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I've got the exact same issue. And I've figured out why and how to fix it.

1) To fix it, remove the stock ram, put your HyperX in slot 1 and 3 only, run as 32 gb. You will get 2400 MHz.

2) I've done a number of test and found running with HyperX 32gb only has the highest performance, infact, my iMAC has the highest score for 2017 model on Geekbench, better than stock 64gb.

https://browser.primatelabs.com/v4/cpu/search?dir=desc&q=iMac18,3&sort=score

3) The reason is HyperX has faster CL, only 14, where stock Apple ram has 17, therefor when you mix different CL speed, HyperX will slow down to 2133 MHz. Actually, it raised factory RAM's CL to 14, however, factory ram will slow to 2133 when CL@14 and force hyperx to 2133 as well. (Please correct me if I'm wrong)

4) If you want to run 4 rams, then ditch the Apple ram and get another set of 32gb Hyperx, you'll have 64GB and it will run at 2400 MHz.

5) I've done many test and 32GB HyperX alone is almost 10% faster than 40GB of mix factory RAM and Hyper X.

6) On my machine, if I leave factory ram in slot 1,3 and HyperX in 2,4, the iMac would not boot up, fan is working but black screen, waited 2 minutes. It does work if I put Hyperx in 1,3 and factory in 2,4. But will only run at 2133 MHz and much worse Geekbench scores.

Conclusion: Only run the same RAM across all slots. Remove your stock ram and run it with 32GB only. Or buy 2 more HyperX if you want 64gb. And HyperX is damn amazing. I think only people with 64GB HyperX can beat my Geekbench score.

What was your Benchmark scores? Do you have a before and after of your scores with the stock ram before and now the 32gb HyperX ram after
 
What was your Benchmark scores? Do you have a before and after of your scores with the stock ram before and now the 32gb HyperX ram after

It stands to reason. Because the HyperX is overclocked, when it is used in combination with Apple's stock RAM all installed RAM clocks back to 2133MHz. That is if you can even get it to boot at all.

HyperX on its own is not a problem.
 
Anyone know the difference of the different RAM I posted earlier? I plan on buying some soon since I just got a new iMac and am a stickler on having matching pairs. Plus I have no need for anything more than 16GB hence why I prefer to just buy some matching OEM RAM.
 
Hi guys,

FYI ;). As some people are saying in this thread they are using the HyperX with no problems, I can confirm from the HyperX twitter team, that the 32GB Kit is fully tested by them and thus compatible. They don't tested the new iMac with the 64GB Kit or two 32GB kits...so they confirmed at least that 32GB is compatible. They don't know yet if they will test the system with 64GB.

Myself am not sure which RAM to buy...is anyone using 64GB of the HyperX and fully tested it, for example with memtest86? I've read that 64GB of Ballistix is working fine without any errors after doing a memtest...So any suggesting for me? Just buying the one which is cheapest? Or maybe the memory specific ram? Thanks!
 
Hi guys,

FYI ;). As some people are saying in this thread they are using the HyperX with no problems, I can confirm from the HyperX twitter team, that the 32GB Kit is fully tested by them and thus compatible. They don't tested the new iMac with the 64GB Kit or two 32GB kits...so they confirmed at least that 32GB is compatible. They don't know yet if they will test the system with 64GB.

Myself am not sure which RAM to buy...is anyone using 64GB of the HyperX and fully tested it, for example with memtest86? I've read that 64GB of Ballistix is working fine without any errors after doing a memtest...So any suggesting for me? Just buying the one which is cheapest? Or maybe the memory specific ram? Thanks!

I have been happy with 64 GB Ballistix in my iMac, running great, and have tested them with memtest86 with no error. Video below is one I made after updating from 40GB to 64GB

 
My crucial memory arrived a short while ago. Everything working smoothly as expected. Very pleased with the setup!
 
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What version was this mate?

