Doesn't matter what grade of gas, The lower the grade the higher tha flash point thus pre detonation accurs. I run a a 50/50 mix of 87 and race fuel in any modified motor I have. And maximum of 30* timing on any motor. I determine that with a degree wheel and dail indicators. AND please don't ask me to explain it. Took me long enough to learn it. Would take even longer to type the explantion out.