Results

I have had my iMac A clock-chipped to 300 MHz since the beginning of November, 1998. The first week it ran unreliably for 45 minutes to two hours at a time while I experimented with different cooling solutions. On 11/8/98 I installed my modified K6 heatsink & Pentium fan, and it has been running very reliably since. I have had it running all day, every day for the last several weeks, including several 6- or 7-day stretches without restarts, without problems. I really have only had a handful of crashes since clock-chipping: once in a great while Quicken 98 hangs the system when launching, a couple (and I do mean only two or three) when using Sherlock, and a few instances where the only way I could establish a reliable TCP/IP connection was by restarting. (My iMac has had that problem since I got it, first with FreePPP and now, to a lesser extent, with ARA 3.1.2. Limiting the modem with Apple's 33.6 connection script has helped tremendously).

I have upgraded to 8.5.1, now 8.6, and have applied the modem updates and all ROM/firmware updates up to and including the Firmware Update 1.2, all without incident. Apple's System Profiler still identifies my iMac as an iMac, albeit at 300 MHz.

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.
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 (mouse speed, General Controls settings, disk cache size & startup disk) are remembered.

Performance is fast. Very fast. I have to admit that in day-to-day use of the iMac, I don't really see much of a difference from before -- I don't use it for page-layout, lots of Photoshop work or complex number-crunching (that's what my 7300/200 at work is for, and WOW can I tell the difference there -- slow!).

The benchmark results are impressive: processor and FPU scores are about 29% better, and even the disk score improved slightly when using the System Info benchmark-standard 128k disk cache. I'm running 8.5 on my iMac, and when I set my disk cache to the "recommended" size (5120k in my case, with 160MB of RAM), my disk score improves tremendously. My RC5-64 "Bovine" client now cracks up to about 977 kkeys/sec, from 755 kkeys/sec maximum before (29.4% faster). See below for some benchmark results using System Info from Norton Utilities 4.0. The hard drive was optimized before each run; all test parameters were standard.

Please note that all G3's below are beige G3's. I hope to change that soon.

You can see from the above benchmarks that an iMac "A" at 300 MHz is not quite as fast as a beige G3/300, due to the latter's 1 MB of Level 2 cache (the iMac has 512k) and its Rage Pro graphics chip (the iMac "A" has a Rage IIc chip, which doesn't perform quite as well). Still, the gains over an unmodified iMac are significant.

The beige G3/300 and my iMac were both running OS 8.5; thus the wide variation in the disk scores for each (OS 8.5 significantly improves disk cache performace when it is set to the "recommended" size). The wide variation in disk scores between the unmodified A model and unmodified B model above is due to OS 8.1 on the A and 8.5 on the B.

Click here to download a stuffed & binhexed file containing my iMac System Info files (24k), as well as those I made for a beige G3/300, and those made by Yoshi of an iMac B.

If anyone has the time to make a file for the Rev C model, please e-mail me a copy!

I have just received confirmation from a reader that clock-chipping the Rev. B models works the same way as the A models. Yoshi's iMac B is running at 266 MHz now, with no extra cooling employed. Thanks, Yoshi! I have no confirmed information on the Rev. C (266 MHz) or Rev. D (333 MHz) models, except to mention once aagain that Lars in Norway has taken the processor card from a Rev. D, overclocked it to 400 MHz, and stuck it into a Rev. A, with no apparent trouble.

If you have clock-chipped your iMac, Rev. A, B, C or D, please e-mail me at danb@thelittlemacshop.com and let me know how fast you got it to go, whether you needed extra cooling, and what you did to get that extra cooling. I'd like to get a rough tally of the number of people who have managed to do this, what speeds they have achieved, and what they did about the heat problem. I'll post whatever information I learn.

If you have an iMac, or any other PowerPC-based Macintosh, be sure to install Motorola's LibMotoSh library (unless you're running System 7.5.5). It can speed up some floating-point operations tremendously. Here's the link for the download page, and here's a link to a FAQ page. The benchmarks above were all done with this library installed, but, for some reason, Norton's System Info doesn't seem to register a difference.

Other peoples' results:
(only those who have contacted me directly)

• 2 people have modified their Rev. A's to run at 266 MHz; neither was stable at 300 MHz.
• 2 people have modified their Rev. A's to run at 300 MHz, and are experiencing no problems.
• 2 people have modified their Rev A's to run at 266 MHz (did not attempt faster).
  One of the above is also using a MicroConversions GameWizard card, without problems.
• 1 person attempted to modify his Rev. to 300 MHz, where it would not start up. He is now running at 266 MHz, with a GameWizard, without problems.
• 1 person has modified their Rev. B to run at 266 MHz (did not attempt faster).
• 1 person has modified his Rev. B to run first at 266 MHz, then at 300 MHz.
• 1 person has modified his Rev. B to run at 300 MHz, without problems.
• 1 person has modified his Rev. A to run at 400 MHz using a processor card froma Rev. D, and also has a GameWizard card installed. Wow! 35 fps in Unreal.

Link summary:

• My e-mail address: danb@thelittlemacshop.com
• Takashi Imai's Mac modification pages
• theiMac.com's guide to drive removal
• PowerLogix' software download (G3 Cache Control & SpeedMeter), & a beta of PowerLogix' AutoCache. (Note that AutoCache is still in beta testing)
• An alternative to PowerLogix' software, the G3Strip
• iMac2Day's Visual Guide to RAM upgrades
• Marc Schrier's Clock-Chipping Home Page
• The Heatsink Guide, and the specific page with links to manufacturers.
• The CPU Overclocking Guide

Some other iMac links of interest:

Adding a floppy drive the old-fashioned way (no USB here!)
WARNING! With the latest iMac Update (1.1), it appears that Apple may have removed software support for the floppy-drive hack. I don't have any info on whether this affects the ADB hack. Anybody know how to fix it?

Adding an ADB port (again, no USB ... just solder)

Apple's guide to installing memory (illustrates how to remove the processor & drive chassis)

Click here to download my modified 33.6 connection script (7k), which will wait several seconds (up to 8) for a dial tone -- handy if you have a voicemail system on your line. If you need it to wait longer, open up the script with a text edtior such as BBEdit Lite and change the value for the S6 register (replace the number after "S6=" with the number of seconds you want it to wait). Once downloaded and decoded, place the file in your "Modem Scripts" folder inside the Extensions folder (inside your System Folder), and then use the Modem control panel to select the script.

Interested in the ultimate iMac gaming machine? Well, in addition to overclocking it, you can add a Voodoo2 accelerated graphics card (the GameWizard) from MicroConversions for $199.

And don't forget about Formac's UltraSCSI card for Rev. A and B iMacs. You can get it from MacTell or BottomLine.

General Mac pages I love to visit:

Macintouch | iMacintouch | Mac OS Rumors | The Macintosh News Network | MacCentral

Apple Insider | MacSpeedZone | iMac2Day | MacsOnly | MacTimes | O'Grady's PowerPage

And a neat idea you might want to check out:

distributed.net's RC5 project