Stoichiometric operation of Spark Ignited (SI) Heavy Duty Natural Gas (HDNG) engines with a three way catalyst results in very low emissions however they suffer from bad gas-exchange efficiency due to use of throttle which results in high throttling losses.Variable Geometry Turbine (VGT) is a good practice to reduce throttling losses in a certain operating region of the engine. VTG technology is extensively used in diesel engines; it is very much ignored in gasoline engines however it is possible and advantageous to be used on HDNG engine due to their relatively low exhaust gas temperature. Exhaust gas temperatures in HDNG engines are low enough (lower than 760 degree Celsius) and tolerable for VGT material. Traditionally HDNG are equipped with a turbocharger with waste-gate but it is easy and simple to replace the by-pass turbocharger with a well-matched VGT.By altering the geometry of the turbine housing, the area for exhaust gases can be adjusted and results in the desired torque. Because of this the turbo lag is very low and it has a low boost threshold. Low boost threshold means that VGT can cover a big operation range of the engine from low engine speeds to high. In this operation range the throttle can be fully open and VGT is used instead of the throttle to control the desired torque which results in eliminating the throttling losses.This paper presents experimental results which show the feasibility of reducing throttling losses by means of VGT. The operating region which is appropriate for controlling the desired torque by VGT instead of throttle is specified. The gains in terms of gas exchange efficiency are quantified. Furthermore the dynamics of using VGT is quantified and compared with throttle. The experiments were performed successfully and the results showed at least 2 unit percent improvement in gas-exchange efficiency. A comparable dynamic to throttle is observed.