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Harro2

RAM? OC i5 750

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If I was to buy an i5 750 on a P55 motherboard what RAM would best suit an over clock to say 3.6GHz.

Im thinking 1333MHz RAM like Patriot Viper II 'Sector 5' series DDR3 4GB (2x2GB), PC3-10666 (1333MHz), 1.65v, 7-7-7-20 DIMM kit would be a good selection?

 

Also I just noticed this RAM is rated at 1.65v, would it be a good gamble to get some cheeper RAM rated at 1.5v and try and OC the ram with a voltage increase to 1.65v to try and get a lower CL say to 777-20?

 

Im a little confused, how does PC3-10666,PC3-12800 or PC3-20000 effect a CPU overclock?

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RAM speeds and ratings can be a little confusing.

 

To explain, lets start at the CPU. It's clock speed is determined by a multiplier and a bus speed (be it FSB or BCLK etc). For the i5 750 that's a multiplier of 20 and a bclk of 133 (20*133=2666MHz). That 133 is what the RAM works from.

 

Now, Intel "quad pumps" the RAM, so with a bclk of 133MHz the RAM is running at 533MHz (assuming 1:1 memory ratios).

 

Now seeing as the RAM you'll be using runs at a double data rate (DDR) it will send/receive 2 bits per clock cycle so it "effectively" runs at double the RAM speed, which in this case will be 1066MHz. This is the value most RAM will advertise.

 

As for the PC3-10666 type ratings it's just marketing to make it look impressive, but this is basically the bandwidth rating of the RAM in megabits per second (I think). The PC3 part just tells you it's DDR3 (PC2 for DDR2, PC for DDR1). The number is calculated by multiplying the RAMs effective speed by 8 (bits in a byte). So stock RAM for your CPU requires 1066MHz rated RAM, so 1066.6*8 gives you PC3-8533. So PC3-10666 RAM is 10666/8 = 1333MHz effective.

 

 

How do those ratings apply to your CPU overclock? Basically the higher the rating the higher speed you can run your RAM at and thus the BCLK. This is good for achieving high overclocks on CPUs with low or locked multipliers.

 

If you're aiming for 3.6GHz (you could go for 3.8GHz) which should be easy as the i5 750 already can turbo itself up to 3.2GHz (using a 24x multi). So with that knowledge assuming you disable turbo feature in BIOS, with a 20x multi 3.6GHz needs a 180MHz BCLK. 180*4=720MHz RAM speed (1:1) times that by 2 to give an effective rate of 1440MHz. Times 8 gives you PC3-11520.

 

So using the base multiplier 1333MHz RAM is not enough. However if your motherboard lets you lock in the turbo multiplier of 24x then you only need RAM rated for 3600/24*4*2=1200MHz. In which case 1333MHz RAM will be ok.

 

I think I've explained enough for you to work out any other variations you could do, including going up to 3.8GHz which should be easily possible.

 

And remember not to put more than 1.65 volts into the memory, Intel states that your could cause long term damage to the CPU if using more than that.

Edited by mark84

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So if I were to run an i5-750 with a bclk of 200MHz on a 20x Multi. This would mean I would need 1600MHz RAM. I think I understand that, but would i need to overclock my RAM as well to be sure that im over the 1600MHz requirement rather than dead on?

 

Sorry if it doesn't make sense, I'm new to overclocking and am looking at purchasing some parts and would like to get an i5-750 to about 4GHz.

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but would i need to overclock my RAM as well to be sure that im over the 1600MHz requirement rather than dead on?

Not exactly sure what you're asking there.

 

No you won't have to go over that limit to hit 4GHz on the i5 750.

 

RAM rated at 1600MHz has been tested and "binned" as such by the manufacturer so they are guaranteed to work at that speed. Typically you'll get maybe and extra 1-10MHz (at the BCLK) more out of them at the max rated voltage (1.65v).

 

If you're aiming for 4GHz 1600MHz RAM is all you need. Up around 200MHz BCLK you start pushing the limits of what the memory controller/mobo can do anyway, so unless you're doing more advanced OCing you'll be fine.

 

Besides if you have trouble getting the BCLK up that high (if your mobo supports it) you could set a 21x multi and go for a 191MHz BCLK setting.

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Thank you mark84, your answer was exactly what I was looking for. I used to know this but I had forgotten, hav'nt OC for over 2yrs.

 

So PC3-12800(1600MHz) should be what Im looking for with low case latency.

 

PC3-1600(2000MHz) would be a waste of money unless you wanted to get greater than +4.2GHz OC with some extreme cooling :P

 

Cheers m8 thank you.

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Thanks heaps for your information Mark84. I just thought that if i were able to get the RAM to even a slightly higher frequency, it would be a more comfortable setting than the EXACT amount. Sorry to be confusing...

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