The intake and exhaust open/close specs are given before tdc or after bdc.
If you map out the cam, you see that the Intake valve opens at 6° before TDC and closes at 42° after BDC. There is 180° of rotation to get from TDC to BDC. So you take the intake open and intake close specs and add 180 to it to get the duration.
6 + 180 + 42 = 228°
This also matches the spec for intake valve duration.
Now to find the centerline of the intake valve, divide the duration in half and subtract the intake valve open point.
228/2 = 114°
Now subtract the intake valve open point:
114-6 = 108°
This is 108° AFTER TDC.
To find the intake open, you have to convert 108° AFTER TDC to define it to BEFORE BDC. To do this, you just subtract the intake centerline from 180°:
180-108 = 72°
So now the intake valve is open for 72° BEFORE BDC, and you have to find out when it closes. Just subtract the before BDC number from half the intake duration:
114-72 = 42° AFTER BDC
The same applies for the exhaust side....
Once you find the intake and exhaust lobe centerlines, you then calculate the lobe centers (how far apart the exhaust valve centerline is from the intake valve centerline). To do this, you just add the two centerlines together and divide by two (the crank spins twice as fast as the cam).
Overlap is simply the intake open point plus the exhaust close point. In this case:
6+12 = 18°
Hopefully you can follow this. Ask if you need more explaination...
