A cheap and easy comparator and a good one at that is to use your shop vac as the vacuum source and take a pressure (vacuum) reading on the side of the device to be evaluated. The measurement can be something so simple as a u-tube manometer with water in it. The water u-tube is measured against a plain ruler. 1" of movement is really two inches of water vacuum. The better the flow characteristics, the less the vacuum draw on the shop vac. Use a 12" ruler and the u tube can be made of plastic tubing that is about 1/4" id. You fill up the u tube with water up to the 6" level and you will have a very accurate measurement for up to 12" of water vacuum.
Unfortunately there are no super simple formulae to apply that would be as easy as that described above. It can surely be calculated and then would need to be flow tested anyway, so the previous description is really super easy to apply. The output of the shop vac is directly dependent upon the power (electrical) at the plug, so assuming that there is not a great deal of power fluctuation at your shop space, it will work as described.
TIP: Before fabricating a complete intercooler, you can measure the sections to select the best one to use. The same can be said of the tanks and the piping. You can first make up models of cardboard and tape to decide on a design before committing to welding.
Some handy equivalents are: 1"H2O = 13.6"Hg 1"Hg = .49psi
Enjoy. Good Luck.
Regards,
HB2
