found this from a link on MR awhile ago, don't remember where
Edit:
Here's the link.
The following are instructions to use OpenFirmware to overclock the
Front Side Bus of Apple's new Dual 867 Power Mac from 133 Mhz to 167
mhz. This overclock will also indirectly overclock your CPU's from 867
Mhz to 1083 Mhz.
NOTE: All <return>'s are places where you are to push the return key.
1. Boot into OpenFirmware ("Command-Option-O-F") and at the prompt enter the NRVRAM editor.
nvedit<return>
2. Line by line enter in the script just as it appears here including the spaces.
" /" select-dev<return>
09ef21aa " clock-frequency" get-my-property 2drop !<return>
" /cpus/PowerPC,G4@0" select-dev<return>
09ef21aa " bus-frequency" get-my-property 2drop !<return>
408e4b46 " clock-frequency" get-my-property 2drop !<return>
09ee81bc " config-bus-frequency" get-my-property 2drop !<return>
027ba06f " timebase-frequency" get-my-property 2drop !<return>
" /cpus/PowerPC,G4@1" select-dev<return>
09ef21aa " bus-frequency" get-my-property 2drop !<return>
408e4b46 " clock-frequency" get-my-property 2drop !<return>
09ee81bc " config-bus-frequency" get-my-property 2drop !<return>
027ba06f " timebase-frequency" get-my-property 2drop !<return>
3. Exit the NVRAM editor by pressing "Control-C".
4. Save the script to the NVRAM.
nvstore<return>
5. Tell the OpenFirmware to enable the use of NVRAM startup scripts.
setenv use-nvramrc? true<return>
6. Tell the OpenFirmware to save and restart your computer.
reset-all
That's it. Pretty easy, and the best part is that it doesn't require any soldering.
KNOWN ISSUES:
Some utilities don't accurately report the speeds of the FSB and the
CPU's. For instance Apple System Profiler reports that the CPU's are
running at 1.83 Ghz instead of the proper 1.083 Ghz. And I have not
found a program yet that can properly determine that the FSB is actually
running at 167 Mhz.
I'm going to try and figure out why this is, but my first assumption is
that it's because this script doesn't overwrite the values stored in ROM
it just overrides them with values stored in NVRAM. Since most programs
only probe directly to the device tree ROM information they won't see
the values stored in NVRAM. Programs which try to actively calculate
the CPU's speed and FSB speed will probably provide a more accurate
figure than programs which just probe for information in ROM. Also bear
in mind that for best results you'll probably have to replace the PC2100
DDR memory in your machine with PC2700 DDR memory as the former is
designed to operate only at the stock 266 Mhz, whereas the latter is
designed to work at the new speed of 333 Mhz.
WARNING:
I'm pretty sure that this will void your warranty. The upshot is that
if you ever want to undo the overclock and get back to factory settings
all you have to do is reset your PRAM by holding "Command-Option-P-R"
at startup. Which will then leave no trace that you've ever performed
this operation.
***USE THIS AT YOUR OWN RISK!!!*** It's possible to render your CPU's
and/or RAM unusable by overclocking them beyond their designed
specifications. It's also possible to render your machine unusable by
altering the OpenFirmware in irrecoverable ways. Though I have had
success and have not experienced failure induced by this OpenFirmware
script I am not going to be liable to others who use this.
