Combining iMac RAM

http://forums.macrumors.com/showthread.php?t=1499124

 

Not sure if it's mentioned in that thread, but this is important:

If you intend to add ram to the Apple memory already installed, you must buy 1600MHz CL11 ram.

You might be tempted to buy something like Corsair Vengeance 1600MHz CL10 ram. CL10 is faster than CL11, so that will be OK, surely? Well no.

The iMac will try to run all memory sticks at the lowest latency any of the sticks can run at. So it will run everything at CL10 if you install that ram. The Apple ram can only manage 1333MHz at CL10, so you will find all the memory runs at 1333MHz is you install CL10 ram.

Clearly this is slower than running at 1600MHz, which is the normal speed.

So you will be slowing your machine down (albeit slightly) if you install faster ram that the iMac ships with. Paradoxical I know, but there you go.

CL11 ram is your only choice.

 

I installed two Kingston KVR16S11/8 DIMMs in my 2012 iMac, alongside the Apple 8GB, in January, for a total of 24GB, and it has been 100% stable. The RAM chips on my Kingston DIMMs are Hynix, as are the Apple OEM DIMM chips.