[Tesla]

I'm talking about the resonator which is normally positioned just after air filter as a branch off the main air intake tube.

In normally aspirated vehicles it often has a dual function, noise reduction and also to improve efficiency of air intake into cylinders by bouncing pressure pulses between the intake valves and the resonator.
In my case I have a turbo, intercooler and about 5' of contorted tubing in between, so I doubt that there is any coherent pressure pulses going on and always just assumed it was there to just reduce intake noise.

Recently I had reason to put a large 4litre chamber in the PCV blow by line, like a catch can, which connects into the intake just after the resonator, seat of the pants assessment indicated a reduction in turbo lag. This was interesting as the mod was just for diagnostic purposes not for performance so the apparant change was a bit of a surprise. I still need to re jig the setup so I can switch between extra chamber and no chamber on the fly to confirm there is a distinct change in behaviour.

In a diesel the air filter is the biggest restriction in the induction line and hence there is always a some vacuum behind the filter which increases with rpm & load.
The effect that I believe is occuring is basically to create an additional volume of induction air between the filter and turbo, when you accelerate from low rpm the injection fuels up, the exhaust then needs to spin up the turbo to push more air in to the cylinders, hence more exhaust and more air again and the turbo spools up. Initially there is a dramatic increase in vacuum in the intake infront of turbo, effectively the engine takes a big gulp of air, but has trouble getting more until turbo gets up to speed and we have turbo lag.

Having a greater volume of air between filter and turbo reduces the rate at which vacuum increases and hence provides more air to engine at that initial point of acceleration, hence allowing turbo to spool up just a bit quicker. The extra volume does not affect steady state running, it only comes into play when there is a change in intake vacuum, MAF is positioned before this air reservoir so under initial acceleration it will, run the engine marginally leaner as MAF will be reading lower airflow than engine is actually getting and on deceleration it will run marginally richer as vacuum is reduced and the reservoir volume is replenished. I don't think that is too much of an issue as acceleration is usually a rapid response whereas on deceleration pedal action is usually more gradual and feathered down as desired speed is reached.

It may have a beneficial effect on economy by moderating down driver behaviour if vehicle response is more effective, i.e. less overkill on pedal required because lag between action and response is reduced.

I'm going to play with this a bit, just want some input as there may be other things I have not considered.
Thanks in advance.

ps - I already have a ram air type of setup on intake which has a beneficial effect on economy by reducing intake vacuum and also partial front skirt, grill block and no radiator fan, have tried wheel covers, no benefit in my case.