Vacuum question!

Ceretianly could be the fuel. The slower burning the furel is the more timing you will need. To get maximum power you need to have maximum cylinder pressure around 20 degrees ATDC, so you need to start combustion early enough for that to happen.

Also, the more efficient you cyliner head combustion chamber the less timing you need. The new magnum heads require less timing for max power, normally less than 35 degrees.