Re: 87 vs 91 octane for better mpg?
That's the "theoretical" compression ratio that you'd get if the closing of the intake valve wasn't delayed. But it is significantly delayed (by 32 degrees in the Camry Hybrid's 2.4-L ICE versus the non-hybrid's 2.4-L ICE). It's the expansion ratio that's represented by this number; the effective compression ratio is around 10:1, as in all low-octane ICEs. Calling it a "compression ratio" is what's throwing you off. The big expansion ratio is what gets more of the thermal energy out of the gases before they're discharged (hence the higher thermodynamic efficiency) — but it also breathes in less fuel per stroke, and so results in the lower power of the "Atkinson"-type ICE for its rated size. It's really more like a 2-L than a 2.4-L ICE power-wise.
You can see the comparison of these two engines on page EG-3 of the attached file from the 2007 Camry Hybrid "New Car Features Guide."
Stan
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