5 Ways for a Better 6800 GT Overclock

[HaLDoL]

I’ve been testing my GT a lot lately and was able to achieve a 473Mhz 3dmark2003 run on aircooling (http://service.futuremark.com/compare?2k3=3649129) which seemed pretty high compared to other GT overclocks ( http://www.xtremesystems.org/forums/...ad.php?t=45876 and http://www.xtremesystems.org/forums/...ad.php?t=49832 ). That’s why I wrote down my tweaks below. I hope it helps other GT users.
When I first got the card, it seemed to be an average card, it could do 415Mhz on the core and 1.1Ghz on the mem with stock cooling and voltage. And that was only benchmark stable. For games I had to drop to 400.
The card is a reference design GT with the mermaid on the cooler from Aopen. So, after weeks of tweaking and testing it could do 473Mhz stable enough for 3dmark 2003. As this overclock is achieved without any soldering or resistor pencilling, a lot of people should be able to reach comparable clocks. These tips could also be useful for other Geforce 6x00 cards.
So here we go:

1. Cooling
It’s obvious that if you want to overclock on air you’ll need better cooling, I’ve put a SLK900u AMD heatsink on it and a powerful fan. On previous generations of Geforce cards this was an easy thing to do, because the holes in these cards were large enough to fit 4 long screws. The placement of the holes around the GT core was different and too small for my screws but with some zipties I’ve managed to put the cooler firmly in place. The effect on the temperature was enormous of course. With 20°C ambient, the gpu never went above 40°C stressed. So the cooling was ok. Any cooler with lots of copper should be sufficient (NV5 Silencer for example).

2. Bios
With better cooling the next logical step is raising the gpu voltage from 1.3V to 1.4V, it can be done by flashing a new bios. You can extract your original bios with nvflash and mod it with Nibitor to 1.4V. Set 2D voltage to the same value as your 3D voltage for better stability. 1.5V didn’t help my overclock, but it’s worth trying. Get all the necessary tools, howto’s and biosses at http://www.mvktech.net/ (Mavke’s site).

3. Power
The biggest problem with GT’s is power, ultra’s don’t come with an extra molex for no reason. So how can we stabilize the power supply to the card, even with only one molex?
One option is to use 2 power supplies. The second PSU is only to provide power to the molex of the card. If you don’t want to fry your card, you’ll have to connect the green and the black wires from both PSU’s to make them boot up simultaneously. If you don’t have a clue what I’m talking about, don’t even try it.
A second way for stabilizing the incoming power is described here (thanks to Reject for the crazy idea, but it works): http://www.xtremesystems.org/forums/...ad.php?t=48577 . It gave me some extra 5-10Mhz.
Also, use clean powerlines, with no other peripherals attached. My Antec TC 550W has a molex at the back of the PSU, using this molex gave me the best results.
A serious overclocked A64 system (vcore > 1.75V) could influence your cards overclock (meaning less). I noticed this especially in 3dmark 2001.

4. AGP
Disable fastwrites
Set AGP voltage to 1.8V at least
Set AGP frequency to 81 (use clockgen).
Tested on a DFI S754 nf3 board and it helped me to get those extra Mhz and stability to complete a benchmark.

5. Rivatuner
Use Rivatuner for overclocking, it has 2 very useful options that can be enabled in the power user tab.
- Disable internal clocktest
- Disable any kind of throttling by forcing 3D performance clocks (http://www.vr-zone.com.sg/index.cgi?i=1582&s=3 thanks to Shamino)
Throttling problems are due to power or temperature. Most of the time it’s a power problem. Disabling the throttling mechanism will help finishing that precious benchmark without performance loss. That is, if you’re lucky, most of the time, it will just freeze and give you a corrupt screen. The Rivatuner author warns that disabling throttling can damage your card, so be careful.

That’s it, I hope it can help you reaching that 470+ overclock without any hardmods, it should be possible now if you follow my tips.

If you do want to hardmod your card, look here : http://www.xtremesystems.org/forums/...ad.php?t=48876
The vcore mod (thanks to shamino) : http://www.vr-zone.com.sg/?i=926&s=1
Vmem mod: http://www.vr-zone.com.sg/?i=1622&s=1

If you have comments or other good tips, let me know.