Intel’s Core 2 Duo E6750 processor

INTEL FIRST UNCORKED its Core 2 processors last July, and believe it or not, relatively little has changed since then. Oh, sure, they’ve introduced quad-core variants of those processors that are essentially two chips in one package and a few low-end parts, but that’s about it. The top-speed-grade Core 2 processor last July was the Core 2 Extreme X6800 at 2.93GHz, and it remains the top speed grade today. That’s a remarkable amount of stability in a year’s time, but then the Core 2 hasn’t needed much help—AMD hasn’t yet caught up to it in performance.

Few things sit still in these parts for long, though, and things are about to change once again, as Intel’s recent introduction of the P35 Express chipset signaled. The new P35 and upcoming X38 chipsets from Intel are capable of supporting front-side bus speeds of 1333MHz, and Intel plans to introduce a slew of Core 2 processors primed for the faster bus. Although those CPUs won’t officially debut until later this summer, we have a preview today of one of those chips, the Core 2 Duo E6750. We have, of course, run the E6750 through our gamut of CPU performance tests, and we’ve also checked out its overclocking potential—which turns out to be rather considerable.

Meet the E6750
The Core 2 Duo E6750 is not a complex, brooding character from a French existentialist novel. Its story is rather simple. Like the E6700 before it, the E6750 is manufactured on a 65nm fab process and runs at a stock clock speed of 2.66GHz. Although its bus speed has jumped from 1066MHz to 1333MHz, its thermal design power (TDP) rating is unchanged at 65W. The E6750 is even compatible with LGA775-style sockets, though it does require a motherboard that supports its faster bus frequency.

I’d be more than happy to tell you what the price of the E6750 will be, but since it’s not officially announced yet, Intel is keeping mum on that front. Here’s a guess for you: it will be priced similarly to the current Core 2 Duo E6700, unless Intel decides to pull the trigger on a rumored across-the-board price cut when the product is announced.

In other words, I dunno exactly, but I wouldn’t expect to pay a premium for the higher bus speed.

We’ve tested the Core 2 Duo E6750 on the Asus P5K Deluxe motherboard, which is based on the P35 Express chipset and has full support for 1333MHz bus speeds. The mobo we chose makes use of DDR2 memory, which is fine for our purposes, especially since the board’s dual channels of DDR2 800MHz memory offer more than enough bandwidth to keep a 1333MHz bus fed. However, DDR3 memory does offer the prospect of even higher bandwidth and some dizzying memory clock speeds. If you’d like to get a look at DDR3’s potential, let me direct you to our P35 Express review.

What you will find on the following pages is an extensive, detailed, and broad-based comparison of the Core 2 Duo E6750 to a whole host of current CPUs from AMD and Intel. We’re able to offer this comparison by plugging the E6750’s scores into a bunch of results accumulated from past reviews, so you have a very clear sense of what the move to a 1333MHz bus yields in terms of performance. With that said, I’ll be the first to admit that our current test suite—widely multithreaded and largely 64-bit though it is—is getting into its rhinestone Elvis phase and will have to be revamped before too long with new applications, new drivers, and the like.

If you’re impatient, you may want to jump straight to our overclocking results, which is where the real fun is. I promise, doing so won’t hurt my feelings. Too much.

Our testing methods
As ever, we did our best to deliver clean benchmark numbers. Tests were run at least three times, and the results were averaged.

In some cases, getting the results meant simulating a slower chip with a faster one. For instance, our Core 2 Duo E6600 and E6700 processors are actually a Core 2 Extreme X6800 processor clocked down to the appropriate speeds. Their performance should be identical to that of the real thing. Similarly, our Athlon 64 FX-72 results come from an underclocked pair of Athlon 64 FX-74s, our Athlon 64 X2 4400+ is an underclocked X2 5000+ (both 65nm), and our Athlon 64 X2 5600+ is an underclocked Athlon 64 X2 6000+.

