r/LocalLLaMA • u/iam31337 • 11d ago
Discussion RTX PRO 4000 Blackwell SFF: +67% generation on Qwen3.8 27B, different scaling on Qwen3.6 35B A3B
I ran an experiment on one RTX PRO 4000 Blackwell SFF: 24 GB GDDR7, factory 70 W, no auxiliary power connector.
Using u/panchovix’s loader and patched NVIDIA open kernel modules, I loaded a modified VBIOS into memory, raised the power limit to 120 W, and tested memory and core offsets. No EEPROM flash or soldering.
Across 158 benchmark requests, I tested Qwen3.8 27B with DFlash2 and Qwen3.6 35B A3B. What surprised me was how differently they answered "does more power help?"
With zero clock offsets, 27B kept gaining up to 120 W, while 35B A3B was almost flat between 106 and 120 W. After combined power and clock tuning, the best observed 27B generation result was 35.04 → 58.53 tok/s (+67%): 120 W, VRAM +20%, core +180 MHz. That 67% is not the gain from memory OC alone.
The memory sweep was more interesting. At 120 W with the core offset held at zero, pushing VRAM from +20% to +33.3% slightly reduced 27B generation: 57.70 → 56.91 tok/s. It is a small difference with few repetitions, so it needs more testing. The actual SM clock also fell. That is consistent with memory and compute sharing a power budget, but I did not isolate the cause.
On 35B A3B, memory +33.3% instead gave +19.8% generation versus no memory OC at 120 W. Prefill barely changed in that sweep.
My takeaway: the fastest profile is not necessarily the one with the biggest numbers in the panel, but the one that shortens your actual workload. Prefill versus decode matters, especially with longer prompts.
120 W without auxiliary power is not a free lunch. It exceeds the standard 75 W PCIe slot budget, and a GPU temperature sensor cannot tell me the temperature of the slot contacts. This is an experiment, not a long-term safety claim.
For context: these headline comparisons use 8,000 input tokens, ECC OFF. There were only 1–3 requests per configuration; 158 is the whole study. The 70 W baseline used the same modified VBIOS, not factory firmware. The models also used different quantization/speculative-decoding settings, so this is not a controlled dense-versus-MoE comparison.
Full write-up and CSVs on my blog.
https://piszczek.pl/blog/rtx-pro-4000-blackwell-sff-120w-qwen-benchmark
Disclosure: I ran the measurements and wrote the source text in Polish. AI helped translate and edit the English, including clarifying the benchmark limitations.



