Oftentimes, you’ll hear terms like “boost threshold” and “turbo lag” get thrown around, sometimes mixed up and sometimes used as the same thing. They aren’t actually, and here’s the difference.
Engineering Explained’s Jason Fenske gives us a detailed walk through of the two terms and what they mean. Basically, boost threshold is the engine speed at which you have enough exhaust gas flow in order to create boost.
Turbo lag is the additional time delay above boost threshold, when the engine’s throttle valve is open and the turbocharger needs to accelerate to deliver positive pressure to the engine.
Fenske spells out the rest.