From Engineering Explained.

Intake valves dipped in gold?! While it sounds cool, those are actually exhaust valves, and… they’re just highlighted in yellow. But you might notice something strange: only the yellow valves open up to release the exhaust gases – a clever strategy Audi has implemented for their twin-turbo V8 engine. You see, at low RPM, there’s not enough exhaust gasses to keep both turbos at full tilt, so only one exhaust valve opens, and all of this exhaust routes through a single turbo. However, as the engine RPM rises, one turbo is simply not enough. A pin drops down, sliding a camshaft collar over that puts the second exhaust valve on a cam lobe, actuating it together with the first exhaust valve. Each exhaust valve is now feeding a different exhaust manifold; the yellow exhaust valves feed one turbo, while the orange exhaust valves route to a second turbo. Thus, actuating the second set of exhaust valves actually activates the second turbo, allowing for maximum power at high RPM. You can think of it like VTEC for turbos, except when this kicks in, you actually have torque.

Full Video (Audi Supercharged Twin-Turbo) – https://youtu.be/k8-_UrfbRBA

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