Cams - Technical terminology

Lift:  This can be cam lift or valve lift.

Duration: This is the length of time, measured in crankshaft degrees that the valve is off its seat. To calculate the duration, add the timing numbers together and add 180.  EXAMPLE: a cam with timing figures of 23/67 added, totals 90, plus 180, gives 270 deg duration.

Overlap:   The number of crankshaft degrees were both the inlet and exhaust valve are open at the same time. To calculate overlap: Add the opening number of the inlet cam to the closing number of the exhaust cam, ie the first and last numbers of the cam timing.   Using our same example of the 23/67 inlet and 67/23 exhaust (usually referred to as 23/67 - 67/23), add together the first and last numbers (23 and 23) and the total (46) is the overlap.  In general terms the larger this number or the greater the overlap, the hotter the cam.
   
  Cam Timing: The position of the camshaft relative to the crankshaft. This is expressed as the number of degrees that full lift occurs after top dead centre (tdc) in the case of the inlet, and before tdc for the exhaust.  To calculate this, take the duration figure and divide by 2. EXAMPLE:  With an inlet cam of 23/76, the duration is the addition of these two numbers, plus 180, equals 270. Then divide by 2 resulting in 135. Deduct the number of degrees before tdc that the valve  started  to open, ie 23 degrees - the result  112. The valve is correctly timed with full lift 112 degrees after tdc.

Valve Timing: The opening and closing position of inlet and exhaust valves relative to the crankshaft as figures before and after TDC and BDC
  Lobe Angle:  The angle between the inlet and exhaust lobe, measure in degrees.

Ramp: The ramp is the part of the profile that takes up the valve clearance and slack in the valve train gradually, before the valve is actually lifted from the seat.  It also rests the valve gently back to the seat after the closing flank. Mechanical profiles use a much larger ramp than hydraulic ones, as the hydraulic cam follower should be in contact with the lobe at all times. The height of the ramp dictates what measurement the valve clearances should be set to.

Flank: This is the part of the profile between the ramp and nose. It is the most important part of the whole design. The flank controls the velocity and acceleration of the valve train.  The acceleration / deceleration rate must be within the working limits of the valve spring, too much and valve float with occur. Generally high acceleration & velocity figures are beneficial to engine performance.  

Nose radius: The larger the nose radius the better. Best to utilise the biggest nose radius possible to keep the stresses to a minimum.

  
   
  Dwell: As the valve reaches full lift it will stop moving for a few degrees before starting to drop back towards the seat, this period is known as the dwell. When checking the cam timing using the full lift figure method the mid-point of the dwell should be taken as exact full lift.

Rocker Ratio: The ratio between valve motion vs cam follower motion. Push rod engines typically use a ratio of between 1.1:1 & 2.0:1. Over head cam, direct operating engines obviously have no rocker ratio as the cam follower motion is exactly the same as the valve motion.

Overall height: The measurement from the nose of the lobe to the bottom of the base circle, in a straight line through the centre of the lobe.

Base circle diameter: The measurement across the lobe, calculated by measuring the overall height and subtracting the cam lift.

Some general info: Maximum lift is 1/4 (25%) of the valve head diameter; eg if a valve is 40mm the optimal valve/cam lift would be 10mm any higher and it would just be putting unnecessary strain and wear on the components.

The higher the duration/overlap the better the engine will be at higher revs, but at the cost of low end torque. This is OK if you want big power at higher revs (3k and above) but not good for town driving of towing. For loads of low down power use a cam with very little duration/overlap BUT get as much lift as possible (within reason)  A low duration/overlap cam will run out of steam as low as 5500rpm while the typical higher duration/overlap will happily rev to 7K and above.

Some typical Ford cam specs:

1.3,1.6,1.8 SOHC Pinto. (Also 1.8 CVH)

22/54/64/12 (256° duration, 34° overlap)

Lift: unknown

2.0 Pinto (carb and EFI)

22/64/70/18 (266° duration, 40° overlap)

Lift 10.12mm in/ex

DOHC (Sierra)

Carb

13/39/43/13 (232°/236° duration, 26° overlap)

Lift in 10.5mm ex 10.9mm

EFI

13/51/43/13 (244°/236° duration, 26° overlap)

Lift in 10.5mm ex 10.9mm

DOHC (Scorpio)

9/51/51/9 (240° duration, 18° overlap)

Lift in 10.83mm ex 10.83mm

An EFI sierra inlet cam and a Scorpio exhaust one 'might' be interesting.

2.9 NON-CAT

24/64/66/22 (268° duration, 46° overlap)

lift 6.72mm (this is cam lift and NOT the valve lift)

2.9 CAT

30/66/76/20 (276° duration, 50° overlap)

lift 6.54mm (this is cam lift and NOT the valve lift)

2.8 MFI

24/72/73/25 (276° duration, 49° overlap)

lift in 9.43mm ex 9.23mm