For the older controllers like the G27
Open the Logitech Profiler:
Click Options...Global Device Setting:
Set Overall Effects Strength to 100%
Set Spring Effects Strength to 0%
Set Damper Effect Strength to 0%
Enable Centering Spring in Force Feedback Games - uncheck
Set Degrees of Rotation to 900%
The newer G29, G920 and G930's for example - we would recommend leaving the defaults.
In the iRacing Software
Launch the SIM (in a Test session for example)
Click Options and in the Drive tab
Set "Enable Force Feedback" to on
Set Force Feedback Strength to 20% (you can adjust to your liking)
Set Damping to 0%
Also please note the auto option for setting the force levels. Although not Logitech specific, it is the quickest way to get your FFB feeling good.