We don't have synonyms for blow-by yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are blowby and blow by.
(n) :
(automotive) In an internal combustion engine, any burnt or unburnt matter escaping from the cylinder, past the (worn) rings on the piston, and into the crankcase.
(n) :
(automotive) In an internal combustion engine, any burnt or unburnt matter escaping from the cylinder, past the (worn) rings on the piston, and into the crankcase.