The air now being sent to the exhaust is blow by and has already been metered for the last compression cycle . The MAF has no way to meter blow by . Blow by is a mixture of gas, burnt gas and oil vapors . I don't want that recirculating back into my engine or oiling up my intake pipes . You would cause a condition if you have PCV valves and vent the front tubes to the atmosphere. This would cause a lean condition when the engine is under vacuum as unmetered air would now be able to enter the intake . This mod will not create an AFR issue but you must run some sort of one way valve in the exhaust line if you wish to keep your PCV valves or you will be sucking exhaust into you plenum through the pcv valves.
