[rotrex]

15 and especially 20 PSI are some serious boost numbers to push through a roots style blower without charge cooling and on petrol.
As Parmas and others did you will indeed need a large amount of ADI fluid to keeps this from having knock issues.
I also suggest in this case to consider pre compressor injection as suggested ans successfully employed by Parmas. Thing will get really hot.
Have you checked your compressor map regarding the expected flow and boost numbers?
Are you still reasonably efficient?