Is it possible to use DDR3-1333 RAM with the 27″ iMac Core i5 and Core i7?
According to the Core i5-750 and Core i7-860 specifications it should be possible to run Apple’s latest 27″ iMac with DDR3-1333 RAM instead of the stock DDR3-1066 RAM it gets delivered with.
It is not the first time Apple has used customized CPUs, so there’s always a chance that the default specs do not match Apple’s implementation. Brave folks went out on a limb and tried if the iMac is able to use more than the standard spec ram.
The answer is, yes, it is possible though you have to be careful. The DDR3-1066 RAM Apple is delivering has a CAS Latency (CL) of 7. CAS Latency is the time it takes from the moment the memory controller accesses the memory to the time the requested data in the memory gets delivered at the output pins. But if you want to use DDR3-1333 RAM you have to keep an eye on the CAS Latency of the RAM. The higher the memory can be clocked the bigger CAS Latency gets. Usually DDR3-1333 RAM has a CL of 9. DDR3-1333 with CL9 does NOT work with the iMac Core i5-750 and Core i7-860!
Testing shows that you can use DDR3-1333 RAM with CL7 (SODIMM, 204pin). This is highspeed RAM which will cost a bit more than the average DDR-1333 CL9 RAM. Kingston for example offers CL7 RAM with their HyperX DDR3 RAM.
Now let us put this into perspective. First you have to swap out all of the stock ram for the DDR3-1333 to get any perfomance gains, second DDR3-1333 CL7 RAM costs quite a bit more than the DDR3-1066 CL7 RAM. How much performance gain will you get for all this?
As you can see there is is little to gain from this investment, especially when you keep in mind that the iMac stock configuration is able to keep up with the current Mac Pro Quad-Core. So while it is possible your money is probably spent better on more RAM instead of faster RAM. Also keep in mind that DDR3-1333 RAM is out of the specs that Apple has outlined for their system.
Results are taken from Macrumors forums user i7QuadCoreMania. Benchmark values are averaged over 5 benchmark runs.
Just a clarification of CL ratings. 1333MHz CL7 is NOT the same timing as 1066MHz CL7.
There is more to this topic than I will mention here. I think you can get the idea of what I am saying though.
CL stands for Column Address Strobe Latency. SDRAM latency is measured in operational cycles not milliseconds. The memory bus expects the RAM to respond on time.
In the case of 1066MHz CL7 RAM this would mean the RAM would reply in (approx) 0.000000006566604 seconds. The 1333MHz CL9 RAM would take (approx) 0.000000006751688 seconds to respond. This is too much delay to operate correctly. 1333MHz CL8 should be fine with a delay of (approx) 0.0000000060015 seconds. 1333MHz CL7 would have a delay of (approx) 0.000000005251313 seconds. There is more to this topic that you can read about online.
Just as a data point, I just installed 8gb HyperX 1333 ram in my iMac i7 and got the following scores in GB: overall 9843 memory 5409
Nick, can you post what kind of Ram have you installed? (Brand, latencies, amount -modules and size-…)
and…
have you removed the “old” 1066Mhz modules provided with your iMac, or are they mixed 1066 and 1333?
Thankyou in advance