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I'm looking to upgrade the RAM in my 2.4GHz i7 (2760QM) MacBook Pro, and I'm wondering which RAM is faster - 1333 MHz CL9 or 1600 MHz CL10?

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2 Answers 2

up vote 4 down vote accepted

The 1600Mhz CL10 RAM has a higher clock speed (800MHz versus 666MHz) and a lower latency (12.5 ns versus 13.5 ns), so it's the hands down winner.

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Thanks, I'll get the 1600 CL10 RAM! –  pacoverflow Nov 24 '12 at 1:14
    
Just a clarification - is the latency the sole factor or did the clock speed also factor into your answer? I found this page link where the guy suggests that the latency (what he calls "memory access time") is the sole factor to determine what is faster. What if I had asked about a hypothetical 1333 MHz CL8 that has a latency of 12.00 ns - would you say that is faster than 1600 MHz CL10 even though the clock speed is lower? –  pacoverflow Nov 24 '12 at 10:49
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The clock speed is the primary factor. The latency only tell you how long until you get the first bit, but generally what you care about in the real world is how long until you get the last bit. (Since you need all of them.) That post is totally incorrect and shows that the poster doesn't understand what CAS is. (Or thinks all modern memory has a "burst" of one bit.) –  David Schwartz Nov 24 '12 at 15:26

A quick shortcut to determine which RAM is faster is to check this ratio:

1600/10 = 160
1333/9  = 148

So, even though CL10 is a higher divisor than CL9, the increase from 1333MHz to 1600MHz in the numerator is more than enough to compensate for that.

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For synchronous RAM the CL value is given in clock cycles which for faster RAM are shorter. The 1333MHZ ram has a clock cycle of 1.5ns and the 1600MHz 1.25ns. –  Brian Nov 23 '12 at 23:37
    
That won't tell you which RAM is faster, it only tells you which RAM has lower latency. –  David Schwartz Nov 23 '12 at 23:47

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