Headshims do set the compression.

To get the right timing on the ignition (self combustion) the compression has a huge impact, with the compression there is the nitro content but also outside temperature and the plug number to have the same impact

All factors shifting the ignition to an early point:
- higher compression
- higher nitro content
- lower plug number
- higher outside temperature

To shift the ignition to a later moment:
- lower compression
- lower nitro content
- higher plugnumber
- lower outside temperature

In basic:
The nitro content is corrected by the headshims
The outside temperature is corrected by the plugnumber

But making the step fron 25% to 16% nitro content I just changed the plug 8 to a 6 and it works very good.

And shifting the ignition does:
- give more torque but lower top RPM when shifted sooner
- give more RPM but less bottom torque when shifted to a later moment.