Quote:
Originally Posted by mrg666
Why does Subaru reduce FA24 compression ratio to 10 for turbo version? Or, Mazda does the same for Skyactive G 2.5 on Mazda 3 turbo. I am just asking your insight, not making a counter argument.
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It's because tuning for dynamic CR is a technical solution to what's really a mechanical problem.
Toyota uses this sort of technique to enable an Atkinson-style Otto cycle for efficiency on the 1NZ-FXE. They let the intake stay open into the compression stroke, pushing some charge back into the intake and lowering the volume being compressed. You get a more complete burn, but total power output is reduced because you're compressing less total A/F mixture.
Subaru puts low compression pistons in because they get to decide what goes in the block before it's put together. They're not targeting 40% thermal efficiency from an NA engine, they're targeting zero knock and passing emissions even though customers are dumping 87 octane in their tank. Mazda's the same way, though now their Skyactiv X engines use 14:1, 15:1, and 16:1 CRs depending on the market.
The 13:1 ratio in the NA FA24 is only possible because of really good tuning, a really fast ECU, and premium fuel. Hell, turboing the FA20 on the stock 12.5:1 block is really only possible because of modern ECUs being so much more capable than those of even ten years ago. Even then power is still limited by your fuel's knock resistance before anything else comes up.