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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 quick update for the Apple 27″ iMacs (late 2009). The first benchmark results have arrived for the 27″ iMac Core i5 and 27″ iMac Core i7. Geekbench 2 shows even faster results in 64-Bit mode.

I’ve updated the chart to correct a small error because the Core i5 result wasn’t taken with the same Geekbench version as the others. Now all results are taken with Geekbench 2 version 2.1.4 and with OS X 10.6.2. I’ve also added the latest Mac Pro models (early 2009) so that you can better compare the results.

Update: Bare Feats now also has benchmark results for the Core i7, Core i5 compared to the early 2009 Mac Pro’s.

Here are the results:

results taken from Geekbench website: iMac Core i5 2.66Ghz, iMac Core i7 2.8Ghz, iMac Core 2 Duo 3.33Ghz, iMac Core 2 Duo 3.06Ghz, Mac Pro 2.93Ghz (1 CPU), Mac Pro 2.93Ghz (2 CPUs)