-Run Riva Tuner v2.24 and go to “Hardware monitoring”. You should be able to see the VRM output voltage now.

 Increasing Voltages & Extreme OC HD 4890 Using RivaTuner

-Right click and go to “Setup”.

  Increasing Voltages & Extreme OC HD 4890 Using RivaTuner

-Click on “More” and now you can increase the vGPU core.    

 Increasing Voltages & Extreme OC HD 4890 Using RivaTuner

I increased mine to 1.35v just to test if it works. Take note that individual results will vary.

 Increasing Voltages & Extreme OC HD 4890 Using RivaTuner

And now we can get higher and more stable speeds.

**Small Disclaimer**

Do not fiddle around with your GPU voltages if you are not sure what you are doing.

Excessive voltage and poor cooling will damage your card in the long run. Make sure to do proper stress testing with your videocard after overclocking. (Furmark is one of them.)

I had about 2-3 Litres of ln2 leftover from the other day’s benching. So I mounted my pot on the card and did a quick test to see if I could do some nice speeds. I got 1130mhz on the core at 1.425v. The remaining amount wasn’t enough to carry on further.

 Increasing Voltages & Extreme OC HD 4890 Using RivaTuner

 Increasing Voltages & Extreme OC HD 4890 Using RivaTuner

That’s all I managed to squeeze from my card with that limited amount of cooling I had.
I kept the temps around -40 degrees only.

I’m very sure many extreme enthusiasts are able to achieve much better speeds.
I’ve already seem 1200mhz on dryice. What’s next?

Well, that’s about all. Have fun with your Radeon HD4890!