Why does the RAM clock rate reduce when adding memory?

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iMac Intel 27" Retina 5K Display 2019

A2115 / 2019 / Processors from 3.0 GHz 6-core i5, up to 3.6 GHz 8-core i9. Released March 19, 2019.

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Lars Olaf Helgestad @hellar

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发帖于: 2020年2月21日

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Why does the RAM clock rate reduce when adding memory?

I have just added 2x8GB memory modules (Kingston HyperX HX426S15IB2K2/16) to the original 2x4GB modules. I put the new 8GB modules in BANK0 and BANK2 (DIMM0) and the original 4GB modules in BANK1 and BANK3 (DIMM1).

I noticed that the speed was reduced from the original 2666MHz to 2400MHz.

Picking out the original 4GB modules gave me a total 16GM of RAM with clock rate 2666MHz.

Mixing 2x8GB modules and 2x4GB modules reduce the clock rate from 2666Mhz to 2400Mhz.

Has anyone seen something like this?

Any suggestions?

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Dan @danj

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Nothing special here! You put in a higher capable SO-DIMMs but the system already had slower SO-DIMMs. The common clock rate will be the slowest SO-DIMM’s which is 2400 MHz is what the system will run at as all of the slots need to run at the same speed.

Technically your system needs PC4-21300 (2666 MHz) DDR4 SO-DIMM’s to get the best performance!

The Kingston RAM you bought is not the correct RAM for your system!

I have encountered mis-marked Kingston RAM before. I’m not sure if this is a Kingston issue or someone playing games reselling RAM by marking the RAM as something other than what it is intended to be.

I would return the RAM.

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Hi Dan!

Thanks for replying!

The thing is that when I run just the 2 new 8GB Kingston modules the clock rate is 2666Mhz.

Mixing the original 2x4GB and the new 2x8GB modules to a total of 24GB, reduces the rate from 2666Mhz to 2400Mhz.

由 Lars Olaf Helgestad 完成的 2020年2月21日

OK I think I see the issue! You are not filling the channel with the same sized DIMMs

ChannelA - DIMM0 & DIMM1 need to be both the same sized DIMM's. Likewise for ChannelB

由 Dan 完成的 2020年2月21日

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Hi Dan!

Thanks for replying!

The thing is that when I run just the 2 new 8GB Kingston modules the clock rate is 2666Mhz.

Mixing the original 2x4GB and the new 2x8GB modules to a total of 24GB, reduces the rate from 2666Mhz to 2400Mhz. — Lars Olaf Helgestad
OK I think I see the issue! You are not filling the channel with the same sized DIMMs

ChannelA - DIMM0 & DIMM1 need to be both the same sized DIMM's. Likewise for ChannelB — Dan

iMac Intel 27" Retina 5K Display 2019

Why does the RAM clock rate reduce when adding memory?

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