Dell's BIOS fan settings are crap.

I just upgraded my Vostro 1510 with a T8300 cpu. I had a T5670 previously.

I upgraded for faster speed and cooler temperatures - and less fan noise to boot.

Now ironically, the fan kicks on even more than before! and stays on longer than it used to!

Yes I installed the proc and heatsink properly (the old cpu had as5 on it too) and i've done the copper mod on the gpu to keep that cooler.

Dell seriously needs to learn how to cool a laptop better...

 

Perhaps your T5670 was at the low end of the power consumption and voltage range while the T8300 is higher up the range (there can be significant differences between individual CPUs within the specified range). It would be worthwhile trying RMClock to see if you can undervolt it.

John