On performance engines, valve timing can be fine-tuned by advancing or retarding the cam, to achieve desired power characteristics. The camshaft must be timed with the crankshaft so that valve opening and closing is perfectly synchronized with piston movement.
The crankshaft drives the cam(s) using a timing chain and gears or timing belt and sprockets, at ½ crankshaft speed. Single Over Head Cam (SOHC) engines operate the valves using followers or pivoting rocker arms, while Double Over Head Cam (DOHC) engines, which have separate cams for the intake and exhaust valves, employ buckets between the cam and valves. On Over Head Valve (OHV) engines, the block mounted cam actuates the valves via lifters that ride on the lobes, pushrods that transfer motion up to the cylinder head, and rocker arms that pivot on shafts or studs. A single cam can be located in the cylinder block, or one or two cams can be mounted on the cylinder head. Most production cams are iron castings that are supported on journals that rotate in bearings, with individual eccentric lobes to actuate each valve. The camshaft, or cam for short, opens and closes the intake and exhaust valves, allowing the fresh air/fuel mixture to enter the cylinder for combustion, and the spent gases to flow out. 350 Chevy Small Block Gen II (4.00 Bore Family).350 Chevy Small Block Gen II (3.90 Bore Family).
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