As you set the motor timing lower, you get more torque.
This makes that the car will accelerate better. Than you need to find the spot for the start RPM.
This is the spot ad almost the top RPM of the motor without the Boost or Turbo.
As you set the motor timing lower, this spot will also go lower but I would first find a gooed gearing with a lower motor timing.
Than look if the Boost comes in smooth.
So play a bit with the RPM settings. I think 40 Boost will do.
Start RPM around the 5000. It depends on the gearing. The heavier you gear the higher the Start RPM.
As you set the End RPM higher like around the 2000, the car will accelerate smoother when the Boost comes in. As you set it lower like 10000, you will have more acceleration power. The car accelerates more.
Just play around with the RPM settings.
Set the End RPM on 18000 and play with the start RPM to see what it does in the different settings like a low start RPM from 3000 till a high Start RPM from 9000. This way you will see the difference and from there you will now witch works better for you.