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Cam timing

  1. Get cyl1 to TDC

  2. Attach a protractor to the crank and set it to read 0'

  3. Measure the angle (on the protractor) of two identical cyl1 inlet valve lift points on the rise and fall. When measuring the two lift points turn the cam clockwise and anticlockwise. Tappet friction can give different results each way.

  4. Add the angles together and halve the answer. This gets the angle past piston TDC that the inlet reaches full lift.

    This is called the Lobe Centre-line Angle (LCA) and should be quoted for your cam. If not look it up in eg: David Vizard's "Tuning BL's A series engine". A cam timed at this position is said to be 'straight up'. This angle should be around 100-120 degrees. Most Kent cams are 106'. MG Metro cam is ~112'. Some cams benefit from a deviation from the LCA. Kent 266-296 cams gain a little extra hp at higher revs by advancing the cam by up to 4' to say 102' ATDC (remember ADVANCED is LESS degrees)

  5. Each tooth on either sprocket is 18' of CRANK rotation (as seen on the protractor). Often simply skipping back or forth by a tooth on the sprocket will be a far too coarse adjustment. In order to fine tune the cam timing you will need either an offset woodruff key, or a vernier (adjustable) sprocket. For road use often just lining up the dots will be ok, but for race applications you want to aim for the exact timing.

Degrees per tooth

ALL teeth are 18 crank degrees each. Sure, the CAM sprocket is half the 'degrees per tooth' for THAT sprocket as it has twice as many teeth, but we are talking CRANK degrees. Think about it.... If you tippex a dot on the block next to the crank sprocket and another dot on the block next to the cam sprocket, then rotate the crank sprocket half a rotation (180')...How many teeth have passed the dot next to the crank sprocket? Answer : 10 ...the cam sprocket will have rotated half that amount (90') as it is twice the size...  How many teeth have passed the dot next to the cam sprocket? Answer : 10  Therefore BOTH sprockets moved round by the same number of teeth, and therefore MUST have the same amount of CRANK rotation per tooth. = 18'

Advancing Kent cams (276,286,etc)

Kent cams are 'straight-up' at 106'. This is the Lobe Centre Angle (LCA) and is the angle between lobe centrelines, and is more easily remembered as the degrees after TDC that the inlet reaches full lift.

Vizard says the power curve is increased and shifted up towards the top end if the cam is advanced by up to 4'.

Most BL cams (eg 998MG Metro) have an LCA (straight-up) of about 112'.

These do benefit from 1 or 2' advance, but not much more than that.


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07 June 2003