I order two of these 32GB kits from Amazon. I paid USD 285.99 per kit. Just checked Amazon; seems the price has dropped to USD 260.99. Link: http://a.co/6naY2dB

Crucial 32GB Kit (16GBx2) DDR4 2400 MT/s (PC4-19200) DR x8 SODIMM 260-Pin for Mac - CT2K16G4S24AM
 
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Hi. So I just ordered 27" i7 8GB and as the rest want to upgrade. Reading all the pages on here I haven't seen a definite winner.
Has anyone said yes go for this....

In the runings:
  • HyperX Impact DDR4 CL14 2400MHz 32GB
  • Corsair Vengeance DDR4 CL16 2400MHz 32GB
  • Ballistix Sport LT DDR4 CL16 2400MHz 32GB

In theory HyperX with the lower CL14 puts it slightly above the others.
 
Hi. So I just ordered 27" i7 8GB and as the rest want to upgrade. Reading all the pages on here I haven't seen a definite winner.
Has anyone said yes go for this....

In the runings:
  • HyperX Impact DDR4 CL14 2400MHz 32GB
  • Corsair Vengeance DDR4 CL16 2400MHz 32GB
  • Ballistix Sport LT DDR4 CL16 2400MHz 32GB

In theory HyperX with the lower CL14 puts it slightly above the others.
Corsair (Ballistix or standard) seems to be best, if you want to mix with standard Apple memory. HyperX is best with same memory, since they are overclocked and will run slower with Apple memory.
 
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In theory HyperX with the lower CL14 puts it slightly above the others.

I agree with what whcvip wrote above.

If you are out for the fastest performance and don't plan on continuing to use the Apple stock RAM then the Hyper-X is probably the best choice.

Other than that, I recommend whichever is least expensive and readily available between the Corsair and Ballistix.
 
I'm having stability problems running with 2 x 8gb Hyper-X kit (without original apple ram)
The iMac just completely freezes up, about 1 or 2 times a day.

I have done memtests but they don't report any problems.
I don't have this problem if I just run with the apple memory.
Anybody else has this?
 
I'm having stability problems running with 2 x 8gb Hyper-X kit (without original apple ram)
The iMac just completely freezes up, about 1 or 2 times a day.

I have done memtests but they don't report any problems.
I don't have this problem if I just run with the apple memory.
Anybody else has this?

What model of Hyper-X is it exactly? Freezing/crashing/kernel panics are often a symptom of bad RAM. Are the modules seated firmly?

I recommend downloading Memtest86 and making a boot Flash drive. If you've got bad RAM chances are you will see errors.
 
Hi. So I just ordered 27" i7 8GB and as the rest want to upgrade. Reading all the pages on here I haven't seen a definite winner.
Has anyone said yes go for this....

In the runings:
  • HyperX Impact DDR4 CL14 2400MHz 32GB
  • Corsair Vengeance DDR4 CL16 2400MHz 32GB
  • Ballistix Sport LT DDR4 CL16 2400MHz 32GB

In theory HyperX with the lower CL14 puts it slightly above the others.

Ok so iMac arrived this morning and I did a before and after Geekbench on the RAM.
iMac (Retina 5K, 27-inch, 2017)
4.2 GHz Intel Core i7
32 GB 2400 MHz DDR4 - HyperX 2 x 16GB
Radeon Pro 580 8192 MB

Before:
Single Core - 5372
Multi-Core - 17793

After:
Single Core - 5824
Multi-Core - 20674

A great increase in Multi-Core score.
 
After:
Single Core - 5824
Multi-Core - 20674

Glad to hear you got your new RAM and nice scores. :)

I posted my own benchmarks for the Ballistix a few pages back but just for comparison's sake, with only 32GB of the Crucial (2x16GB) installed in the same spec'd iMac:

Single Core: 5818
Multi-core: 20206

The Hyper-X seems marginally faster but Geekbench is a synthetic benchmark so the results are probably close enough to call it a draw.
 
Has anyone noticed any real world benefits in apps like FCP, Premiere, Cinema 4d, After Affects when running the Hyper X with the better CL rating?