Our test systems were configured like so:

Processor Core 2 Duo E6300 1.83GHz
Core 2 Duo E6400 2.13GHz
Core 2 Duo E6600 2.4GHz
Core 2 Duo E6700 2.66GHz
Core 2 Extreme X6800 2.93GHz
Core 2 Quad Q6600 2.4GHz
Core 2 Extreme QX6700 2.66GHz
Core 2 Extreme QX6800 2.93GHz
Core 2 Duo E6750 2.66GHz Athlon 64 X2 3600+ 1.9GHz (65nm)
Athlon 64 X2 4400+ 2.3GHz (65nm)
Athlon 64 X2 5000+ 2.6GHz (65nm)
Athlon 64 X2 5000+ 2.6GHz (90nm)
Athlon 64 X2 5600+ 2.8GHz (90nm)
Athlon 64 X2 6000+ 3.0GHz (90nm)
Athlon 64 FX-70 2.6GHz
Athlon 64 FX-72 2.8GHz
Athlon 64 FX-74 3.0GHz
Core 2 Duo E4300 1.8GHz Athlon X2 BE-2350 2.1GHz
System bus 1066MHz (266MHz quad-pumped) 1333MHz (333MHz quad-pumped) 1GHz HyperTransport 1GHz HyperTransport
Motherboard Intel D975XBX2 Asus P5K Deluxe Asus M2N32-SLI Deluxe Asus L1N64-SLI WS
BIOS revision BX97520J.86A.2618.2007.0212.0954 0311 0903 0205
BX97520J.86A.2747.2007.0523.1129 1004
North bridge 975X Express MCH P35 Express MCH nForce 590 SLI SPP nForce 680a SLI
South bridge ICH7R ICH9R nForce 590 SLI MCP nForce 680a SLI
Chipset drivers INF Update 8.1.1.1010
Intel Matrix Storage Manager 6.21
INF Update 8.3.0.1013
Intel Matrix Storage Manager 7.5
ForceWare 15.00 ForceWare 15.00
Memory size 2GB (2 DIMMs) 2GB (2 DIMMs) 2GB (2 DIMMs) 2GB (4 DIMMs)
Memory type Corsair TWIN2X2048-6400C4
DDR2 SDRAM
at 800MHz
Corsair TWIN2X2048-6400C4
DDR2 SDRAM
at 800MHz
Corsair TWIN2X2048-8500C5
DDR2 SDRAM
at 800MHz
Crucial Ballistix PC6400
DDR2 SDRAM
at 800MHz
CAS latency (CL) 4 4 4 4
RAS to CAS delay (tRCD) 4 4 4 4
RAS precharge (tRP) 4 4 4 4
Cycle time (tRAS) 12 12 12 12
Audio Integrated ICH7R/STAC9274D5
with Sigmatel 6.10.0.5274 drivers
Integrated ICH9R/AD1988B
with Soundmax 6.10.2.6180 drivers
Integrated nForce 590 MCP/AD1988B
with Soundmax 6.10.2.6100 drivers
Integrated nForce 680a SLI/AD1988B
with Soundmax 6.10.2.6100 drivers
Hard drive Maxtor DiamondMax 10 250GB SATA 150
Graphics GeForce 7900 GTX 512MB PCIe with ForceWare 100.64 drivers
OS Windows Vista Ultimate x64 Edition
OS updates

Our Core 2 Duo E6400 processor came to us courtesy of the fine folks up north at NCIX. Those of you who are up in Canada will definitely want to check them out as a potential source of PC hardware and related goodies.

Thanks to Corsair for providing us with memory for our testing. Their products and support are far and away superior to generic, no-name memory.

Also, all of our test systems were powered by OCZ GameXStream 700W power supply units. Thanks to OCZ for providing these units for our use in testing.

The test systems’ Windows desktops were set at 1280×1024 in 32-bit color at an 85Hz screen refresh rate. Vertical refresh sync (vsync) was disabled.

We used the following versions of our test applications:

The tests and methods we employ are generally publicly available and reproducible. If you have questions about our methods, hit our forums to talk with us about them.

