testor guage. You only need one guage. When the output hose is disconnected so no air leaks. The psi should be adjusted to read 100 psi on the guage. Most if not all compressor tanks have an adjustment tool similar to a manual boost controller to regulate output pressure. Once you have the psi adjusted. Hook up the output hose to the cylinder and see who far your guage needle changes. Be sure to have someone hold the crankshaft pulley because it is going to torque instantly when you hook up the pressure.
On my cold engine I recorded 2-3 % across all cylinders.
