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FSB:DRAM ratio help

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December 21, 2008 7:55:27 PM

I've just bought an E8500, Asus P5Q-E, and G.Skill DDR2-1066 (2x2gb).
I heard that a FSB/DRAM ratio of 1:1 is the best but I want to know how I would achieve this.
I'm looking to overclock my CPU to 4.0ghz with the help of my Xigmatek S-1283.
My ram would be running at 533mhz; would I have to set my FSB to 533 and multiplier
to 7.5 to achieve 1:1 ratio and maintain 4.0ghz overclock. Would this be stable. Thanks for the help.

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December 21, 2008 8:26:22 PM

You are correct FSB 533 is DDR2-1066 at a 1:1 ratio. FSB 533 (I can only do FSB 400) is hard to achieve on any chipset if i'm remembering correctly. On my asus board i need to set the memory ratio to "sync", the "1:1" option is actually "1:2" (for my asus board anyway). You'll also need to adjust the chipset voltages and probably the cpu voltages and lower you cpu multiplier to at least 8 (4.264GHZ cpu) or 7.5 if you're trying for FSB 533.

I would first try for something more manageable like FSB 400 (3.8GHz CPU) and running your ram at DDR2-800 with tighter timings (say 4-4-4-12 1T)

*Edit: Of course start with loose timings and a low multiplier and increase the FSB gradually if you're going for the max FSB overclock.
December 21, 2008 8:44:04 PM

Thanks a bunch pshrk. I think I'm not even going to bother trying to achieve an FSB 533 with E8500 and ASUS P5Q-E.
Is it really important for me to have a 1:1 ratio?
What I was hoping to do was set the FSB 432 x 9.5 to equal 4.105 Ghz OC but have my DDR2-1066 really be DDR2-864.
Is this a smart move or does anybody have any other insight.
Thanks for the help guys :) 
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December 21, 2008 8:53:26 PM

I think it's a good idea, but maybe someone else here with more overclocking experience will have more insights.

I currently run my DDR2-1000 memory at DDR2-750 and a 1:1 memory ratio.

You can always try different setups and benchmark them yourself, it's all part of the fun of overclocking. (just don't use superPI as a benchmark or at least not the only benchmark)
December 21, 2008 10:30:20 PM

Read up on an OC how-to guide first (like the one on this website) so you know what you're doing.

Once you've done that, then once you OC your CPU to 432 FSB, your DDR2 will be running at DDR2-864 if the memory divider remains at 1:1. This is well within the parameters of G.Skill DDR2-1066, so it shouldn't be a problem. You run memory at 1:1 "by default".

Once you've settled on the CPU OC, then your next step is to increase the memory divider to take advantage of the extra specified headroom your RAM provides (otherwise you would have bought lesser RAM). This is all fine and dandy, but usually the performance increase from OC'ing RAM is pretty small. Therefore, 1:1 is usually fine. Running a higher memory divider is squeezing out every last bit of performance from your system.

At FSB 432, if you use a memory divider of 1.2 (or 6/5 in fractional terms), then your RAM runs at DDR2-1037, which is still within the spec'd parameters of the RAM. Meaning, the RAM can run at that speed for sure.

Now at that same 432 FSB, if you increase your memory divider to 1.25 (or 5/4 in fractional terms), then your RAM runs at DDR2-1080, slightly above its rating. This is not guaranteed by the manufacturer, but it is the nature of overclocking your RAM. You should probably be able to do this if you tweak it properly--you'll have to get your voltages right too.

Here's a set of OC'd speed parameters that would probably work with your system (I'm just calculating backwards from maximum specified RAM speed):

RAM speed: 1066
Memory divider: 1.25
FSB: 426
CPU speed: 4.047 GHz

You would run the RAM at G.Skill's specified voltage, most likely 2.1V, and will need more CPU voltage to hit that speed.

This is the relationship between FSB, CPU speed and RAM speed. You'll have to juggle the various parameters of your system to get a stable overclock, but therein lies the fun. Since you have bought "factory OC'd" RAM, you've got plenty of headroom to run a higher memory divider, so why not? Hope this helps.

Btw, despite the fact that your BIOS will allow you to input an FSB value up to 800, you will not be able to run FSB 532 with your setup. ;) 
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