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. Most production cams are iron castings that are supported on journals that rotate in bearings, with individual eccentric lobes to actuate each valve. A single cam can be located in the cylinder block, or one or two cams can be mounted on the cylinder head.

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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. 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.

The crankshaft drives the cam(s) using a timing chain and gears or timing belt and sprockets, at ½ crankshaft speed. The camshaft must be timed with the crankshaft so that valve opening and closing is perfectly synchronized with piston movement. On performance engines, valve timing can be fine-tuned by advancing or retarding the cam, to achieve desired power characteristics. Along with timing, valve lift and duration are also great engine power production factors and these too are determined by the cam’s lobe profile.

Duration is the length of time that the valve is open. Duration is increased on performance camshafts to increase flow for more high rpm power. Lift is the distance the valve is raised from its seat. On vehicles with rocker arms the lobe lift can be increased by the rocker arm ratio, so there is a lobe lift and a valve lift specification. Lobe profile can also differ between solid lifter and hydraulic lifter camshafts. The former require a certain amount of clearance, or lash, during operation and require periodic adjustment. Hydraulic lifters have a plunger and spring inside an oil-filled cavity to compensate for expansion.

Lifters can be further segregated according to whether they are flat tappets or rollers. Flat tappet cam lobes are ground with a taper to allow the lifter to rotate, otherwise the lobe and bottom of the lifter would quickly wear. Because the wear pattern is unique to each lobe, if components are reused they must be reinstalled in the same position. Flat tappet cams should always be installed with new lifters. The significantly reduced friction enables roller lifter cams to be much more durable than flat tappet designs, and as long as the roller wheels and bearings are in good shape, roller lifters can be reused.

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Nice product.
EPosted by Elanier (Hackensack, NJ) /
2001 Lexus IS
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