I recently purchased an OWC Merucry Extreme 1.467-1.5GHZ 7455b upgrade for my Quicksilver mutant mac. (Its a digital Audio Case, with QS 2001 Power Supply, QS 2001 Logic Board, and until a week ago, a 2002 1ghz DP board.) It's been a great upgrade, very fast, and far cooler running then the dual 1ghz. In my endless quest for faster, I've been experimenting with higher clock rates. I'm successfully running the board at 1.6ghz with no problems, and little change in temp. I've tried both 1.66 and 1.73, but in either case I've had no luck getting the system progress much beyond POST if at all. The board I have is the one with the low profile copper cooler and the two small fans.
I've been trying to investigate increasing the core voltage, but I have some questions regarding this. There are two sets of DIP switches on the OWC board. One used for adjusting the clock multiplier, and the other isn't documented. From the research I have done, I am 99% sure this board and the giga 7455b share the setting for core voltage. In comparing with the Gigadesigns 7455b upgrades, it appears to be based on a similar reference design. Accelerate your mac has a review of a Giga 55b upgrade, the 1.25, in which they detail the Core voltage jumper settings.
I made a very broad assumption about the orientation of the OWC dip switch, and the giga jumpers, but near as I can tell I am correct in assuming the layouts are the same. J7 is nearest the heatsink, J10 the edge of the board. This would seem to indicate the shipped configuration was for 1.65v or J7:OFF J8:ON J9:ON J10:ON. I tired setting for 1.70v and left the multiplier alone. With my hand on the power cord ready to yank it at the first sign of trouble, I fired up the system and was greeted with a startup tone and a display. Cool. So, next I tried for 1.70v and 1.73 ghz. The system started began to load and then quickly threw a Kernel Panic. So I tried 1.75v, and 1.73ghz, and the system went a little further, and then promptly failed in the same manner. At this point I wasn't sure I wanted to go much further, and set it back to 1.65, and 1.6ghz. My question, for any who many have played with these settings, is, how high can you safely go with the core voltage before you risk cooking something? Xbench indicates 7455b stepping 3.3. I am unsure as to the exact clock rating for the chip, but I imagine its a 1.33 or maybe a 1.4. I couldn't find any indication on Freescale's site that they are shipping anything beyond a 1.25 in a 7455b, so I could be really wrong, but a ~400mhz overclock on a part like this seems unlikely. Any experiences others have had tweaking the OWC 7455b cards would be a great help!
Otherwise, I have been really thrilled with this card. I totally recommend it for anyone looking to eek a little more out of their machine. It's a bonus to be able to run at 1.6 and have the L3 cache, which isn't very common with the 7455s. As for being able to get the card running stable at 1.6ghz with stock cooling and no real effort, I am totally pumped. I ran xbench 1.2 (i need to get 1.3) and it reports the cpu at 61.37, vs 41.48 with the dual 1ghz. I am now limited by the Geforce 4 TI in halo, which is cool, and otherwise, its made the system much more snappy. It seems that the most recent production of the 7455 are more tolerant of clocking, I may get curious and remove the heat sink to see what the chip's rated clock is, as I can't help but wonder just how hard I am pushing it.
Thanks for any help or advice!