There is more to it than that. the rpm and bpr sets the sweet spot of sorts. the compressor is designed to run ballanced at a certain rpm and BP stetting. I am not sure what the rpm may do for it but the BPR sets the pressure for the last stage to match the force ( psi and area of the piston) of he other cylinders. At that pressure the compressor runs smooth and does not vibrate. below or above that pressure the compressor will vibrate. It is best to have that sweet spot n the upper 2000's so that at the top off of the tank the final stage does not have the larger force on it. The compressor will last longer if it is designed with a ballanced bp at say 2800 rather than 1500. The bpr has little to do with the final stage floating piston chattering as it will chatter until the BP gets to maybe (like my SA6) 50-100 psi and is good after that. Any bpr immediately after the final stage is good for that. However they are not normally put there. They are put after the first external moisture separator to pull the water out. In that use,,,,, the higher the BP the better,, as it mechanically removes the water and not chemically in the filter stack. Compressor rpm ,,, as I have seen determines the output of 3 or 6 cuft/min. I think that on that SA6 the BPR is right after the internal final moisture separator,,,,, however that separator is a very small volumn so the clatter stops after a couple of seconds. The larger the separator the longer the chatter if your first BPR is placed after it.
Now I really like this, way off topic I know but such an important consideration.
But I have again one question as I'm not sure if we are all talking about the same thing or if the Anglofiles on the forum understand the term "sweet spot" or you lot understand what us lot think of balancing a compressor
I have heard before on the forum this term "balancing" or the compressor being "balanced" and never got around to asking before I'm just not sure what we all mean by the term "balanced" or if we even fully understand the term
Maybe we should use the term balancing just for describing mechanical vibration reduction
and the term "Pressure Balancing" for what your describing above the "Sweet Spot"
If so the "Pressure balancing" on your SA-6 design is 3000psig or 207 bar.
But the mechanical balancing is adustable by the counter weight length at the front end (fan side)
and at the back end (driven pulley) by adjusting the PCD position of the three 19mm weight balancing holes
Or by using a standard industrial SPZ twin groove pulley and adding another counter weight at the back end
in the opposing face to the front one (upside down) The crankshaft has a drilling at the back end pulley side for this purpose.