Skylar Robinson, son of famous American racer Chip Robinson, puts on a show in a raucous Porsche 962 making a peculiar series of noises. The 2.8-liter, twin-turbo six in the middle of his IMSA GTP racer squeaks and roars, and when the full 750 horsepower arrives in one big lump, it fires the car down Laguna Seca's straighter sections at a pant-wetting pace. More exciting than the raw speed, going by the comments beneath the video, is the distinct "choo-choo-choo" noise echoing in the cabin, often referred to as 'wastegate chatter'.
What's A Wastegate?
The wastegate is, essentially, a valve which diverts exhaust gasses away from a turbocharger to keep it from spinning too quickly and, therefore, increasing boost pressure too much. The valve is controlled by a pressure actuator, which is linked to the turbocharger, and the wastegate is held shut by a spring inside the actuator. When the boost pressure exceeds the pre-set maximum, it compresses this spring, progressively opening the wastegate to bleed off excess pressure, regulating the impeller's speed, and ensuring the turbocharger and the motor aren't damaged.
How Does it Differ From Compressor Surge?
Though the fluttering noise made by this 962's engine might be referred to as wastegate chatter, it is technically known as compressor surge. When the throttle plate closes, the combination of the lack of airflow, consequent rise in pressure, and the high speed of the compressor cause the compressor blades to effectively lose their “grip” on the air. Aerodynamically speaking, the air separates from the back of the compressor blades, allowing some air to escape back out through the compressor. The resulting pressure drop in the intercooler piping allows the blades to “grip” again which, under the residual momentum of the turbo, will increase the pressure and the cycle repeats. This is what causes that remarkable flutter sound that will crack a smile across the stoniest of faces.