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I want to increase the RAM in my MacBookPro 15", 2.53 GHz, Mid-2009. I currently have 4GB, and I want 6GB (and upgrade to 8GB later - I think this should work with my model:

Now I see two offers:

  • 1x4GB DDR3 1333 CL9 9-9 (149 EUR)
  • 1x4GB DDR3 1066 CL7 7-7 (192 EUR).

Both are from the same brand (Transcend), both are labeled "For MacBook/MacBookPro".

Now I have no idea, which of these would work better with the 2GB RAM module from my currently installed memory (Manufacturer: 0x80AD, Running at 1067 MHz, according to System Profiler - but with what CL setting?)

It's clear that both together will only run at 1066 MHz. But what about the latency?

  • Would they both run with CL7? Maybe causing incompatibility?
  • Or both with CL9?
  • Or would one run with CL7, and the other with CL9? Maybe causing incompatibility?

  • How can I find out the latency setting of my current RAM? (There's no BIOS screen on Mac. And System Profiler doesn't show it.)

I think I've read a few years time ago, that it's possible to decrease the latency setting for RAM, when running it at a lower clock rate (MHz). Would that still be true in this case?



PS Should I expect problems when I later upgrade to 8GB with a different module (in case the same module won't be produced anymore)? Is there a big risk, that two different modules won't work together very well?

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Just wanted to add, that I eventually bought the "1333 CL9", and it runs fine at 1066 MHz. Don't know with which CL setting (probably CL7). Also not sure, if dual channel is on or off now (probably off). – Chris Lercher Apr 13 '10 at 17:04
I didn't get it. If the memory says CL9 isn't that slower than CL7 and thus can't run at CL7? – cregox Dec 5 '10 at 3:12
@Cawas: 9 clock cycles at 1333 MHz take approximately as long as 7 clock cycles at 1066 MHz. The only question is: Does the computer allow a CL7 setting, or is it too overcautious? – Chris Lercher Dec 5 '10 at 12:40
Oh I see... Now, maybe you want to know more about my similar issue:… I tried 2 banks with 1333 CL 9 on my similar macbook and, though it ran, it kept crashing. Seems like you're going to try same thing, so I wish you good luck! Maybe I just happened to get bad modules after all... Neway, thanks so much for the explanation! ;) – cregox Dec 5 '10 at 13:41
up vote 2 down vote accepted

According to Apple your machine has the 1066 MHz CL7.

HOWEVER Like the other answers say, higher RAM specs are often backwards compatible with slower mother boards ("logic board" in Apple's case). So you could go with the higher speed/lower cost option and do fine.

But I'd say that having a shop with a well known return policy might be better. Check the fine print.

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Thanks - how do you know, it's CL7 (I don't find that info on the page you mentioned)? – Chris Lercher Mar 18 '10 at 14:12
The important bit is that it's 1066MHz Quoted: Processor and memory - 2.53GHz or 2.66GHz Intel Core 2 Duo processor with 3MB on-chip shared L2 cache running 1:1 with processor speed; or 2.8GHz or 3.06GHz Intel Core 2 Duo processor with 6MB shared L2 cache - 1066MHz frontside bus - 4GB (two 2GB SO-DIMMs) of 1066MHz DDR3 memory; two SO-DIMM slots support up to 8GB Also: Both 1066 MHz and 1333 MHz support the 7-7-7 timing. – Kio Dane Mar 18 '10 at 14:45
Good link. With the info on that page, I calculated, that CAS latency is approximately the same for both (about 6.7 ns) - so at least if there's no ROM setting that will unnecessarily switch to CL9, I should be fine with the 1333 module. – Chris Lercher Mar 18 '10 at 15:25

I'm not 100% sure when it comes to Apple logic boards, BUT I would purchase the CL9 stick since USUALLY motherboards downgrade higher frequency RAM to the lowest DDR memory installed anyway.

I can't imagine the CL9 stick doing anything but simply downgrading to 1066MHz, and since it's cheaper and offers faster cycles (once you ditch the 2GB 1066MHz module), why not? Not to mention, when you get around to buying another 4GB module, you can buy the CL9 and get the 1366MHz performance (dual-channel as well if the logic board is smart).

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Thanks! I'm just a little bit worried, that when I install the 1333 CL9 4GB together with my 1066 CL7 2GB, that I might end up with CL9 for both, of course running at 1066 MHz (CL9 is slower than CL7)? – Chris Lercher Mar 18 '10 at 0:34

CAS latency doesn't affect clock speed. They are different. If the box can run the memory at 1333, and you add some 1333 and some 1066 memory, the box will run all the memory at 1066. (No, it will not run memory faster than it's specified for, because it's not safe to assume that it will work reliably.)

The 1333 RAM is more expensive because it's 1333, not because of its CAS latency.

If the RAM already in your MacBook is 1333, buy the 1333. If it's 1066, buy 1066 (you can buy faster, but you will just be wasting money, because it would be run at 1066 in that case).

How do you know what RAM your machine already has? System Profiler would tell you, or you can just pop it out and read the label on it.


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Actually, in the case of the offers I have, the 1333 is cheaper. So either that's because it has worse latency, or the price is just related to supply and demand. – Chris Lercher Mar 18 '10 at 12:33
Sorry, I misread the two prices. Yes, because of supply and demand -- have you priced PC133 memory lately? Wow. – Alexander Burke Mar 18 '10 at 18:36

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