Maximum memory speed using a non-overclocked Intel Sandy Bridge (socket 1155) Core i5

Question

I've read a couple of similar questions on this subject here, but I'm still confused: I'm about to buy a new PC and I'm interested in a Core i5 (2500). Apparently the CPU supports DDR3 1066/1333 memory. I have seen several motherboards (socket 1155) using the P67 chipset which support memory speeds higher than that.

I'm not going to overclock the system. I assume that if I place DDR3 memory with a speed higher than 1333 on this setup, the memory will be used at 1333 regardless of the mobo support, due to the CPU limitations. Which means that I should opt for 1333 RAM, because anything faster than that is a waste of money.

Is this correct?

Answer 1

Yes if you are not overclocking go with the 1333. Since the memory controller is integrated on older core series chips as you up the multiplier/FSB/BCLK you will also be overclocking the RAM. Most AMD over-clockers are very familiar with this. Buying RAM with higher clock rate gives you head room when increasing the Multiplier so you won't have to them lower the speed on the RAM as you overclock to keep your system stable. With the new Generation of Sandy Bridge processors this is no longer the case, the memory and CPU clocks are controlled independently. Just get the 1333 if you are not overclocking because it will certainly lower the clock rate on anything faster to 1333 anyway.

EDIT: On the K series chip the memory clock and cpu clocks can be controlled independently and the memory speed will not increase with the FSB. From Clunk:

Things have changed somewhat, regarding memory on Sandy Bridge. On previous platforms, when you increased the BCLK/FSB, you would automatically increase the memory frequency, so the chances were, that you'd either be under or over the rated speed for your modules.

Because the BCLK is "fixed" to 100MHz, the memory frequency that you choose in the BIOS, will stay the same, regardless of the overclock that you end up with (Unless you have a locked CPU, more on this later), and this is because you have adjusted the CPU's multiplier, instead of the BCLK, this should all become less confusing as we progress through the guide, but in short, Sandy Bridge offers the following memory frequencies which are selectable, regardless of the CPU speed, but be aware that selecting the top two whilst running the CPU very fast, will require some extra tweaking, and we will look at that in a separate article or add on to this guide

Again the above only applies to K series chips for non K series chips Toms hardware has some great info:

If you don’t buy a K-series chip and instead grab a Core i7-2600, Core i5-2500, -2400, or -2300 (along with a P67-based motherboard), you’ll still have access to “limited unlocking.” This basically means you can set clock rates up to four speed bins above the highest Turbo Boost frequency setting available at any given level of processor activity.

So, take a Core i7-2600 as an example. The chip’s base clock is 3.3 GHz. With four cores active, it gets one bin worth of additional performance—3.4 GHz. Four bins above that would be 3.8 GHz. With two cores active, Turbo Boost bumps it up two bins, to 3.5 GHz. Limited overclocking makes 3.9 GHz available in that case. In a best-case scenario, only one core is active. Turbo Boost adds four bins of frequency, yielding 3.7 GHz, and Intel’s overclocking scheme lets you run at up to 4.1 GHz.

Answer 2

Overclocking on these chips is a little different than on the older ones; on last gen chips you had to increase the FSB/BLCK/clock generator/whateveryouwanttocallit to overclock the processor, and this increases the frequency of the RAM accordingly. On Sandy Bridge chips, you can't really get more than a couple MHz increase in the BCLK and have the system be stable, so that's out the window - to overclock the processor you're altering the processor multipliers ONLY, which doesn't touch the RAM at all. (EDIT: I should point out that this only works on unlocked chips, SKUS with the 'K' identifier.)

By the same token, you can clock up the RAM without touching the processor. (EDIT: This doesn't matter if it's a K SKU or not, you can adjust the memory ratios.) So, if you want to buy DDR3-1600 and you're okay with turning up the memory frequency a bit, you can use that effectively without touching the clockspeed of the CPU whatsoever. That said, if you DON'T want to do this, and I get the impression you don't but I want you to understand the options here, then yes - you should use DDR3-1333, because anything faster would be a waste.

EDIT: I would just add that, while I don't have hard numbers, I suspect you'd see little to no difference between 1333 and a higher memory clock using stock clockspeeds on the processor. It's really only at higher clocks that you need the extra memory speed, so even if you WANT to do this it's probably a waste of a bit of money.