But I might be wrong. My gut feel says strongly that you want high air velocity. But gut feel can lead us astray, so I just worked it out from a head loss perspective. Going from 4" round duct to 6" round duct will save you about half a PSI. The 4" will have about 0.7 PSI of loss, and the 6" will have about 0.1 PSI. For a shop vacuum that may only make -3 PSI (guessing here), that may be significant. You might very well get 10-20% more suction at your endpoint by increasing the duct diameter.
It's a tradeoff. All I know is I don't want to mess around with removing sections of duct to try to unclog them, so I'm installing 4" in my shop.
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