Gas options

A factory spec ‘73 318 shouldn’t even need 87 octane. At any elevation above sea level that still has a road.

Compression ratio from the factory in ‘73 was advertised at 8.6:1, and in reality they were probably closer to 8:1, the pistons were almost always further down in the hole than spec. I ran the 318 in my Challenger exclusively on 87 octane from sea level to over 8k feet, detonation was never an issue. The rebuilt 340 in my Duster has a calculated compression ratio of 9.8:1, and it does ok on 91 octane. I could probably throw a little more timing at it with more octane, but that’s 9.8:1. None of that is ethanol free, just standard pump fuel in California which always has some percentage of ethanol. It even varies from the summer to winter blend too, never even noticed on the 318.

If a factory compression 318 won’t run on 87, there’s something seriously wrong with the tune. Unless of course the compression ratio isn’t factory. Even then though, you shouldn’t need much more than 87 until you’ve gone higher than the mid-9’s for compression.