Overclocking: What limits the FSB?

Kierrox · 5 Aug 2007 at 15:07

Just a quick question. When overclocking what component is it that limits how far you can push the FSB? Is it the motherboard or the CPU? Or is it a mixture of the two?

The reason I ask is because my current motherboard supports 1333mhz and im currently using a 1066mhz CPU. I was wondering if buying a 1333mhz CPU would allow me to push the FSB further?

MumboJumbo · 5 Aug 2007 at 17:18

The motherboard can limit it if it cant function stably at higher FBS. I dont beleive the CPU directly limites it. But the heat it produces does. If you can cool it sufficiently then the sky is the limit i beleive. Well they used liquid nitrogen to clock one past 11GHZ so thats my reason to believe its mainly heat which limits it. Ram can also limit it.

HangTime · 5 Aug 2007 at 17:58

Kierrox said:
The reason I ask is because my current motherboard supports 1333mhz and im currently using a 1066mhz CPU. I was wondering if buying a 1333mhz CPU would allow me to push the FSB further?

No, if anything a 1333mhz FSB cpu will (potentially) restrict your overclock because it has a lower multiplier, meaning that you will need a higher FSB for a given speed. At which point you could hit the limit of your motherboard.

FreeStream · 5 Aug 2007 at 17:59

8igdave said:
The motherboard can limit it if it cant function stably at higher FBS. I dont beleive the CPU directly limites it. But the heat it produces does. If you can cool it sufficiently then the sky is the limit i beleive. Well they used liquid nitrogen to clock one past 11GHZ so thats my reason to believe its mainly heat which limits it. Ram can also limit it.

11ghz? Fastest I've seen is a P4 @ 7....

And the limit is the limit of the silicone, 500/550 for 965 and 600 for the new boards.

paradigm · 5 Aug 2007 at 20:04

8igdave said:
The motherboard can limit it if it cant function stably at higher FBS. I dont beleive the CPU directly limites it. But the heat it produces does. If you can cool it sufficiently then the sky is the limit i beleive. Well they used liquid nitrogen to clock one past 11GHZ so thats my reason to believe its mainly heat which limits it. Ram can also limit it.

The world record is a shade over 8GHz (might be closer to 9 now), so that 11GHz figure is just laughable.

Secondly, my board can quite comfortably do 500+ FSB (as proven with a mates E6600 stuck in my machine), however with my e2140 in, it won't go higher than 422MHz, no matter what you try.

The machine is fully stable at the 3.376GHz that puts me at, but even lowering the multi to 6 (from default of 8) still won't enable me to run 423MHz FSB, even though the chip is now down at 2.5GHz.

This alone indicates the existance of CPU FSB walls.

Greebo · 6 Aug 2007 at 11:01

I concur. There is definitely something relating to the cpu and the fsb achievable.

Not sure whether different on the intel platforms but I noticed it on my AMD setup. With my 3200+ single core processor my fsb max'd out at 274 no matter what multiplier I tried.

With my dual core 4200+ I can max out at 297.

weescott · 6 Aug 2007 at 15:12

Greebo said:
With my 3200+ single core processor my fsb max'd out at 274 no matter what multiplier I tried.

Ye old memory controller limit.

Other limiting factors include:

voltage available / and or used
RAM slots used
BIOS used
BIOS settings
revision of components
cooling
luck

Kierrox · 7 Aug 2007 at 11:12

paradigm said:
Secondly, my board can quite comfortably do 500+ FSB (as proven with a mates E6600 stuck in my machine), however with my e2140 in, it won't go higher than 422MHz, no matter what you try.

Hum, I tried pushing my E6600 to 400mhz before with a 6x mulitplier (to make sure it wouldn't get too hot) and it was very unstable. It kept locking up and giving visual distortions.

The reason I specficially want to push my FSB and not my multiplier is because I want my FSB:RAMBus to be 1:1 sync.

Would getting a g0 CPU allow me to achive 400mhz? Or is my motherboard (P5N32-E SLI Plus) going to limit me?
Just to point out, my motherboard is one of those that were orignally 1066 but were increased to 1333 via bios updates. Is this why I cannot push it so far?

simonnance · 7 Aug 2007 at 11:40

HangTime said:
No, if anything a 1333mhz FSB cpu will (potentially) restrict your overclock because it has a lower multiplier, meaning that you will need a higher FSB for a given speed. At which point you could hit the limit of your motherboard.

Likewise, a 1333Mhz CPU might it itself have a higher FSB wall...... i know my E6400 has a wall at 485/490, even on 6x, which means the max i could ever get off it is less than 4Ghz if it could run at 490x8 (not got the cooling to try it). Maybe Phase could pull off some more (looks like v cold temps can let you past some FSB walls), but i'm not about to go down that path (though kudos to those that do).

My P5k-dlx should be able to hit higher than that without too much fuss, so the limit is the CPU, not the Mobo as far as i can see.

As mobos get higher and higher FSBs to cater to the OCing crowd (i've seen 666Mmz FSB at XS forums recently) the limit is more likely to be the CPU, rather than the Mobo.

paradigm · 7 Aug 2007 at 12:08

simonnance said:
Maybe Phase could pull off some more (looks like v cold temps can let you past some FSB walls), but i'm not about to go down that path (though kudos to those that do).

It isn't a certainty that's for sure. Sticking my e2140 under phase did nothing to alleviate my FSB limit