Clock-chipping the iMac — introduction

400 MHz is (are?) better than 233, yes?

These pages will cover how to modify your iMac to run at speeds faster than stock. I have achieved a stable 300 MHz with my Revision A, but with some determination, fortitude, a good electronics supply store and a cash reserve, you may be able to go even higher. Other machines, however, may not be able to accelerate successfully at all.

BREAKING NEWS: Lars Hansen in Norway has managed to modify his Revision A iMac to a speed of 400 MHz, using an overclocked processor card out of a Revision D (333 MHz) model! Lars has used no extra cooling measures, and also has a MicroConversions GameWizard Voodoo2 graphics card installed. He says it is stable (and of course very fast). He claims 35 fps in Unreal. Wow! Now if only we could convince Apple to start selling that part on the open market!

The basics of clock-chipping

Clock-chipping, also known as overclocking, is basically just stepping up the pace at which the microprocessor inside your computer processes information. All microprocessors are tested at certain speeds, so they're guaranteed to operate at a minimum speed -- but some chips, if not most, can operate at speeds higher than they were tested at. In some cases, they can go a lot higher. This depends on variety of factors, and the only way to really know for sure what your chip can do is to try it. A typical limit might be 10-25% faster; 50% faster is not unheard of, however.

A more technical explanation is that clock-chipping involves changing either the timing oscillator, on older models, or reconfiguring the processor PLL to work at a higher ratio on newer models. On "regular" desktop & minitower G3's (both beige and blue-and-white), this is as simple as moving some jumpers around; on the iMac, unfortunately, there is soldering involved.

G3's aren't the only Macs that can be clock-chipped; many Macs, old and new, can achieve significant speed increases with this relatively inexpensive technique. Keep in mind that it's all relative -- a IIfx running at 50 MHz rather than 40 is not going to eat G3's for breakfast; it's going to be approximately 25% faster than it was before. For more information on clock-chipping your Mac, see Marc Schrier's excellent clock-chipping home page.

Peek ahead

Running at 300 MHz, my iMac's benchmarks are very nice numbers. CPU and FPU scores are up about 29%, video was up about 43%, and my disk score with the standard 128k disk cache was up almost 6%. With the "Default Setting" in OS 8.5's Memory control panel, 5120k in my case, my disk score was up 231%. Yes, 231%. Keep in mind that's just a benchmark, though, and large disk caches tend to impact benchmark tests more significantly than real-life tasks. Peek ahead here for some screen shots from Norton's System Info.

I have noted two quirks since the modification: 1) my PRAM no longer seems to function properly, in that my date of manufacture is not constant (advances every once in a while, and not related to PRAM zapping) and my hours of use remain at zero, and 2) the Energy Saver function that allows you to start up and shut down your Mac at preset times doesn't work. The monitor sleep function of Energy Saver still works OK. #2 may be related to #1; not sure if that is something that would be stored in PRAM.

TechTool Pro 2.1.1 reports that the PRAM itself tests fine, and other PRAM functions like mouse speed, General Controls settings, disk cache size & startup disk are remembered.

Disclaimer

Modifications of this sort will almost certainly void Apple's warranty as well as any extended service plan you may have bought from Apple or a reseller. This means that even if something totally unrelated breaks for reasons that have nothing to do with clock-chipping, you probably will have to pay for the repair yourself. And if you mess up the modification, you won't be able to get it repaired or replaced for free. You may not even be able to obtain a replacement processor card, in which case your iMac might make a nice mantle piece.

These modifications were made to my early "Revision A" iMac. I have heard from several owners of Rev. B iMacs who have successfully modified their machines, and the method is identical to that documented in these pages. I cannot say with any certainty, however, that these same modification techniques will work with your iMac -- they should, but you have no personal guarantee from me! I have no information on modifying either the Rev. C 266 MHz or Rev. D 333 MHz models. If you have any, please drop me a line.

These modifications are also rather difficult, and the end result could be that you produce a 39-lb. Bondi Blue paperweight. Neither I nor anyone else whose name might be mentioned in these pages assume any responsibility whatsoever for the future of your iMac, nor do I/we make any claims regarding the feasibility of this modification being made by you -- it's difficult, and you might mess it up, in which case you have only yourself to blame. If you don't want to risk your cute little machine's future, leave it as-is.