Memory subsystem performance
Since we’re dealing with a faster front-side bus, we’ll start things off by looking at memory performance. The Core 2 Duo must traverse its FSB in order to reach main memory, and that bus is theoretically a clear bandwidth constraint. At 1066MHz, the Core 2 Duo’s bus offers 8.5GB/s of peak throughput, while the two channels of DDR2 memory available on most motherboards top out at 12.8GB/s of aggregate bandwidth. The E6750’s 1333MHz bus raises the ante to 10.6GB/s—not quite enough to match the memory subsystem, but a substantial improvement nonetheless. Here’s how the E6750 handles our synthetic memory bandwidth and latency benchmarks.

The Athlon 64 remains the leader in these tests by virtue of its integrated memory controller, which does away with the front-side bus altogether. The E6750, though, easily outclasses any of the 1066MHz-bus Core 2 processors in memory bandwidth, with no additional cost in latency.

The Elder Scrolls IV: Oblivion
We tested Oblivion by manually playing through a specific point in the game five times while recording frame rates using the FRAPS utility. Each gameplay sequence lasted 60 seconds. This method has the advantage of simulating real gameplay quite closely, but it comes at the expense of precise repeatability. We believe five sample sessions are sufficient to get reasonably consistent results. In addition to average frame rates, we’ve included the low frame rates, because those tend to reflect the user experience in performance-critical situations. In order to diminish the effect of outliers, we’ve reported the median of the five low frame rates we encountered.

For this test, we set Oblivion‘s graphical quality to “Medium” but with HDR lighting enabled and vsync disabled, at 800×600 resolution. We’ve chosen this relatively low display resolution in order to prevent the graphics card from becoming a bottleneck, so differences between the CPUs can shine through.

Notice the little green plot with four lines above the benchmark results. That’s a snapshot of the CPU utilization indicator in Windows Task Manager, which helps illustrate how much the application takes advantage of up to four CPU cores, when they’re available. I’ve included these Task Manager graphics whenever possible throughout our results. In this case, Oblivion really only takes full advantage of a single CPU core, although Nvidia’s graphics drivers use multithreading to offload some vertex processing chores.

Rainbow Six: Vegas
Rainbow Six: Vegas is based on Unreal Engine 3 and is a port from the Xbox 360. For both of these reasons, it’s one of the first PC games that’s multithreaded, and it ought to provide an illuminating look at CPU gaming performance.

For this test, we set the game to run at 800×600 resolution with high dynamic range lighting disabled. “Hardware skinning” (via the GPU) was disabled, leaving that burden to fall on the CPU. Shadow quality was set to very low, and motion blur was enabled at medium quality. I played through a 90-second sequence of the game’s Terrorist Hunt mode on the “Dante’s” level five times, capturing frame rates with FRAPS, as we did with Oblivion.

The first thing we should say about these results is that most of these CPUs don’t appear to be a performance constraint in either game. That was certainly my experience while playing these games during testing. The E6750 does appear to offer a measurable performance boost over the other Core 2 Duo processors in Oblivion, although it’s very slight.

Supreme Commander
This game is multithreaded and can actually take advantage of more than two processor cores, making it a rare commodity indeed. We tested performance using Supreme Commander‘s very nice built-in benchmark, which plays back a test game and reports detailed performance results afterward. We launched the benchmark by running the game with the “/map perftest /nosound” options. (Normally, we prefer to test games with audio enabled, but we made an exception here.) We tested at 1024×768 resolution with the game’s default quality settings.

Supreme Commander’s built-in benchmark breaks down its results into several major categories: running the game’s simulation, rendering the game’s graphics, and a composite score that’s simply comprised of the other two. The performance test also reports good ol’ frame rates, so we’ve included those, as well.

The faster bus on the E6750 confers only a minuscule advantage over the E6700 in Supreme Commander, another game whose performance doesn’t seem to be especially CPU-bound with today’s processors. That small boost is all the E6750 needs to place it near the top of the pack of dual-core processors, just below the Core 2 Extreme X6800 and Intel’s two quad-core Extreme Edition processors.

Valve Source engine particle simulation
Next up are a couple of tests we picked up during a visit to Valve Software, the developers of the Half-Life games. They’ve been working to incorporate support for multi-core processors into their Source game engine, and they’ve cooked up a couple of benchmarks to demonstrate the benefits of multithreading.

