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Oh I just remembered, knock is definitely possible at 17:1 since that's still close to stoich flame temperature. That's why lean burn engines from years past would jump straight to 20+ AFRs.
That makes lean burn retrofit an interesting problem then. Do you have electronic control over an external EGR system? The EJ motors have that right? Reducing the oxygen content in the charge would reduce knock, but the margin is probably very thin before you lose efficiency to poor combustion. You can hijack the factory EGR pipe with an EGR cooler from another car, piping coolant to it, and then hopefully opening it up at cruise somehow.
This mod would make sense because if you trick the engine into running lean, you're also reducing EGR because there's less manifold vacuum. Increasing the fraction of exhaust gas to fresh air would just be bringing it back to normal.
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