Camshafts

Camshaft The camshaft rotates ½ times the crankshaft – or – once per four-cycle stroke. The camshaft may operate the: Valve train Mechanical fuel pump Oil pump Distributor

Camshaft Major function - operate the valve train. The lobes on the cam open the valves against the pressure of the valve springs. Bearing journal can be internally or externally lubricated (oiled).

When installing externally oiled cam bearings it is essential that the holes in the bearings lineup with the oil passages in the block

Camshaft Pushrod engines have the cam located in the block. Cam is supported by the block and the cam bearings.

Cam may or may not be held in place by a thrust plate.

Overhead Camshafts Overhead camshafts are either belt or chain driven and are located in the cylinder heads.

Either design may or may not use separate cam bearings Overhead Camshafts May be housed within a bore in the cylinder head or: May be supported within a cradle held in place with caps. May or may not utilize a thrust plate. Either design may or may not use separate cam bearings

Overhead Camshafts May have a one piece lifter – rocker design Will use one of the following: Cam followers Rocker arms May have a one piece lifter – rocker design A bucket design

Bucket Design

Camshaft Followers

Rocker Arms

Design A cam casting will include A cam casting may include Lobes Bearing journals Drive flanges A cam casting may include Drive gear(s) Fuel pump eccentric

This designation is actually determined by the lifter design. Classification Camshafts are of one of four types: Hydraulic flat-tappet Hydraulic roller Solid flat-tappet Solid roller This designation is actually determined by the lifter design.

Hydraulic flat-tappet The lifter is “spring” and oil loaded to allow for compensation. Traditional O.E. style (1950’s – mid 90’s) Used with flat or convex-faced lifters Generally cast iron or hardened steel Requires a “break-in” period to establish a wear pattern

Hydraulic flat-tappet Cast-iron cams are finished with a phosphate coating. Steel cams (SAE 4160 or 4180) are hardened by: Induction hardening – heated cherry-red in an electric field then oil cooled. Liquid nitriding – hardens to .001 to .0015 Gas nitriding– hardens to .004 to .006 thickness

Flat tappet Lifters

Hydraulic flat-tappet Most cams are coated at the factory with manganese phosphate . This gives the cam a dull black appearance. This coating is to absorb and hold oil during the “break-in period”.                          

Hydraulic flat-tappet Most late model designs use a convex bottom (.002”) to encourage lifter rotation. This rotation helps reduce lifter and (or) bore wear. The Cam lobe will also be slightly tapered (.0007” - .002”). This provides for a wider contact pattern.

Hydraulic flat-tappet Camshaft “break-in” The lobes of the cam and the bottom of the lifters must be coated with a molydisulfide lubricant often called “cam lube”. This insures that the cam is properly lubricated during “break-in”.

Hydraulic flat-tappet Camshaft “break-in” Typical procedure – Maintain 1,500 RPM for 10 - 20 minutes Drain the engine oil a immediately afterwards Check the recommended procedure and lube for your particular cam!

Hydraulic Roller The lifter is “spring” and oil loaded to allow for compensation. The contact between the cam and lifters are separated by a steel roller. This roller reduces friction. Lifters cannot be allowed to rotate within the lifter bore.

Hydraulic Roller The camshaft is generally made of non-hardened steel. The lobes must be “finished” by the manufacturer prior to assembly there is no “break-in period”.

Hydraulic Lifters (tappets) Hollow cylinders fitted with a plunger, check valve, spring and push-rod seat.

Hydraulic Lifters (tappets) Engine oil pressure forces oil into the lifter through the oil inlet holes. A check valve and ball hold most of the oil inside the lifter “hydro-locking” the plunger inside the cylinder.

Hydraulic Lifters The oil passed through the check valve exits through the hole in the push rod seat. The oil then passes through the pushrod to lubricate the rocker arms.

Hydraulic Lifter Preload Also called valve lash. The distance between the pushrod seat and snap-ring when the lifter is resting on its base circle. Typical values range from .020 to .045”. Check manufacturers specifications.

Hydraulic Lifter Preload Adjusted by: Adjustable rocker arms Often referenced by “turns past zero lash” Non-adjustable rocker arms Longer or shorter pushrods Shim or grind rocker stands

Hydraulic Lifter Preload Necessary if: Cylinder head has been decked Cam has been changed Altered head gaskets Camshaft is worn An engine rebuild

Hydraulic Lifter Valve-float NOT GOOD The lifter fills with oil faster than it can purge it. This raises the lift of the camshaft. Usually caused by excessive RPM. May damage valves, pushrods, pistons etc.

Solid Flat-tappet and Roller No internal hydraulic absorption. Allows for a more consistent valve lift, especially at high RPM. Noisy when cold, more frequent and precise valve-lash adjustments required.

Solid Flat-tappet and Roller Oil is diverted through the pushrods via a pushrod seat.

Solid Flat-tappet and Roller No lifter preload – valve lash only. Lash values may be given hot or cold Typical values range from .002 - .005”.

Composite Approaches Composite camshafts of medium- and high-alloyed powered metal lobes mounted on a hollow tube are popular as they have the capacity of withstanding high contact stresses. Composite camshafts can be 50% lighter than cast iron or steel shafts; can have lobe material tailored to the application; and they can have lobes molded to near-net shape, which means that the amount of grinding stock, and consequently grinding time, are both reduced.

Cam Specifications Lift Duration Valve overlap Lobe center (separation angle or lobe spread)

As lift increases the forces on the entire valve train also increase. Lobe Lift The amount the cam lobe lifts the lifter Expressed in decimal inches As lift increases the forces on the entire valve train also increase.

Lobe Lift Asymmetrical design – the amount of lift between the intake and exhaust lobes is different. Symmetrical design - the amount of lift between the intake and exhaust lobes is the same.

Duration The number of degrees of crankshaft rotation for which the valve is lifted off of the seat. If the amount of degrees that the intake and exhaust valve are open differ – it is of an asymmetrical design.

Usually expressed as one of two values Duration Usually expressed as one of two values Duration (at zero lash) Duration at .050” lift – preferred method Compensates for tappet styles and clearances

Duration More duration = rougher idle and better high RPM performance Less duration = smoother idle and better low RPM performance

Valve Overlap The number of degrees of crankshaft rotation that both valves are off of their seat (between the exhaust and intake strokes). Lower overlap = a smoother idle and better low RPM operation Higher overlap = better high RPM operation

Valve Overlap Having the exhaust valve still open when the intake starts to open uses the exhaust "pull" out the exhaust port to help start the intake charge entering the chamber -- before the piston has started down and has generated it's own vacuum.

Valve Overlap

Lobe Separation Angle The difference, in degrees, between the center of the intake lobe and the center of the exhaust valve. The smaller the angle the greater the overlap The larger the angle the less the overlap

Lobe Separation Angle

Camshaft Degreeing Advanced cam timing Retarded timing The camshaft is slightly ahead of the crankshaft More low speed torque less high RPM power Retarded timing – The camshaft is slightly behind the crankshaft More high RPM power Reduced low RPM torque

Adjustable Camshaft Gear