Camshaft Design and Theory

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Camshaft Design and Theory
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Camshaft

• The brain of the engine
• Controls valvetrain operation
• Rotates at ½ crankshaft speed
• Along with the crankshaft it determines firing
  order
• Along with the induction and exhaust systems it
  determines the useful rpm range of the engine

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Camshaft Design

• Features
• Max lift or nose
• Flank
• Opening clearance ramp
• Closing clearance ramp
• Base circle
• Exhaust opening timing figure
• Exhaust closing timing figure
• Intake opening timing figure
• Intake closing timing figure
• Intake to exhaust lobe separation

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Camshaft Measurements

• Lift
• Duration
• Lobe separation angle
• Valve overlap
• Intake valve opening (IVO)
• Intake valve closing (IVC)
• Exhaust valve opening (EVO)
• Exhaust valve closing (EVC)

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Lift

• Lobe lift is the distance the lifter moves in one
  direction
• Lobe lift is the difference in measurement
  between the nose of the lobe and the base circle
  of the lobe
• Valve lift is what most people are taking about
  when they refer to lift and is simply lobe lift
  multiplied by the rocker arm ratio

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Lift

• Increasing the lift opens the valve further.
  This reduces the restriction to airflow at the
  valve and allows air to flow more freely into the
  cylinder.
• At some point the valve can be opened to a point
  at which the port is the valve is no longer the
  greatest restriction to airflow, and at that
  point opening the valve further will not increase
  airflow.
• The distance a valve can be opened is limited by
  duration, rocker arm ratio, lifter design,
  camshaft design and valve to piston clearance.

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Duration

• Duration is the length of time (measured in
  degrees of crankshaft rotation) that the valve
  remains open
• The point at which a valve is considered open
  will be given along with the duration figure
  (i.e. .004, .006, .050)
• Example (240 degrees _at_ .050)

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Duration

• At higher engine speeds the valve opens and shuts
  in a shorter amount of time. This limits how
  completely the cylinder can be filled.
• Longer duration camshafts hold the valve open
  longer, increasing the amount of time the
  cylinder has to fill.

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Duration Vs. Time

• At 2000 RPM a camshaft with 200 degrees of intake
  valve duration, the intake valve is open for
  0.0166 seconds/cycle
• At 6500 RPM the valve is only open for .005128
  seconds/cycle
• If we substituted a camshaft with 245 degrees of
  duration this time would increase to .0204
  seconds/cycle _at_ 2000RPM and .00628 seconds/cycle
  at 6500RPM
• This is a 22 increase in time available to fill
  the cylinder

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Lobe Separation Angle

• Lobe separation angle (LSA) is the number of
  degrees separating the point of peak exhaust lift
  and peak intake lift.
• Lobe separation angle directly impacts the amount
  of valve overlap.
• Because of this, production vehicles usually
  employ a wide LSA to reduce valve overlap and
  increase idle quality.

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Valve Overlap

• Valve overlap is the time in which both the
  intake and exhaust valves are open.
• Valve overlap is affected by LSA and duration.
• Valve overlap is used because of the principle of
  exhaust scavenging (the exiting exhaust gases
  help pull in the fresh intake charge,
  especially at higher rpm when fill time is
  limited).
• At low RPM when intake port speed is low, a long
  valve overlap period will cause reversion into
  the intake port (the cylinder pressure exceeds
  the force of the air in the intake port and
  exhaust gasses are forced into the intake port).
• This causes the lumpy idle associated with big
  camshafts.

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Intake Valve Closing

• Most critical valve opening/closing point
• To early of an IVC and the cylinder may not have
  time to fill completely
• To late of an IVC and the cylinder pressure will
  overcome the inertia of the incoming airflow and
  revert flow back into the intake port
• This causes a serious disruption to flow and
  destroys any pressure waving tuning

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Exhaust Valve Opening

• 2nd most critical valve opening/closing event
• Determines the balance between power event
  efficiency and exhaust pumping losses
• To early of an exhaust opening will reduce the
  amount of energy converted from cylinder pressure
  to mechanical force on the piston
• To late of an EVO will cause an increase in the
  amount of power needed to expel the burned
  exhaust gases from the cylinder

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Exhaust Valve Closing

• Along with IVO it is the least critical valve
  timing event
• Along with IVO it determines the amount of valve
  overlap and exhaust scavenging
• Too early of an EVC will not allow the exhaust
  gas to be fully purged from the cylinder
• Too late of an EVC will allow fresh air/fuel mix
  to be purged into the exhaust system

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Intake Valve Opening

• Along with EVC it is the least critical valve
  timing event
• Along with EVC it determines the amount of valve
  overlap and exhaust scavenging
• Too early of an IVO will allow exhaust gas
  reversion into the intake ports causing major
  flow disruption and intake charge dilution
• Too late of an IVO will limit the time available
  for the cylinder to completely fill

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Cam Phase Graph
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Cam Card