The RON measurement (International standard) for octane shall always be 4 or 5 greater than the averaged octane calculation for the same fuel.
US/Canadian …………. International
(RON + MON) ÷ 2 ……. (RON)
85 ……………………….…………… 90
87 ………………………...…..……. 91/92
89 ……………………………………. 93
91 …………………………...………. 95
92 …………………………...………. 96
93 ……………………………………. 97
94 ………………………………...…. 98
95 ………………………...…………. 100
Pump gas rated 91 octane in the US and Canada is what was once referred to as premium pump gas. Pump gas rated 93 to 94 octane in the US and Canada is what was once referred to as "super premium" pump gas.
The 1970/1971 versions of the 351C 4V manufactured in the US were built to operate on “premium fuel” off the showroom floor. The two versions of the 351C 4V with the highest compression ratios, the 1970 351C 4V and the 1971 Boss 351 both had dynamic compression ratios of 7.6:1.
The Cleveland cylinder walls are nominally 0.160 inch thick (roughly 5/32 inch, or 4mm), which isn’t very thick. The actual cylinder wall thickness of any given block will be thinner than 0.160 inch in places due to core shift during the casting process. When pushed beyond their limits the Cleveland cylinder walls crack.