The 'ideal' condition you're looking for in a gasoline combustion engine is a mixture of 14.7:1 - I believe this is called Stoich or Stoiometric condition. Basically, it's 14.7 parts air to one part gasoline. This is where the optimum explosion occurs. However, from what I've seen, most Stangs seem to run a little richer than this, say around 12.5:1 or so.
Running too rich can be bad too. Too much fuel and over time you can end up washing the walls of the cylinders with fuel and there go your rings. You can also get massive amounts of carbon like build up on your valve and pistons.
Some of the engineers on this site could give you a more technical answer, I'm just giving you the layman terms.
E
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1991 5.0 LX Coupe - 40,750 miles
331 cu. in. / Tremec 3550 / BFG Drag Radials
12.22 @ 114.31 mph - w/1.89 60'
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