Warning boring, engineering post; might want to skip to bottom:
Octane is a hydrocarbon molecule – C8H18. The Octane Rating System defined pure Octane as having value of 100 and Heptane (C7H16) as having a value of 0 when run in the standard variable compression engine – much like water boils at 100°C and freezes at 0°C. The rating system only defines how susceptible the fuel is to knock, it has nothing to do with heat content.
For Research Octane Number (RON), the engine is run at 600 rpm, for Motor Octane Number (MON), the engine is run at 900 rpm/fuel mixture is preheated and ignition timing varied which provides a more accurate number on how fuel will burn in carbureted engine. The number at the pump is average of 2.
So, 87 octane fuel, theoretically would be 87% Octane and 13% Heptane. But practice rarely matches theory.
At refinery, unless you have pure feedstock, you are going to have to modify the product to meet the specification (and you are going add detergents, stabilizers, water, etc). And you are going to want to spend the minimum you can to meet the requirement. So, if I can slip in a little Hexadecane (C16H34) – which would have a negative Octane number (about -30), but add a little more Octane, i.e. a mix of 88% Octane/9% Heptane/3%Hexdecane, you could deliver a cheaper product, that still meets the specification. Alternatively, the spec could be met with some Hexane (C6H14) which has Octane number of about 25. e.g. 85% Octane/7% Heptane/8%Hexane. And refineries get super complex with a whole list of hydrocarbons to make up your gasoline.
Now, Octane has enthropy of combustion of ~5.5MJ/mol, Heptane about 10% less, Hexane about 20% less, and Hexadecane about double. So, solution two has 10% more energy/volume than theoretical solution while solution three has 10% than theory.
The theoretical gas is very consistent in its knock properties; the practical, with their varied mixtures, not so much. The better the companies engineers, the worse product you are getting at pump…
In the good olde days, refineries cranked out sludge and added tetraethyl lead (CH3CH2)4Pb – 1g/liter was worth 10+ octane points…
To OP: I would stick with a 331 – in fact I have. 3.4” stroke is just too long for a 5.0 block to enjoy longevity (IMNSHO).
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