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.