There is a short answer to this question and a long answer. The short answer is that if you want to have an 8 pack you need to change your genetics which is impossible. Some people will naturally show an 8 pack as they lower their body-fat and it gets below 5%.
If you know anything about the anatomy of the body then you will know very well that the Rectus Abdominus are similar to 2 pieces of flat beef jerky running parallel to each other. These two pieces of muscle run from the lower pubic area to the sternal area inserting under the rib cage.
Each strip of muscle has bands of connective tissue running horizontally through them. For most men – the top 3 bands (not including the top band/connective tissue where you see the muscles attach to the front of the rib cage) are what creates the six pack abs look.
When we see an 8 pack on a man it is the bottom band that inserts into the pubic bone that we see, and is at the exactly the same level as the muscle, or very close to the same level. So the reason that you do not see the bottom portion on the average man is that this 8 pack is hidden by being the same level as the muscle.
The bottom line is that you cannot specifically train for an 8 pack as the chances of you showing an 8 pack is closely related to your own specific genetics that you have as well as the body-fat that you carry. There is no way that you can change your 6 pack into an 8 pack so you need to be satisfied with your 6 pack.
For more info on how to get 6 pack abs fast go to How To Get A Perfect Six-Pack Of Abs ASAP