Back in the 1970's when I owned a Triumph motorbike one of the things we were told we could do to them was to clean up any curves in the inlet tract by removing a small amount of metal, a so called 'gas flowed' cylinder head as they called it then. The benefit was supposedly better mid range tractability.

Also some engines do benefit from a smoothed inlet, while others reckon a rough cast inlet is better to help mix the charge, do any of these mods apply to 2cv heads?

Not especially,

rough surfaces hold a cushion of gas and so let the other stuff flow better, thats why golf balls are dimpled. rough up the ports with 80grit emmery or a carborundum

take the sharp edges off the machining make sure there isnt a step between the head and the manifold use the gasket as a templare

dont be tempted to "open up" the inlets as the inlets are too wide to start, some of us had beeen welding up the heads to increase gas flow but it caused a furore within the raceclub

overall it just moved peak power up or down the rev range. with no real net gain

Sean

Thanks Sean.