There is a seal at the firewall that is supposed to keep engine compartment air out of the HVAC system. It clips over the rainshield. If this seal is not doing it's job on your car then any whiff of oil smoke, etc will find its way into the cabin, and if you have an open breather hose you're going to get a lungfull, especially at high RPM. When I first drove my car, the leakage from the cam towers dripped onto the exhaust and I could smell it coming through the HVAC. My seal is fine, and I verified that it seals against the rear of the hood. So even if this seal is good some engine compartment air gets inside.