|If you need a calibration tool for doing your own flow testing I have a file in the Onshape public domain for a Helgesen calibration plate. You can make you own flow bench pretty easy and use an air pump source.
I made one from wood and sealed it. Bought some big orings to match cylinder size to seal to box and used a TI pressure trasnducer and multimeter to measure flow. Worked pretty well but my vacuum couldn't keep up at higher flow rates so it was only useful for low lift assessment.
I did all this for my beautiful work on a set of VH45de heads. Polished intake and exhaust runner, polished chambers, smoothed out the bowl areas with some light porting. All to find no meaningful improvement at low lift. ??
It was a learning experience with the biggest lesson being I should have listened to the forum folks who said the head castings were already very well done and only marginal gains were to be had unless runners were opened considerably. Live and learn I guess.
I still ended up with a nice mantle piece because those heads really did turn out nice.