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Thread: To Overclock or not to Overclock, that is the question...

  1. #1
    Join Date
    Jan 2001


    Ok, I've had an XLR8 ZIF carrier in my Powermac 8600/200 for two months now with an IBM 500Mhz/1MB G3 (10x of course), running at the rated 500Mhz. I have the itch for a bit more speed, how about overclocking(not to mention its time to void some warranties)? How high should I push it? How unsafe is it overclocking an XLR8 carrier from experiences? (will the ZIF just go dead sometime, Carrier get fried?, etc.) And then to extra cooling, I've just got the XLR8 finned heatsink and Arctic silver on it now, and there does not seem to be much room for a Pentium heatsink/fan. Ideas?

  2. #2
    Join Date
    Jul 2001
    Las Vegas, NV, USA


    Well¨÷ Your 8600's maximum bus speed is rated at 50MHz; What you're currently using. It's unlikely you'll be able to push that up very much. Perhaps, if you're lucky, the motherboard may be able to handle up to 52.5MHz. That would give you 525MHz for the processor and perhaps, if your backside cache can't handle the extra speed, force it to 1:2.5 the processor speed (210MHz) as opposed to its current 1:2 (250MHz). In either case you'd likely get no more than a 5% increase in speed. Not really enough to be noticeable.

    It's your call, but I don't see much incentive to take the risk.

  3. #3
    Join Date
    Feb 2001
    Austin, TX


    In general, there is very little risk to *careful* overclocking. However, very little, is not zero. There is some finite risk that damage could occur to your motherboard and/or your CPU.

    My experience is that the ability to overclock depends as much on the CPU as on the motherboard. For example, my (7) S900s and my (2) PowerTower Pro boards will all do about 62 MHz bus speed with an old PowerBoost Pro 604e board. However, with my 220/110/512 G3 upgrade (ancient stuff, yes?) they won't manage above 49 MHz. This is very consistent.

    Now, what that says to me, is that it is the CPU cards limiting the maximum bus speed, not the motherboards. If it was the motherboard, I would expect to see some variation with the same CPU and different motherboards. But I get identical results provided I use the same CPU card on a wide variety of motherboards.

    I have a friend with an S900 and an XLR8 300/200/1 MB G3 upgrade. Her G3 will do 58 MHz in her machine. If I install my older G3 upgrade, bam, back down to 49 MHz goes the limit. Install my PowerBoost PRo and suddenly it's happy at 61.6 MHz.

    Now in your machine, your motherboard may actually limit the bus speed more, because you have an extra chip on the CPU bus (CHAOS chip) that none of the above machines have and the CHAOS may not be so tolerant of high bus speeds.

    Here's the general MHz testing procedure I like to use. You have three numbers to determine. The first is your maximum bus speed. The second is your maximum cache speed. The third is your maximum CPU speed.

    So, e.g., on my G3 220/110 upgrade, my maximum bus speed is 49 MHz. My maximum CPU speed was 308 MHz but I upped it to a little above 315 MHz with some overvoltaging (don't try that at home). My maximum cache speed is 149 MHz.

    Now, those three numbers are your individual maximums. However, the values of those three numbers that you can use in practice are related to each other by fixed multipliers. So, e.g., I can't run my card at 49 MHz bus and 308 MHz CPU because 308/49 = 6.28 and the available bus multipliers are 6 and 6.5. So I could run at 47.33 X 6.5 = 307.6 or 49 X 6 = 294. These are the types of compromises you must choose once you've found your max numbers.

    Further, you'll usually use a cache ratio of 1:2 or 2:5 so in my case I could run at a cache speed of 294/2 = 147 (close to max, but the CPU speed is low) or 308 X 2/5 = 123 MHz -- max CPU speed but limited cache speed. I can't use 1:2 ratio on the cache at maximum CPU speed because that would give me 308/2 = 154 MHz and my cache won't do better than 149 MHz.

    So in determining each maximum the thing to do is to set the other two values low and tweak the third one high until you find the maximum for the value you are currently testing. Then, once you have the maximums, you can choose the usable values that seem best to you.

    So, to determine your maximum bus speed, I would switch your CPU multiplier down to about 8X instead of 10X and then adjust the bus speed up by maybe 2 MHz at a time. So 52, then 54, then 56 etc. When you reach a point where it's unstable, go back to the last speed that worked, and increase it in smaller increments like .5 or .66 MHz at a time. Instability will manifest as a failure to bong at power on, or bonging, but staying black screen or starting to boot and then freezing or generating a bus error or similar message.

    You could get some disk corruption from this, so I recommend either backing up your data before hand, or use a volume that is just for testing which you will reload the system on afterward. This is not always necessary, but it's another one of those super safe precautions.

    You can test your max cache speed in a similar fashion. Probably the easiest way is to set your CPU to run at whatever speed your cache is currently running at. So, e.g. if your cache is doing 250 Mhz now, set your machine to 42 MHz bus, and 252 CPU and then set your cache ratio to 1:1. Then increment your speed until you start having problems again. That way you'll know your maximum cache speed.

    The reason for doing it in the above manner is that if you test your cache at 1:2 and up the speed to 520 MHz in order to see if yoru cache will do 260 MHz, you won't know if any problems are because the cache doesn't like 260 or because the CPU doesn't like 520 MHz or because the bus doesn't like 52 MHz.

    Finally, you can test your max CPU speed. Set your cache ratio low so that you won't bump into your maximum cache speed, so 2:5 or 1:3. In this case you'll be pushing your maximum bus speed because you don't have any multipliers above 10X to work with. Then up your bus speed either until you have problems with stability or until you reach your previously determined maximum bus speed. That will give you your maximum CPU speed.

    Once you have those three numbers you can pick the best combo for you. In general, you're best off backing down from the maximum bus speed a few megahertz to give yourself some margin. Your computer might have a hot (temperature) day or something and not be as tolerant of high speeds, so leave a little breathing space.

    On my friend's computer, I set her G3 to run a 55 MHz bus. She could do a 58 MHz bus and about 375 CPU, but a 55 MHz bus at a 6.5 ratio gave 357/55 which left a nice margin.

    On my card, I cheated a little, did some surgery, found that I could overvoltage the CPU into doing 330 MHz, then I dropped the voltage down to where the top speed was about 320 MHz, and I run it at 315/45. But I wouldn't recommend this as I had to do a bunch of soldering for a very tiny little advantage.

    And as Lasvegas wrote, the advantage you get out of the whole involved process probably won't amount to a perceptible difference in performance when you are using your machine. So, it's a form of entertainment that can eat up an entire evening (you spend a lot of time rebooting) but it doesn't really gain you that much. But it's fun if you enjoy that type of thing.

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