ah, but there is. Running at higher compression ratios give you higher efficiencies. As audikillsbmw points out, the 4.2 has an 11:1 compression ratio versus a more typical 9:1 compression ratio on cars that run 87 octane.

"A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency."

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Jim
formerly FtWorthS4

1997 S6