Short version: The R640 and R740 (14th gen) take Intel Xeon Scalable 1st and 2nd gen on socket LGA3647. The R650 and R750 (15th gen) take 3rd gen Xeon Scalable on socket LGA4189. Nothing crosses between them — not the CPU, not the memory, not the heatsink.
Within the 14th gen, the upgrade that actually works is Skylake-SP → Cascade Lake, same socket, subject to a BIOS floor. The upgrade that quietly fails is dropping a high-TDP CPU into a chassis that has the wrong heatsink — and on the R740 that is a hard, documented rule: 1U heatsink up to 125 W, 2U heatsink above 125 W. Get that wrong and you have bought a processor your server cannot cool.
The generation wall
Start here, because it kills most of the bad ideas immediately.
| R640 / R740 / R740xd (14G) | R650 / R750 (15G) | |
|---|---|---|
| CPU family | Intel Xeon Processor Scalable Family — 1st gen (Skylake-SP) and 2nd gen (Cascade Lake) | 3rd Generation Intel Xeon Scalable (Ice Lake) |
| Socket | LGA3647 (Socket P) | LGA4189 (Socket P+) |
| Max cores per CPU | 28 | 40 |
| Memory channels per CPU | 6 | 8 |
| DIMM slots (2 CPU) | 24 | 32 |
| Max memory speed | DDR4-2666 (Skylake) / DDR4-2933 (Cascade Lake) | DDR4-3200 |
The socket changed. LGA3647 → LGA4189. An Ice Lake CPU will not physically seat in an R640 or R740, and a Cascade Lake CPU will not seat in an R650. There is no BIOS, no adapter, no workaround. If someone offers you a "R740 upgrade to Ice Lake," they are selling you a different server.
The memory topology changed too. 6 channels → 8 channels, 24 slots → 32 slots. Even though both generations use DDR4, a memory config optimised for the R740's 6-channel layout is wrong for the R650's 8-channel layout. Populating an R650 with a 12-DIMM config carried over from an R740 leaves two channels empty per CPU and costs you real bandwidth. Different platform, different rules.
The heatsink rule (R740 / R740xd) — Dell's own table
This is the single most valuable thing on this page, because it is a hard mechanical constraint that most secondary-market listings do not mention.
Dell's thermal restrictions documentation for the R740 specifies:
| Configuration | CPUs | Heatsink required | CPU / DIMM blank | Air shroud | Fans |
|---|---|---|---|---|---|
| R740, single CPU | 1 | 1U standard heatsink for CPU ≤ 125 W | Required | Standard | Four standard fans + one blank covering two fan slots |
| 2U standard heatsink for CPU > 125 W | |||||
| R740, dual CPU | 2 | Two 1U standard heatsinks for CPU ≤ 125 W | Not required | Standard | Six standard fans |
| Two 2U standard heatsinks for CPU > 125 W | |||||
| R740 with GPU | 2 | Two 1U high-performance heatsinks | Not required | GPU air shroud | Six high-performance fans |
Three things fall out of this that cost people money:
1. The 125 W line is the one to check. Most mid-range Xeon Scalable SKUs sit at or below 125 W. But the moment you step up — a Gold 6248 (150 W), a Gold 6254 (200 W), a Platinum 8280 (205 W) — you need the 2U heatsink. If your R740 shipped with mid-range CPUs, it almost certainly has 1U heatsinks in it. Buying a 150 W CPU without buying the matching heatsinks means the CPU arrives and the server cannot run it properly.
2. Single-CPU R740s need a processor blank AND a fan blank. Dell requires a CPU/DIMM blank in the empty socket, and the fan configuration is four fans plus a blank covering two fan slots — not six fans. Pull the blank out "to improve airflow" and you have made it worse: you have broken the designed airflow path. If you are populating the second socket, you need the blank out, two more fans in, and a second heatsink.
3. A GPU build is a different chassis spec entirely. High-performance heatsinks (not standard), high-performance fans (not standard), and a GPU-specific air shroud. You cannot bolt a GPU into a standard R740 and expect it to work — the cooling parts are different SKUs.
The R640 is 1U and has its own thermal envelope, which is tighter. Check the R640's own thermal restrictions table before specifying a high-TDP part in 1U; the headroom that exists in a 2U R740 is not there.
Skylake → Cascade Lake: the upgrade that works
Within the 14th generation, this is the real upgrade path. Same socket, same DDR4, meaningful gains — Cascade Lake brings higher memory speeds (DDR4-2933 vs 2666), higher core counts at the top of the stack, and in-silicon mitigations for several of the speculative-execution vulnerabilities that cost Skylake performance in software.
The requirement is BIOS. Second-generation Xeon Scalable support requires a BIOS that includes the second-gen processor and memory reference code plus the corresponding microcode. Dell shipped this in the 2.x BIOS line for the R740 / R740xd / R640 / R940 — Dell's release notes for BIOS 2.21.2, for example, explicitly cite updated processor and memory reference code for the second-generation Intel Xeon Processor Scalable Family and updated processor microcode for it.
Do this in the right order:
- Flash the BIOS first, with the old CPU still installed. This is the step people skip. If you swap the CPU first and the BIOS does not know about it, you may not get far enough into POST to flash anything.
