[Rossp]

I still disagree completely I'm afraid.

My information comes from Alan Allard's book on Turbo and Supercharging. He specifically mentions compressor stall. http://www.allardturbosport.co.uk/

General opinion is compressors do stall. Frequently. I've certainly heard it on every car i've had when I've run with an incorrectly setup dump valve, or no dump valve at all. And the effects are obvious when driving. MUCH more lag between gearchanges.

Wikipedia - http://en.wikipedia.org/wiki/Blowoff_valve

99% of all modern turbo cars as standard are fitted with dump valves for this very reason. Your engineer seems to have a different opinion to everyone else, including some very respectable sources (Such as Alan above).

For example: Graham Goode - http://www.grahamgoode.com/dump.htm:

Why do manufacturers fit a dump valve?

It is for a number of reasons. Without a dump valve, when the throttle was closed, the build up of boost pressure would be immense, very similar to closing a quickly flowing water tap. This excess of pressure, (which could be over five times the running boost pressure), would put the components of the system under a great strain. It would try to burst the intercooler and pipework. More significantly, it would put a large strain on the turbocharger itself, firstly stalling the compressor shaft, then trying to force the compressor wheel out of the turbo, against its bearings.
So, the dump valve prevents mechanical damage to the intercooler & turbo pipework, prolongs turbocharger life, and prevents excessive turbo - lag that would be caused by the compressor shaft stalling.

Turbo Technics - http://www.turbotechnics.com/turbo/faq.htm

What is a dump valve ?

A valve which relieves back-pressure between the compressor outlet and engine as the throttle is closed. Only sometimes required on engines producing in excess of 14psi boost pressure.

HKS - http://www.hksusa.com/categories/?id=1062

Blow-Off & Bypass Valves have one goal, prevent compressor surge. Compressor surge occurs when the throttle plate of a turbocharged engine closes and the pressurized air is forced back into the turbocharger’s compressor housing causing the turbo to slow down. Compressor surge not only decreases the spool of a turbo but can also damage its center cartridge.

TurboSmart - http://www.turbosmartonline.com/index.php?id=624

When a gear change occurs, the throttle which allows air to flow into the engine is closed.

This results in a large pressure spike as the turbocharger is still trying to flow air due to the inertia of the compressor and turbine but there is nowhere for the air to go as the throttle is closed. The pressure within the intercooler piping continues to increase until the compressor reaches its pressure limit. When it reaches this limit, it cannot flow the air any more and the built up air pressure inside the intercooler and pipe begins to flow backwards through the compressor which is trying to flow air forwards.

The result of this reversal flow of air is the immediate deceleration of the turbocharger and a high load on the bearings which support the compressor/turbine shaft. At low turbo speeds and low pressures, the deceleration of the turbo and the load on the bearings is low, i.e. a small amount of fluttering at low engine speeds and throttle movements is negligible.

At high turbo speeds and high pressures, compressor surge during a gear change can be damaging to the bearings of the turbocharger as the deceleration rate of the turbocharger is high and the reversal airflow through the compressor is high.

Greddy - http://www.greddy.com/products/turbo...ow-off-valves/

[GReddy] Blow Off Valves [are easily adjustable to] eliminate compressor surge and preventing premature boost leakage, while increasing boost response between shifts.

And finally, Garrett themselves - http://www.turbobygarrett.com/turbob...o_tech101.html

The Blow-Off valve (BOV) is a pressure relief device on the intake tract to prevent the turbo’s compressor from going into surge. The BOV should be installed between the compressor discharge and the throttle body, preferably downstream of the charge air cooler (if equipped). When the throttle is closed rapidly, the airflow is quickly reduced, causing flow instability and pressure fluctuations. These rapidly cycling pressure fluctuations are the audible evidence of surge. Surge can eventually lead to thrust bearing failure due to the high loads associated with it.

With respect, your engineer at garrett has it wrong.