I have a machine in use with a 16gb set of ram. 2x8gb, 1866mhz, 10-11-10-30, 1.5V. There are two available slots in the mobo, and I recently remembered that I have an 8gb set sitting around collecting dust. Don't know the exact timings off hand, but 2x4gb, 1600mhz, Cas latency 9, 1.65V. Link to them below. I can't find the mobo in question right now as I don't have access to the machine at the moment, but it is some FM2 and/or FM2+ MSI microATX board. I believe it is an A78, but I could be wrong. Anyway, ram speed isn't terribly important, but capacity is, which is why I am looking into this. Is it possible to downclock the faster set to 1600 or something so that the 8gb set can be used as well? Would the difference in voltages cause a problem? Can I expect stability issues if I try this? As you may be able to guess, I have never mixed ram sets before, so advise is appreciated.
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231611
http://www.newegg.com/Product/Product.aspx?Item=N82E16820104173
I would assume if the timings are identical then there shouldn't be too much of an issue. That being said, I've never done it before either so real world results may contradict my hypothesis.
The answer here ranges from probably not to definitely yes. Timings are the important bit here, mixing ram with very different timings will certainly cause issues, but, mixing ram with identical timings probably won't cause problems.
Of course it's possible to downclock the faster set. Typically you'd have to make the ram run at 1866mHz during setup in bios with xmp as ram defaults to run at 1333. The speed it says on the ram is the highest certifiably stable speed that the manufacturer has determined. It's possible to overclock past this speed (but with the typical drawbacks of overclocking) and it doesn't run at this speed (1866mHz) out of the box.
I have done two different ram sets before with no issue. There may be some motherboards that don't like it but as long as each set is two of the same ram then it should more than likely work on ddr 3 based motherboards.
If they are compatible it shouldn't be a problem, most MBs will fall back to the slowest ram so the speed also shouldn't be a issue, having said that if there are some incompatibilities they may not show up at post, they may not show up at any point until the system is stressed, normally there are a whole list of things that happen from random lockups, crashes, and general instability, memory problems are some of the hardest problems to trace down because the symptoms vary from crash to crash never pointing to one specific thing causing the problem.
If the memory is compatible give it a try, if you start having problems remove it and see if the problems continue or disappear, different chip sets and different motherboards react differently to mixed types, speeds, and capacities, no harm in giving it a try as long as the type is listed in the manual of your motherboard as being supported and there's not any footnotes about mixing types of modules, most MB manuals are pretty clear as to what is supported and what isn't.
Good luck.
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1600Mhz on FM2? I would just buy more if you're using the internal GPU. It's not exactly expensive. The stuff I have is still $120 for 16GB.
I said in the OP. Speed isn't important. This isn't a gaming build. I am only concerned about capacity.
Fair enough. I would put in the 1866Mhz sticks and manually set the clock and cas to the same as the other sticks. Then put those in. Since it's FM2 it might also screw with the GPU though.