The first of those tests runs a particle simulation inside of the Source engine. Most games today use particle systems to create effects like smoke, steam, and fire, but the realism and interactivity of those effects are limited by the available computing horsepower. Valve’s particle system distributes the load across multiple CPU cores.

Valve VRAD map compilation
This next test processes a map from Half-Life 2 using Valve’s VRAD lighting tool. Valve uses VRAD to precompute lighting that goes into its games. This isn’t a real-time process, and it doesn’t reflect the performance one would experience while playing a game. It does, however, show how multiple CPU cores can speed up game development.

The faster bus doesn’t work wonders in either of our Valve benchmarks, as the E6750 places only an eyelash ahead of the E6700. We should probably pause to consider another factor that hasn’t changed much: the E6750 is still miles ahead of AMD’s top dual-core processor, the Athlon 64 X2 6000+.

3DMark06
3DMark06 combines the results from its graphics and CPU tests in order to reach an overall score. Here’s how the processors did overall and in each of those tests.

The E6750 actually drops behind the E6700 by a handful of points in 3DMark’s overall score. That’s due to it giving up a few hundredths of a second to the E6700 in 3DMark’s graphics tests, which are largely GPU-bound, as one would expect. The differences here are inconsequential. The E6750 does make up some ground in 3DMark’s CPU tests, but not enough to put it ahead of the E6700.

The Panorama Factory
The Panorama Factory handles an increasingly popular image processing task: joining together multiple images to create a wide-aspect panorama. This task can require lots of memory and can be computationally intensive, so The Panorama Factory comes in a 64-bit version that’s multithreaded. I asked it to join four pictures, each eight megapixels, into a glorious panorama of the interior of Damage Labs. The program’s timer function captures the amount of time needed to perform each stage of the panorama creation process. I’ve also added up the total operation time to give us an overall measure of performance.

picCOLOR
picCOLOR was created by Dr. Reinert H. G. Müller of the FIBUS Institute. This isn’t Photoshop; picCOLOR’s image analysis capabilities can be used for scientific applications like particle flow analysis. Dr. Müller has supplied us with new revisions of his program for some time now, all the while optimizing picCOLOR for new advances in CPU technology, including MMX, SSE2, and Hyper-Threading. Naturally, he’s ported picCOLOR to 64 bits, so we can test performance with the x86-64 ISA. Eight of the 12 functions in the test are multithreaded, and in this latest revision, five of those eight functions use four threads.

Scores in picCOLOR, by the way, are indexed against a single-processor Pentium III 1 GHz system, so that a score of 4.14 works out to 4.14 times the performance of the reference machine.

The E6750 splits these image processing tests with its predecessor, scoring slightly higher in picCOLOR but falling short when stitching together a panoramic photo.

Windows Media Encoder x64 Edition
Windows Media Encoder is one of the few popular video encoding tools that uses four threads to take advantage of quad-core systems, and it comes in a 64-bit version. For this test, I asked Windows Media Encoder to transcode a 153MB 1080-line widescreen video into a 720-line WMV using its built-in DVD/Hardware profile. Because the default “High definition quality audio” codec threw some errors in Windows Vista, I instead used the “Multichannel audio” codec. Both audio codecs have a variable bitrate peak of 192Kbps.

LAME MP3 encoding
LAME MT is a multithreaded version of the LAME MP3 encoder. LAME MT was created as a demonstration of the benefits of multithreading specifically on a Hyper-Threaded CPU like the Pentium 4. Of course, multithreading works even better on multi-core processors. You can download a paper (in Word format) describing the programming effort.

Rather than run multiple parallel threads, LAME MT runs the MP3 encoder’s psycho-acoustic analysis function on a separate thread from the rest of the encoder using simple linear pipelining. That is, the psycho-acoustic analysis happens one frame ahead of everything else, and its results are buffered for later use by the second thread. That means this test won’t really use more than two CPU cores.

We have results for two different 64-bit versions of LAME MT from different compilers, one from Microsoft and one from Intel, doing two different types of encoding, variable bit rate and constant bit rate. We are encoding a massive 10-minute, 6-second 101MB WAV file here, as we have done in many of our previous CPU reviews.

