Sorry, missed I needed to look again!

I guess it's that the tube needs to fill with hot fast moving gas, to get the good stuff coming out the back! The time taken to fill this affects acceleration. I'm no tail pipe expert, but we fitted a P60 on the end of a 2' long 3 3/4" tube and the thing has no acceleration! It takes two circuits to get on the step, thottle back and slow down and it is slow to get going again.
A no pipe set up has no real drag/losses.
Dave