Overclocking Core i7: A Tale of Two Retail Processors

Anandtech.com looks into the varying differences between Intel Core i7 processors in terms of overclocking – Uncore voltage (CPU VTT) and memory overclocking.

As most Core i7 overclockers would know by now, higher the bclk and memory frequency clocks you go, the more Uncore voltage (CPU VTT) is needed. Now the issue is each Intel Core i7 processor will have different optimal Uncore voltage (CPU VTT) requirements to reach those specific bclk/memory frequencies. I’ve experienced this first hand when comparing 2x i7 920 3836A756 batch processors on DFI UT X58-T3EH8 motherboard.

While this processor matched the ‘international average’ for core voltage scaling, I discovered that I had to apply ridiculous levels of VTT/Uncore voltage to get it to boot at anything over 185BCLK. A 3.8GHz clock speed fell with relative ease, but pushing up to 4GHz, well that’s where things became real tricky. This processor seems to stop in its tracks with VTT voltage levels over 1.36V (just outside the warranty maximum by the way) or so, causing both of the motherboards to halt during POST with a C1 error.

Manipulating voltages within the OS using motherboard specific software tools can circumvent this condition to an extent, but the voltage has to be ramped up in small steps. Either way, 4GHz stable on this particular processor is way more hassle than it could ever be worth in a 24/7 system so I am stuck at 3.8GHz. Ok, this seems a bit demanding of me, a free 1.2GHz overclock from stock and I’m nowhere near happy! In fact, at this point I was pretty much convinced that both of the motherboards I was testing were duff, not the CPU, especially when I looked over at some of Gary’s early results on the same boards, as well as results of forum members.

I conferred back with Gary on his results, he’s got three retail 920s in his repertoire (with a fourth on its way) and his results are erratic in this department too. One of his processors needs high levels of Vcore to make 4GHz possible, well in excess of 1.50V, and refuses to POST with Bclk ratios set higher than 200, regardless of VTT/Core voltages. The other two (3837A)are better than mine for IMC VTT scaling and can also handle 6GB triple channel memory at 2000+MHz with a little persuasion. Something I found impossible on my 3838A processor with 3GB of memory let alone 6GB. Also, both of his processors allow for 3.8GHz operation at stock or below stock core voltages with VTT near 1.15V. Both processors allow clock speeds to reach about 4.3GHz on 1.45V of VCore, but VTT required is near 1.425V on air cooling with a 2:8 memory ratio, change the memory ratio to 2:10 for DDR3-2000 and VTT requirements hit 1.50V, which also happens to be the maximum amount his processors will allow before throwing up a C1 code on POST.

Based on this, I decided to purchase another 920 from a different retailer in the UK, again purchased as a consumer – so no cherry picked possibilities. This one was an OEM unit, 3835A batch, a little earlier in the production timeline. Onwards to testing using the exact same components as before and much to my surprise, 4GHz + on this particular CPU is not an issue. Further, the VTT required in comparison to the 3838A processor is miniscule.

Take a look at the voltage screenshots below – VTT required for an effective BCLK of 210 Prime95 8 thread stable is a mere 1.20V under full load, Vcore in the region of 1.332V (real) for just shy of 4GHz CPU speed. If I stick the 3838 retail processor into the same board on these settings – it won’t even post, in fact I can barely get it to post at 195BCLK using 1.36VTT, which then proceeds to throw a BSOD as soon as the system is presented with a load – regardless of how much I tinker with voltages.

Bottom line here is that the integrated memory controller in the i7 Core processors is highly variable. We’ve seen results much better than ours in some instances, and we’ve also heard of users finding it hard to get processors to post much beyond 3.6GHz. Based on our research to date, lot codes 3835, 3836, and 3841have the best opportunity of receiving a processor that will overclock well while offering both excellent VCore and VTT voltage rates. It is still luck of the draw for the most part, but chances are these lot codes will offer improved clocking rates at lower voltages.

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