While both ATI and nVidia use a form of 'thermal throttling' in brand of GPU to
stop them from being damaged by excessive heat due to overclocking (or other
circumstances), nVidia's implementation of this feature tends to be a bit more
sensitive than that found on ATI cards.
It's extremely rare to see artifacts or visual imperfections on a current or
previous generation nVidia card. This is because the video core will
manually slow itself down before a state stressful enough to cause video
artifacts is reached. As readers might imagine, this feature makes
determining the maximum overclock attainable by an nVidia card more frustrating
than it otherwise would be.
When using the Coolbits hack to overclock an nVidia videocard, there is no
indication that this throttling has been activated, so the only way to tell if
you've pushed the card beyond its temperature limits is by noticing a decrease
in benchmarking scores. If at x MHz the benchmark score increases as
normal, but at x+5MHz it suddenly drops several hundred points, the thermal
throttling has just kicked in. The frustration really sets in when trying
to determine the precise point that the feature activates. Since the
throttling is heat-triggered, it won't necessarily be the same value every
Best Practices for Overclocking an nVidia
To fully exploit an nVidia card's overclocking potential, you should follow
1. Using the set of instructions above, enable overclocking on the nVidia
2. Increase the videocard's core speed only (do not raise the memory speed
yet) by 3-5MHz increments, benchmarking at each increment and recording the
score. Look for 'snow' (small white particles appearing intermittently) during
each benchmark run as a sign the core's maximum speed has been reached. When you
see snow, the benchmark refuses to complete, or when the benchmark score
mysteriously decreases, the core has been pushed too far.
3. Power down the system for a few seconds to allow the card to cool, then
reboot and return the card to its previous fastest 'safe' settings. Increase
core speed from here in 1MHz increments until snow appears, the benchmark
refuses to complete or the benchmark score decreases. The fastest stable score
here is the nVidia cards maximum core speed. Note that due to nVidia's sensitive
thermal throttling, the maximum core speed reachable with 5xxx or 6xxx series
cards may vary with external conditions. With nVidia cores, it's usually best to
'ballpark' the figure slightly, backing off a few MHz to ensure that throttling
does not occur. Don't forget to make a note of the maximum core speed and return
the card to its default settings.
4. Following the same procedure as steps 2 and 3 above, increase the card's
memory speed in 5-10MHz intervals, leaving the core speed at stock. This time,
look out for 'artifacting' while running the benchmark. 3D artifacts will often
manifest as jagged lines or flashing objects appearing briefly at certain points
of the benchmark. Though thermal throttling should not be an issue when raising
the memory speed, keep an eye out for decreasing benchmark scores anyhow. Once
problems manifest themselves, return to the highest 'safe' memory speed and
increase it in 1MHz increments until the memory's maximum speed is determined.
5. Now for the real test. Raise both the core and memory speed to the maximum
values determined in steps 3 and 4, then run the benchmark. If snow, artifacts
or problems running the benchmark occur, back both the core and memory up 1-2MHz
then try again until the problems clear up. Once the card is running the
benchmark stably, record the settings, reboot and start trying out some
favourite games. If these appear to run ok, your nVidia video card's maximum
overclock has been determined!
Due to nVidia's sensitive thermal throttling mechanism, it's also a good idea
to reboot, back the core speed up about 10-20MHz from your maximum value then
benchmark the card again and compare the scores. This will make sure that the
thermal throttling has not triggered.