try reading the Cast XMR thread. Your answer is there.
https://bitcointalk.org/index.php?topic=2256917.0if you can't find it there.
it's in the SRBminer thread.
If you still can't find it there.
it's in the XMRig thread.
And if you still can't figure it out...
in summary, V8 has less hash than V7. if you are looking for more hashrate, mine v7. If you are looking for more profit, hash v8.
and you'll need to shut off HBCC for Vegas for cryptonightV8 to get optimal hashrate (still less than v7... see line above)
v7 forked to v8