vray motion blur correct settings

[super gnu]

i was going mad thinking my tyflow object bind had a slight lag compared to target geometry. small particles dissapearing  momentarily inside one side or other of object as it changed direction. 

turns out its the vray motion blur.

with it off, particles behave.. with it on, they  lag slightly. 


i tried changing interval centre of moblur to both start and end of frame, but it didnt seem to help much.  any suggestions?