I am constantly amazed at the amount of people that I run in to that A) do not know what a Porsche 911 Turbo has or what it is, B do not know what a VTG is once explained the car is a twin turbo with VGTs. If you are reading this then the odds are that you at least know that the 997TT is, well a twin turbo. So we will forget the definition and description of A).
Now what is all this VTG bla bla…The goal of a regulated turbine is to expand the usable flow rate range in practical applications while maintaining a high level of efficiency. In other words trying to get the exhaust to the turbo as fast as possible so that it can hit it efficiency range and start to create boost. To accomplish this, the turbine output is regulated by changing the inflow angle and inflow speed at the turbine wheel inlet. In the case of the VTG turbocharger this is achieved using guide vanes located in front of the turbine wheel.
When the guide vanes are in the closed position, the high circumferential components of the flow velocity and a steep enthalpy gradient lead to a high turbine output and therefore to a high charging pressure. When the guide vanes are in the fully open position, the turbine reaches its maximum flow rate and the velocity vector of the flow has a large centripetal component. The advantage of this type of output control over bypass control is that the entire exhaust mass flow is always directed through the turbine and can be converted to output. The guide vanes adjustments are controlled through our ECU directly.
I recently removed my turbos and sent them out for modifications to the compressor and the compressor housing. We can’t make much change to the turbine side due to the veins that are always moving. But between a larger compressor wheel and some more aggressive tuning we have easily raised the HP to over 700 HP. The best part about these types of turbos is they can simulate many turbine sizes if you will to create a fast spooling turbo that delivers boost much sooner than say that of a older GT2 Turbo or X50 car fitted with a K24. In fact the last set of dynos we did proved to make its HP at least 1000 RPM sooner. On the street and for drivability this is huge! The amount of TQ that can be created at low RPMs is staggering.
Porsche continues to give us the leading edge to play with. In 1974 it was the worlds first gasoline fitted Turbo in a production car, in 2006 it was the gasoline fitted production car with VTGs.