The combustion efficiency in a gasoline engine is at least 95%. A 30% increase in efficiency would give me, what, a 125% efficient combustion?
If the combustion was sufficiently ineffecient for something like this to work, most cars would have flames coming out the tail pipe. In fact, when an engine misfires there is a huge stress on the catalytic converter as the unburnt fuel burns inside the catalytic converter. It doesn't take long before it is ruined.