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Camshaft Basics
Lobe Lift
Refers to the distance, measured in thousandths of an inch, that the lobe of the cam is raised from the base circle of the cam. Increasing valve lift allows more air/fuel mixture to enter the cylinders and more exhaust to exit, which generally leads to an increase in engine performance. An increase in lift almost always leads to an increase in duration because increasing lobe lift requires more distance to create the opening and closing "ramps" on the lobe. However, today's computer designed, asymmetrical profile cams have a high acceleration opening ramp, while the closing side sets the valve down softly, helping longevity.

Valve Lift
Refers to the distance, measured in thousandths of an inch, that the valve is lifted off its seat at the cam lobe's highest point. Remember, the cam's rotary motion is converted to linear motion thru a lifter, pushrod, and finally a rocker arm. Therefore, valve lift equals lobe lift multiplied by the rocker arm ratio.

A cam lifter, aka a follower, aka a tappet, makes direct contact with the cam lobes and follows the contour, or profile, of the cam.

Types of lifters:
Hydraulic flat-tappet
This type of lifter is self-adjusting, thanks to a valve controlled plunger inside its lifter body. engine oil pressure maintains preload against the pushrod, and since no lash allowance is needed, they run quieter than mechanical lifters. On the other hand, they can perform poorly at high rpm due to an inability to bleed down excessive oil pressure

Solid (Mechanical) flat-tappet
Essentially a solid link between the came lobe and the pushrod. Allows for greater rpm potential, since bleed-down is not a concern, but do require that lash, or clearance, be set between the valve and rocker arm to allow for expansion as the engine gets hotter.

Mechanical roller-tappet
Allows for the most aggressive lobe designs, due to the roller follower, and lends itself to high rpm operation, since its a solid body design. Like a solid flat tappet lifter, it requires a lash setting that must be readjusted over time.

Hydraulic roller-tappet
Used in OEM small blocks since 1987, allows for aggressive lobe designs along with self-adjustability an dquiet operation. They can, however, also suffer from bleed-down at high rpm.

One of the roller cam's greatest advantages is that rolling frictional forces are less than those caused by the "sliding" of a flat-tappet cam, which frees up some power. In general, roller-tappet profiles can be more aggressive for a given duration. On the other hand, a flat-tappet cam actually accelerates the lifter more quickly in the initial portion of the lift curve. With a sort-duration cam, a flat-tappet cam can actually get to a higher lift faster than a roller cam.

(pic from Gen-IV)

As lift refers to how far a valve opens, duration refers to how long it stays open. The opening time is expressed in degrees of crankshaft rotation, so a cam spec'd to 230* duration means the valve is being held open for 230* of crank shaft rotation.

usually referred to by two types of duration
Advertised Duration
The number of crankshaft degrees that the cam follower is lifted more than a predetermined amount off its seat. the SAE standard is 0.006 inch. So advertised duration is the number of crankshaft degrees that the valve is open at least 0.006 inch. ***Not all manufacturers use SAE standard***

Duration at 0.050 inch
Measures the movement of the cam follower, in crankshaft degrees, from the point where it is first lifted 0.050 inch off the base circle on the opening side of the lobe to the point where it decends to 0.050 inch off the base circle on its closing side. ***Common industry standard ***

Duration has a dramatic effect on engine performance. Stock camshafts usually offer relatively short duration and lift figures. This is done in the interest of a smooth idle and optimal part-throttle response. Longer intake duration opens the valve sooner in the cycle and closes it later. At lower engine speeds, this means the intake valve is still open as the piston begins to move upward, pushing the fresh air/fuel mixture out of the cylinder. On top of that, cylinder pressure can't build until the intake valve closes. The result is an engine with poor low and midrange respons. But the late-closing intake valve becomes an advantage at higher rpm, when the higher air inlet speeds keep filling the cylinders, even with the late closing valve. Basically, increasing duration will move the torque curve higher in the rpm range. Choosing a performance cam becomes a big compromise, you have to give some to get some.

Overlap is the amount of time, measured in crankshaft degrees, that both the intake and the exhaust valves are open. This happens at the end of the exhaust stroke and the beginning of the intake stroke. Increasing duration or decreasing LSA, or doing both, increases overlap. A cam with an exhaust closing point of 4* ATDC and an intake opening point of 8* BTDC has 12* of total overlap

Lobe-Separation Angle (LSA)
Indicates the angle, in camshaft degrees, between the maximum lift points on the intake lobe and the exhaust lobe. Lobe separation is usually calculated by adding the intake centerline and the exhaust centerline, then dividing by two. For example, a cam with a 106* intake centerline and a 114* exhaust centerline has an LSA of 110* [(106+114)/2=110]

Narrower LSA
Increased overlap
Increased low-rpm torque
Narrower powerband
Reduced idle quality
Increased cranking compression
Decreased piston-to-valve clearance

Wider LSA
Reduced overlap
Improved top-end power
Wider powerband
Improved idle quality
Reduced cranking compression
Increased piston-to-valve clearance

Single and Dual pattern cams

Single Pattern
In this type of cam, the intake and exhaust lobe profiles are identical, using equal amounts of duration and lift.

Dual Pattern
This type of cam has different intake and exhaust profiles. In general, the exhaust lobe will employ more lift and duration to help evacuate exhaust gas from the cylinders.

more info

photo thanks to UsedTaHaveA68

LS3 Cam specs
204/211 @.05 .551/.525 117

L99 Lift specs
Camshaft Lobe Lift - Exhaust - Non Active Fuel Management Cylinders
7.48 mm
0.294 in

Camshaft Lobe Lift - Exhaust - Active Fuel Management Cylinders
7.61 mm
0.300 in

Camshaft Lobe Lift - Intake - Non Active Fuel Management Cylinders
7.48 mm
0.294 in

Camshaft Lobe Lift - Intake - Active Fuel Management Cylinders
7.61 mm
0.300 in

Created by Milk 1027, 08-12-2009 at 06:08 PM
Last edited by Milk 1027, 08-12-2009 at 06:28 PM
Last comment by PQ on 12-09-2009 at 12:00 AM
1 Comments, 24,156 Views

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