In mechanics, the escape of gases (especially from combustion) past a piston ring in an engine, reducing efficiency.
From 'blow' + 'by,' a technical term describing how gases bypass the intended seal or barrier.
Engine blowby is why old cars smoke and modern cars have crankcase ventilation systems—it's a constant physical battle between sealing piston rings tightly while allowing them to move freely, and it's one reason why maintaining proper oil viscosity matters so much.
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