Project's basic scheduling formula is:
Duration = Work/Units
Depending on what you are trying to accomplish, you can fix any one of the three variables. In your case you are trying to see the effect on duration given a fixed amount of work (i.e. 86 days worth or 688 hours given a standard 8 hour workday). Set the task as Fixed Work type, by default it will be effort driven (i.e. more resources [units] shorter time). Then assign a resource. As you assign more resources you should see the duration decrease and if you remove some resources, the task duration will increase.
Hope this helps.