The HEMI has a 10:1 Compression Ratio. The owners manual recommends a minimum of 89 octane. The same as with my old 94 Intrepid ES with the 3.5L (10:1 & min. 89 Octane). I ran the Intrepid on 91 Octane (sometimes 93 or 94 for a treat) and got great performance and amazing fuel mileage (37mpg Hwy vs 33mpg Hwy EPA). Since day one (I bought my Ram brand new in 2017) I have run it on 91 octane and on occasion 94 octane). The engine runs clean, smooth, great performance, very responsive (the 3.92:1 gears help) and I get pretty good gas mileage both hwy and city (when driving efficiently, as has been the case lately with the fuel prices).
Can you run the engine on 87? Yes. Will you damage he engine on 87? Not likely
The engine has knock sensors. With these, the computer adjusts the ignition timing to compensate for the octane and eliminate engine knock (ping/detonation/pre-ignition) by retarding the ignition timing. When running 91 octane, the computer advances the timing to maximum efficiency/performance.
Also, most 91 fuels don't have Ethanol. 87 has up to 10%, 89 up to 5% and none in 91 and higher. However, some oil companies are starting to add up to 10% ethanol to all grades of gas (with the exception of E85 which has up to 85% ethanol).
The bottom line: My engine runs better on 91 octane, gives me better and more responsive performance and better fuel economy. While driving efficiently, it is actually cheaper for me to run 91 octane than 89 or 87 octane as the tank lasts me much longer than with the lower octane fuels. Yes, it costs more to fill up but in the long run, it's cheaper. Of course that goes right out the window when you drop the pedal to the floor and use all 395 Hp, then it is going to drink like a thirsty pig in the desert.