Shifting Planned Start and End Dates

Hi,

I have a very small project that lasts for a week with 3 resources. Due to resource crunch i've asked the two resources to stretch the working hours (more than 8 hours) and complete the task in one week itself.

But though i set "Fixed dates" mode and assigned resource with days (7.5/week), after "Calculate activities planning" the dates are shifting to adjust 8 hours / day and the Project's planning dates are moving to 1.5 weeks. I don't want to change working hours >8, because it impacts other projects too.

Is there any workaround for this?

Thanks
Raj