If you have already installed existing sound mods please go into the section of the turbo and replace it with the following for more accurate turbo simulation, which is based on actual engine load.

<retarder file="sounds/turbo_loop.gls" linkNode="0>" innerRadius="23.0" outerRadius="90.0"  pitchScale="1">>

<volume indoor="0.5" outdoor="1" >

</volume>

     <loopSynthesisRpm>

                    <modifier type="MOTOR_LOAD" value="-0.00" modifiedValue="0.0" />

                    <modifier type="MOTOR_LOAD" value="0.00" modifiedValue="0.0" />

<modifier type="MOTOR_LOAD" value="0.25" modifiedValue="0.1" />

<modifier type="MOTOR_LOAD" value="0.70" modifiedValue="0.45" />

                    <modifier type="MOTOR_LOAD" value="1.00" modifiedValue="0.65" />

     <modifier type="MOTOR_RPM_REAL" value="500" modifiedValue="0.50" />

                    <modifier type="MOTOR_RPM_REAL" value="1000" modifiedValue="1.0" />

                </loopSynthesisRpm>

<lowpassGain indoor="0.7" outdoor="1"/>

</retarder>