| The engine doesn't decide to make boost when it needs it. It's more like, when you need boost you give it more throttle (which leads to flow which leads to boost). So it's lack of flow that prevents boost. No load on the engine = no power required to rev. The engine is throttled so you aren't sucking much air in meaning you don't have much exhaust hitting the impeller. (Even if you think you're revving the crap outta your engine, you're really just briefly blipping the throttle but keeping it 90% closed in between...otherwise you'd be bouncing off the rev limiter like the illegals at my apartment complex trying to warm their ricer Toyotas when it's -5 out). People tend to assume you should have some fixed boost at 5000 rpm, but with no load it only takes 1/10th (or whatever) the power to spin the engine that fast so you're using 1/10th the throttle and putting out 1/10th the exhaust.
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