Hi and welcome to the forums.

The simple answer is No. Each WU can only be processed by one core. The only way to get to 100% CPU usage on multicore machines, is to have at least one WU/core, in your case, at least eight. There's no way around this.

Try selecting more projects, or the option "If there is no work available for my computer for the projects I have selected above, please send me work from another project" in the device profile.

DainBramaged ,
Only programs which have been designed and coded as multithtreaded can use multiple cores to do their job, and most programs are not eligible for such a design.

As P . P . L . said, check your processor settings in your profile(s), and if you have ever used them check the same parameters in the Local Preferences of your BOINC client.
Be careful to not confuse "Use no more than xxx % of processor time" and "On multiprocessors, use xxx % of processors". For your problem the latter is the important one.

This processor number field is redundant to any client of the 6.10 family which will imminently be adopted by WCG for Windows. To test this I set the quad profile to 3 processors on the website (field not available in clients 6.2 and up), and hit update project in agent. BOINC continued running on 4 cores although the log indicates that the new web profile was loaded... client 6.10.58 on Linux.

29/08/2010 12:21:00 World Community Grid General prefs: from World Community Grid (last modified 29-Aug-2010 12:20:05)

If the post 6.2 client is still responding to that field it's a bug in the individual build of the client.

This is an option that is NOT impacted by Run Always/Run Based on Preferences. The Activity menu as the name says are strictly for when a client runs / not runs i.e. the behavioral and network scheduling part of the controls.

Here an extract WCG Web device profile explains the effects of processor number and %. I've inserted the defaults!
Want to restore sanity when things act different than the web prefs prescribe, than clear the local preferences and only the web prefs will be used. It's the easiest way to manage multiple devices attached to WCG from 1 screen (there are others web based easy ways, but not discussed here).

Glad you were able to confirm doing this from another project website. In principle BOINC only listens to the web prefs of the last website where changes were made. I think those values proliferate to all the grid websites, but not sure about that. Certainly when attaching to a new projects, the new website device profiles pop in the last applicable website device profile settings, which I guess pulls it from the client... sort of deep under the hood fringe interesting to the average cruncher.

To amplify my previous post, the first part of the test was to leave the local prefs in place. Then cleared the local prefs and 4 cores continued running i.e. the processor number field is ignored in 6.10.