Prevent costly air box explosions! Porsche 911 engines from 1973 on have had the bad habit of backfiring upon cold-starting. And blowing a hole in the air box. When this pop valve is installed in the air box and a backfire occurs, the valve will instantly open and release the backfire pressure. Afterwards, the pop valve then automatically closes and the engine continues to run. The air box pop valve conforms to all emission regulations and has no effect on performance or gas mileage.