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I understand the engines are not identical, but as stated - close. From a few quick calculations, I can conclude the shape of the torque curve is different. We know that the Subaru engine generates 151 lb-ft @ 6,400rpm. We also know that the Mazda engine produces 155 hp @ 6,000 rpm. From that we can calculate that the Mazda engine must also produce 135 lb-ft @6,000rpm. At high rpm, the boxer engine has more torque. (But then again, at 4,000 rpm the Mazda engine produces 148 lb-ft while the boxer engine probably produces about 135 lb-ft. So its a give and take. )
But can anyone help me understand why one engine with a 12.0:1 compression ratio is fine on 87 octane fuel and why another engine with a 12.5:1 ratio requires 93 octane. Does the answer lie in valve size, valve angle, camshaft profiles, cylinder bore, etc.? (And if the concept of efficiency enters the equation, why does the heavier, larger and less aerodynamic Mazda3 get 39mpg hwy compared to the estimated 30 mpg hwy for the BRZ?)
I realize these are technical questions. Perhaps I should have selected an existing thread in the “engine” section of the forum rather than the “general” forum to post these questions. But since it is primarily an octane related question, I selected this thread.
EDIT: Could it be that the Mazda engine use “straight” direct injection and the Subaru engine uses a combination of direct and port injection? Could the direct/port combination provide less cylinder cooling than straight direct injection? If cylinder temps are higher, it would contribute to pre-detonation and the need for higher octane. Any thoughts on that?
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