Potentially this question is dumb or I'm missing something blatantly obvious; also feel free mods to transfer this to the tuning section if that's a better fit.

Why is it that a turbocharger seems to only make peak boost at WOT under engine load (e.g. not free revving in neutral)?

I've heard the classical response to this as "you are adding more fuel at WOT which increases cylinder pressures and thus exhaust gas velocity". Is it that fuel maps are set such that when the engine isn't under load that less fuel is added than when the engine is under load? That's the only reason I can think of that would explain why a turbo generates minimal boost when revving in neutral.

 

I to would also like to know the answer.

 

Its about energy, and engine with no load has little energy. To create tons of energy you need some type of load, otherwise a engine is literally just turning itself over.

So in regards to a turbo, it needs enough energy to get the wheel to spin fast enough to create the high boost you are desiring. That 'energy' is in the form of mass = X amount of fuel + X amount of air. Your engine does not make max power with no load, it requires load to start adding all this 'energy'

... this is as simple as I can put it.

 

In simpler terms when is your body (motor) expelling the most air? At rest or during vigorous exercise?

Your body or engine requires the most under exertion (load) and expels more air volume. Likewise a loaded motor develops more air movement providing better boost.

 

An engine with no load on it takes very little fuel and air to free rev. If you gave it enough fuel and air to spool up the turbo it would just rise up to rev limit and start bouncing off the limiter before ever getting close to the amount of air/fuel it can ingest while under load. Thats why people using the two step to launch will only get like ~10-12 psi of boost. There are other things you can do though, evos in other countries came with anti-lag which bypasses the engine somewhat and allows you to inject fuel and air directly into the turbo (its more complicated then this but effectively this is what its doing).

 

Thanks for the replies on this! This makes good sense, both from a fuel/air perspective as well as a physics 'work' sense (in that the engine isn't performing as much work when there is no load).

Also explains why a preload clutch launch can generate boost better than free revving as the engine has additional load applied to it.