at what it takes to feed higher rpms...the higher the exhaust flow there needs to be. ratio is dependent on lots of variables..maybe chamber/burn efficiency/comp ratio..you can cam around this.
but they basically have a rule of thumb, well some do..I'm in the 67-75% for NA club.lol
Its what split pattern cams work around too
There is a point that where if ur not too careful...you'll over scavenge and pull raw fuel right out the exhaust.