My PC just drove me to the brink of madness last night. I think I'm going to pull the trigger on a top end 27 inch iMac, expense be damned. I want to milk every bit of speed I can out of this machine.
 
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I'm interested in upgrade memory too.
I'm using my iMac for video editing, photo editing and gaming. Does 32GB do the job or better buy 64GB to max it out?
I have installed stock 8GB RAM and I think that more RAM would be good for stuff I do but is the 64GB necessary?
I don't want to regret that I bought 32GB only or 64GB if there will be not noticeable difference in efficiency.
 
It's been said on here a few times that non-certified crucial ram works fine in the new iMac. But is there any chance that non-certified ram might develop faults over time??? On the crucial website the Mac ram design does look different, so there must be a difference!? Or maybe they are just representative images anyway. Thanks!
 
It's been said on here a few times that non-certified crucial ram works fine in the new iMac. But is there any chance that non-certified ram might develop faults over time??? On the crucial website the Mac ram design does look different, so there must be a difference!? Or maybe they are just representative images anyway. Thanks!

The only difference in specifications to be noted in Crucial's "For iMac" parts is that they are lead-free and halogen-free which apparently conforms to Apple's sustainability standards but will have zero effect on their performance or reliability.
 
I am going to buy a 2017 5K with 8GB RAM, 512GB SSD. I am not a heavy duty user, just web surfing, mail, finance, word processing, youtube, fb, etc, but nonetheless want more RAM.

I can upgrade to 16GB from Apple or buy 8GB kit from OWC or 16GB kit from Crucial. Is there a problem to mix Apple 2x4 with OWC 2x4 or Crucial 2x8? I have read posts that say no issue, and other posts say there is indeed an issue (even when the specs supposedly match).

So what's the answer? I can upgrade to 16GB thru Apple EPP for $180. An extra 8GB is $90 from OWC. The Crucual 16GB is $152. Is it worth it to stay with Apple and know that there won't be any issues?

Thanks in advance!
 
My recommendation would be to purchase the 16 GB kit from Crucial. Just plug it into the two empty slots on your iMac to give you a total of 24 GB. It will work fine and you will have plenty of RAM for whatever you may need to do in the future.
 
I just installed my new 32(2x16)GB from Corsair. Now I have 40GB of RAM memory in my new iMac. Installation was so easy and I don't have any problems. The machine boot up normally, showing all memory in all slots. I will be testing the system with video editing on macOS and gaming on Windows 10. Stay tuned for more!

Update: As I said in another topic, my Corsair modules were faulty. I experienced crashed in macOS and Windows. I requested a refund from amazon. Old Corsairs are on the way back and the new one are on the way to me. I will tell if refund helped.
 
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I've always felt that Crucial was a top tier RAM brand whereas G.Skill and Corsair were 2nd tier, but also that RAM failure rates overall for these brands weren't uber high.

I finally found a source that corroborates this, although the data is based out of France, and it is several years old now. Still, it's useful information. The numbers are from retail data.

https://linustechtips.com/main/topi...ts-french-but-i-translated-nearly-everything/

Average Failure rates:
- Kingston 0,20% (vs 0,27% year before)
- Crucial 0,39% (vs 0,30% year before)
- G.Skill 0,95% (vs 1,01% year before)
- Corsair 1,18% (vs 1,06% year before)

Average Failure rates:
- Kingston 0,20% (vs 0,20% before)
- Crucial 0,46% (vs 0,39% before)
- G.Skill 0,90% (vs 0,95% before)
- Corsair 1,08% (vs 1,18% before)

http://www.hardware.fr/articles/893-1/taux-retour-composants-8.html
http://www.hardware.fr/articles/911-1/taux-retour-composants-9.html
 
Ok, finally my new Corsairs arrived. I tested them with Apple Diagnostics and everything is fine now. I have 40GB so my computer is prepared for heavy work I'm planning for him. Today we edited short video and it was rendered flawlessly. I hope it will be also so good in heavy projects. Now, it's evening and it's time to rest!
 
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