XJet

Your stack will provide *much* less restriction to the incoming air if you flare the open end into a bell-mouth.

I've done extensive work with intake systems on pulsejets -- which are surprisingly similar because they (like the intake system of a single-cylinder 4-stroke) are not constant flow.

If you don't have the bellmouth, a vortex ring forms at the inside edge of the open end and this ring can significantly reduce the effective cross-sectional area of the tube at this point -- in other words it will choke your engine.

Flaring the end means that the vortex is not only smaller (because the air isn't crossing the edge at 90 degrees) but is also formed outside the diameter of the actual tube itself.

You'd be absolutely astonished at just how restrictive that vortex is when sucking air through an unflared pipe -- it can reduce the effective cross-sectional area by over 25% in some cases.