How can I set the strength of the particle motion blur? I thought I can set it through the Particle Blur parameter in the render POP but unfortunately this parameter has no effect in my renderings. Even if I set it to zero I get motion blur.
Currently the speed lines in the viewport have nothing to do with the rendered motion blur in the output image.

The amount of “blur” is determined by two things, the shutter speed of the camera and the velocity of the particle. Tweaking both these things will change the blur. Changing the shutter speed will effect the blur on all your objects. On the other hand scaling the velocity of the particles can give you more control.

To add to Jim's post…..

You can mess/play with the velocity settings using a point sop after your particle simulation. On the particle tab choose “Add velocity.”

Hope this helps,

/Rick

Wolfwood
The amount of “blur” is determined by two things, the shutter speed of the camera and the velocity of the particle. Tweaking both these things will change the blur. Changing the shutter speed will effect the blur on all your objects. On the other hand scaling the velocity of the particles can give you more control.

Okay. Only the velocity attribute blur does work with particles. Is that right? So if it can be controlled only by the velocity why do I have a particle blur attribute in the render POP? As I understand it modifying the velocity will also change the particle speed. In that case it would be no valid option. So the only way seems to be doing it over the shutter speed. But that's okay, that's exactly what I need.

Yes only the velocity attribute matters when determining velocity motion blur. The Particle Blur attribute on the Render POP is a bit misleading; all that parameter does is increase the length of some of the particle types. (Lines for example.)

Changing the velocity of your particles is usually the best option. Just as Rick W. said use a Point SOP after your Particle network to scale the velocity. Since its a SOP node it won't change your simulation.