Question i want to upgrade my laptop ram but i have problem with timing (CL - cas)

Jun 17, 2019
sorry my english not very good

i want upgrade my laptop ram to 12gb with 8gb but i confused about what timing "CL" should buy

this is my laptop (Acer aspire 5 A515-51G-5400)

i have now 6gb in my laptop 4gb + 2gb i want remove 2gb and add 8gb

this my main ram 4gb HMA851S6AFR6N-UH

this is my secondary ram 2gb 76.A302G.D330B

the specifications of both ram are ddr4 2400Mhz - 1.2v - CL17 but the CPU-Z show me different specifications
speed is 2128 or 2133 not 2400 and CL is 15 not 17

these pictures of my ram and cpu-z and ramexpert

i know my cpu (i5-7200u) doesn't support more than 2133 that why my ram is 2133 - CL15 not 2400 - CL17 because i think when speed decrease also CL decrease (but not very sure)

but now what specifications of ram should i buy

1- 8gb ddr4 2400 1.2v - CL15 (because timing of my ram now is 15 but i worry if CL15 will decrease to 13 because the speed will decrease to 2133)

2- 8gb ddr4 2400 - 1.2v - CL17 (i think it will work because the specifications like my 2gb ram but maybe there is better choice for my laptop)

3- 8gb ddr4 2133 - 1.2v - CL15 (maybe it is best choice but i don't know if it will work fine or not because my specifications of my main ram is 2400 - CL17 not 2133 - CL15)

which one should i buy

and please tell me if there is better than these choices

thank you
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