Originally Posted by blue2000s

Maximum power occurs at a relatively rich level. It's also common to use extra fuel to keep the engine cooler. Also, when going from a small throttle opening to full throttle, the cylinder mixture can run lean so the ECU compensates by injecting extra fuel. Under full throttle and heavy load, the car dumps alot of extra fuel into the cylinders. This is why most cars will puff some black smoke when accelerating hard. It's normal. If it's blue smoke, it's a different story.