Intel’s Core i5-655K & Core i7-875K: Overclocked and Analyzed

Testbed Setup

EVGA’s Classified motherboard was used for all of the overclocking and power consumption plots. We used a DC clamp meter directly on the EPS 12V power line to monitor current draw to VCC and VTT. Fortunately, we happen to know the approximate efficiency curve of the Volterra VRM so have taken the time to account for the switching losses in all figures shown.

Corsair’s Dominator GT 2200MHz kit was used at CAS 8-8-8-24 timings from DDR-3 1333MHz ~ DDR3 1820MHz. Not exactly tight for this kit, but representative of the memory timings many users will run using affordable memory kits on these processors (given the nature of the Clarkdale IMC). 

For comparative purposes, all components used were exactly the same throughout our testing. All memory timings and bus frequencies were matched up as needed (unless specified otherwise).

Clarkdale i5-655K Overclocking

We’ll start off with the basics. First off, the maximum overclock using stock voltages:

Next up, maximum stable overclocks:

We’re often criticized for being a little brief and thin on overclocking results for our processor launch articles so we’re going to break things down a little further today and take a look at sensible operating points and compare the power curves of different processors.

VID, VCC & Relative Frequency Scaling

First up, let’s take a look at the stock voltages programmed by Intel (VID) for each processor on test today, and the required voltage under full Linpack load at stock frequency and 4.55GHz.

Do note, that actual load voltages will vary from board to board depending upon loadline resistance, Voffset and Vdroop. For the uninitiated, Voffset is difference between the applied voltage and the actual idle state voltage, while Vdroop is the level of voltage sag between idle and full load processor states. These safety margins are necessary to ensure that the real time voltage level (VCC), does not breach the applied VID for extensive periods or by excessive levels (acceptable overshoot margins are stipulated in the Intel white papers). Further explanation of these design principals is provided here.

At first glance, the i5-661 stands out for having a relatively low stock VID in relation to its maximum single-threaded Turbo mode speed of 3.6GHz. That’s 160MHz higher than the i5-655K with a  small reduction in voltage. Not only that, but it manages to reach 4.55GHz using the lowest VCC of these three test samples.

Here’s a graphical representation of how each processor scales frequency versus voltage, normalized to each part's stock VCC:

Of course, this is but a mere portion of the story, what we’re interested in is the power consumption of each processor at equivalent frequencies and how that has an impact on frequency scaling given various forms of cooling.