- Check the heatsink against the 125 W rule above. Order heatsinks with the CPUs if you are crossing the line.
- Check the iDRAC/Lifecycle Controller firmware is current too — an out-of-date Lifecycle Controller can block the BIOS update path.
- Then swap the CPU.
- Verify memory speed after boot. Cascade Lake's DDR4-2933 only materialises if the DIMMs are rated for it. Cascade Lake in an R740 full of DDR4-2666 runs at 2666.
That last point catches people. Upgrading the CPU does not upgrade the memory. If the business case for the upgrade rested on the higher memory bandwidth, you are buying DIMMs too.
The mixing rules
These are the ones that produce "it boots but it's wrong":
- Both CPUs must be identical. Same model, same stepping, same TDP. Dell will not run mismatched processors in a dual-socket configuration. "Close enough" is not a thing here.
- Do not mix Skylake and Cascade Lake in one server. Same socket, but they are different processor generations and will not run together.
- Both heatsinks must match the CPU TDP. Two 1U heatsinks and two 150 W CPUs is not a valid configuration even though it will physically assemble.
- Memory must be balanced across both CPUs. Each CPU owns its own 6 channels. Populating DIMMs on CPU1 only leaves CPU2's memory controller with nothing local and forces every memory access from CPU2 across the UPI link. It boots. It performs badly.
- Processor sockets are not hot-pluggable. Dell states this explicitly. Power down fully.
What you actually need to order
A CPU upgrade on a 14G PowerEdge is rarely just a CPU. The realistic bill of materials:
| Scenario | What you need |
|---|---|
| Swapping like-for-like TDP (e.g. 125 W → 125 W) | CPU(s) only. Reuse the heatsinks. Confirm BIOS supports the new part. |
| Skylake → Cascade Lake, same TDP band | CPU(s) + BIOS update. Optionally DDR4-2933 DIMMs to realise the memory gain. |
| Crossing the 125 W line (R740) | CPU(s) + 2U heatsinks + thermal paste. Verify fan config. |
| Adding the second CPU | CPU + heatsink + two additional fans + remove the CPU/DIMM blank. Balance the memory across both sockets. |
| Adding a GPU | GPU + 1U high-performance heatsinks + high-performance fans + GPU air shroud + riser + possibly a higher-wattage PSU. |
The heatsink and fan lines are the ones that get dropped from quotes and then hold up the whole job. Order them with the CPU.
Is a 14G CPU upgrade even worth it?
An honest answer, because the maths does not always favour the upgrade.
It usually is worth it when: you have R740s with low-core-count CPUs (a pair of Silver 4110s, say) and a per-socket software licence — VMware, Windows Datacenter, Oracle. Doubling the cores per socket without adding a socket is often the cheapest core you will ever buy, because the licence does not change.
It usually is not worth it when: you are already near the top of the stack, or the workload is memory-bandwidth-bound rather than core-bound. A 14G platform maxes out at 6 memory channels and DDR4-2933. If that is your ceiling, a bigger CPU does not move it — you need a newer platform.
For the wider generational picture, our PowerEdge generations guide (R620 → R760) lays out what each generation actually buys you. And before you order DIMMs, our R740 memory compatibility guide covers which RDIMMs and LRDIMMs actually work and how they interact with the population rules above.
Where to source the parts
- Server Processors (CPUs) — Xeon Scalable 1st, 2nd and 3rd gen
- Heatsinks & Thermal Solutions — 1U, 2U and high-performance heatsinks for 14G PowerEdge
- Server Cooling Fans — standard and high-performance fans, and the blanks
- Dell Parts — air shrouds, CPU/DIMM blanks, risers
- Server Memory (RAM) — DDR4-2666 and DDR4-2933 RDIMM/LRDIMM
Need a quote?
Tell us the server model, the CPU you have and the CPU you want, and we will quote the complete kit — processors, the correct heatsinks for that TDP, the fans, and the memory if you need it — so nothing is missing when the box is open and the maintenance window is running. We ship worldwide DDP with duties included, accept purchase orders, and return quotes within 24 hours.
Sources: Dell, "PowerEdge R740 Technical Specifications — Thermal restrictions" (dell.com/support/manuals, per740_techspecs_pub) — the heatsink / blank / air shroud / fan table above is transcribed directly from Dell's Table 1. Dell, "PowerEdge R740 Technical Specifications — Processor specifications," which states the R740 supports up to two Intel Xeon Processor Scalable Family processors, up to 28 cores per processor, and that processor sockets are not hot-pluggable. Dell PowerEdge R650 spec sheet and technical guide for 3rd Generation Intel Xeon Scalable support, up to 40 cores per processor, 8 memory channels per CPU and 32 DDR4 DIMMs at 3200 MT/s. Dell BIOS release notes for R740/R740xd/R640/R940 (e.g. version 2.21.2), which cite updated processor and memory reference code and microcode for the second-generation Intel Xeon Processor Scalable Family. Confirm the minimum BIOS revision for your specific processor SKU against Dell's BIOS release notes for your exact model before purchasing. TDP figures for individual Xeon SKUs should be confirmed against Intel's specifications.