Question Mixing two sets of LPX DDR4 4X4 RAM: 2800 + 3000 Mhz

Feb 13, 2021
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Hello!
First of all I Know I shouldn't mix 2 sets of RAM.
However, I built my Rig back in 2014, and bought a LPX 2800 mHz memory kit (CMK16GX4M4A2800C16) that's very hard to find nowadays when building my X99 platform (Rampage V extreme with an Intel 5930K processor).

This MOBO has 8 memory slots, and I was running XMP 2998 on my original kit, running quad-channel on with timings 16-18-18-39 with trc 70 at 1.35V. I had done a little overclocking on the processor too, which was running with a 125 cpu strap and a x34 multiplyer (at 4.25 Ghz at 1.30v vcore stable).
The system was VERY stable. However, trying to squeeze every drop of performance from it, because I'm planning to skip this generation of GPUs, going straight to the next ( I know I will have massive CPU bottleneck with series 40 GPUs, but I am willing to live through it until I can buy a whole new system), I tried to upgrade my original 16 Gb of RAM to 32.

Since I couldn't find the exact same set of memory nowadays, I tried buying the closest I could find: LPX 3000 (CMK 16GX4M4C3000C16)
The problem is I can't make it stable enough. I can barely make the system post, which I managed Making both sets work at 2666 MHz with 1.35v on both sets (I rearranged them on the MOBO, so that I can control voltages independently for each set).

This stable(ish) setting was achieved @2666Mhz with 16-18-18-37. However even though I should be able to reach at least 2800 @1.2V (using timings from the lowest-performing kit,)I can't even get it to boot. I also tried 3000 Mhz to no avail, since both kits have a 3000 XMP profile and the timings seem closer at that profile.
The way things are now, I'm forced to "overvolt" (it's still XMP voltage, but above factory specs) both kits even though I underclocked both way below their factory specs in order to get some stability.

I'm really new to overclocking (as you might have guessed from my terrible results), and can't help but think I'm missing something.
If I could get stable XMP 2800 timings combining these kits it would be a major victory at this point.
One of the things I know might be worth exploring is tRAS timings, which this MOBO allow us to control only indirectly. However I already tried getting those indirect timings really high (as high as the BIOS let me), and still didn't manage to boot.
 
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Scutu Mix

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Mar 26, 2015
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First,... try looking on eBay as well - I could find the CMK16GX4M4A2800C16 module if not mistaken, see here: https://www.ebay.com/c/215830784

Second ... Mixing different speed memory modules, usually will limit all of them at the speed of the slowest - if no other incompatibility or motherboard issues. Which will result in less performance for the cost of your memory and overclocking them will never be the best for stability.

But I wouldn't try to fix it since it will never be as stable as you will want it. So try to sell all the memory modules you have an buy a 32Gb kit with all identical modules and you'll never have any other problems with it! :)
 
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Reactions: BernardoSC
Feb 13, 2021
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Thing is I'm from Brazil. That limits RAM availability by a lot. That's why I would like to make this work... but thanks anyway!
 

Scutu Mix

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Mar 26, 2015
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Oh, I see! Yes,it won't be that easy.
Recently I did a similar upgrade with my PC from 16Gb to 32Gb of RAM, and I found the exact memory I have but from China, and it arrived after 2 months but it works perfectly, no issues at all!
Good luck with your issue, hope you'll fix it soon!
 

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