On the 1.6 gas the stock pipe is tuned to give the best torque between 7000 and 9500 rpm,but the lengt is the same or for the bigger rpm I need to reduce the lenght (and for bigger props the pipe must be longer)?

There is no need to change the pipe length for props between 16x10 and 20x8, which is the entire working range for the engine.
Smaller props will not produce much thrust, and larger props will cause overheating and excessive connecting rod loads.
no need to change anything. All changes will reduce performance.

Paolo,


To add to what Pe wrote, the pipe is installed with just 2-5 mm of 'air', within the silicone coupler.

With the pipe further back, the RPM range will be slightly lower, but will allow slightly larger props.

You don't need them, just install at stock length.