. The hydraulic flat-tappet lifter is the most commonly used, having quiet operation and requiring no adjustment once installed correctly. They also work well in both stock and modified engines to about 6,500 rpm. Mechanical (solid flat-tappet) lifters make a slight ticking, and are generally used in applications where very high rpms (8,000 rpm or more) are needed. They also need periodic adjustment.
Small-Block Chevy Camshaft Selection
Before I bought my 1970 Chevelle, the small-block engine (with a slew of aftermarket pieces installed by the former owner) made great power after 3,000 rpm, but was sluggish at lower RPMs. Since the car was to used as a daily driver, I needed to bring the engine performance back to a streetable level. I took out a 350hp cam and installed a 300hp cam. The cylinder heads, camshaft and gears, intake manifold and carburetor were all changed together.
Although the 327 Chevy, with a short 3.25" stroke and 4.00" bore, responds well in higher RPMs, I selected a stock-grind cam with a short duration and relatively mild lift. This "commuter" cam, GM part #3896929, has .390" lift on the intake side and .410" on exhaust. Duration (at .050) was 195 degrees intake and 202 exhaust.
Selecting a SBC camshaft usually involves a compromise between low or high-end power, but you don't need a wild camshaft to build a strong-running small-block engine. Wade Owens of Cape Girardeau, Missouri, runs a 283ci motor in a 1966 Chevelle, and competes in NHRA's Stock Eliminator class. His car runs in the mid-11s, at or under the national record. Duration on his cam is approximately 258/260 degrees at 0.050 inch, and lift for this engine combination is rules-limited to 0.399 inch, measured at the retainer.
SBC Timing Chain and Gears
Timing chains don't need to be replaced at specific mileage intervals like timing belts do, but they do wear down and stretch. As an engine gets older, a timing chain will develop slack, which affects valve timing and hurts performance. Because the chain is difficult to access, it's a good idea to replace it when you're replacing the cam (unless it was recently changed). A small-block Chevy timing chain and gears set is inexpensive.
SBC Chevy Timing Chain Cover
If you decide to use an aftermarket timing cover, it may not have a timing pointer on it. This will make setting the timing nearly impossible! Many suppliers offer a bolt-on timing tab; two bolts attach them to the timing cover. However, care must be taken that it's the right one for your engine and harmonic balancer. Check that the timing mark on the harmonic balancer is in the right place. Unless the mark is moved to correspond with the new tab, you'll never set the timing correctly.