Moving to a 1333MHz bus grants no advantage whatsoever when encoding MP3s in LAME, but it does shave about 10 seconds off of our video encoding task.

Cinebench
Graphics is a classic example of a computing problem that’s easily parallelizable, so it’s no surprise that we can exploit a multi-core processor with a 3D rendering app. Cinebench is the first of those we’ll try, a benchmark based on Maxon’s Cinema 4D rendering engine. It’s multithreaded and comes with a 64-bit executable. This test runs with just a single thread and then with as many threads as CPU cores are available.

POV-Ray rendering
We’ve finally caved in and moved to the beta version of POV-Ray 3.7 that includes native multithreading. The latest beta 64-bit executable is still quite a bit slower than the 3.6 release, but it should give us a decent look at comparative performance, regardless.

Contrary to my expectations, the E6750 proves to have an advantage over the E6700 in these rendering applications. We’re still not seeing big performance gains, but these are gains.

MyriMatch
Our benchmarks sometimes come from unexpected places, and such is the case with this one. David Tabb is a friend of mine from high school and a long-time TR reader. He recently offered to provide us with an intriguing new benchmark based on an application he’s developed for use in his research work. The application is called MyriMatch, and it’s intended for use in proteomics, or the large-scale study of protein. I’ll stop right here and let him explain what MyriMatch does:

In shotgun proteomics, researchers digest complex mixtures of proteins into peptides, separate them by liquid chromatography, and analyze them by tandem mass spectrometers. This creates data sets containing tens of thousands of spectra that can be identified to peptide sequences drawn from the known genomes for most lab organisms. The first software for this purpose was Sequest, created by John Yates and Jimmy Eng at the University of Washington. Recently, David Tabb and Matthew Chambers at Vanderbilt University developed MyriMatch, an algorithm that can exploit multiple cores and multiple computers for this matching. Source code and binaries of MyriMatch are publicly available. In this test, 5555 tandem mass spectra from a Thermo LTQ mass spectrometer are identified to peptides generated from the 6714 proteins of S. cerevisiae (baker’s yeast). The data set was provided by Andy Link at Vanderbilt University. The FASTA protein sequence database was provided by the Saccharomyces Genome Database.

MyriMatch uses threading to accelerate the handling of protein sequences. The database (read into memory) is separated into a number of jobs, typically the number of threads multiplied by 10. If four threads are used in the above database, for example, each job consists of 168 protein sequences (1/40th of the database). When a thread finishes handling all proteins in the current job, it accepts another job from the queue. This technique is intended to minimize synchronization overhead between threads and minimize CPU idle time.

The most important news for us is that MyriMatch is a widely multithreaded real-world application that we can use with a relevant data set. MyriMatch also offers control over the number of threads used, so we’ve tested with one to four threads.

STARS Euler3d computational fluid dynamics
Charles O’Neill works in the Computational Aeroservoelasticity Laboratory at Oklahoma State University, and he contacted us to suggest we try the computational fluid dynamics (CFD) benchmark based on the STARS Euler3D structural analysis routines developed at CASELab. This benchmark has been available to the public for some time in single-threaded form, but Charles was kind enough to put together a multithreaded version of the benchmark for us with a larger data set. He has also put a web page online with a downloadable version of the multithreaded benchmark, a description, and some results here. (I believe the score you see there at almost 3Hz comes from our eight-core Clovertown test system.)

In this test, the application is basically doing analysis of airflow over an aircraft wing. I will step out of the way and let Charles explain the rest:

The benchmark testcase is the AGARD 445.6 aeroelastic test wing. The wing uses a NACA 65A004 airfoil section and has a panel aspect ratio of 1.65, taper ratio of 0.66, and a quarter-chord sweep angle of 45º. This AGARD wing was tested at the NASA Langley Research Center in the 16-foot Transonic Dynamics Tunnel and is a standard aeroelastic test case used for validation of unsteady, compressible CFD codes. The CFD grid contains 1.23 million tetrahedral elements and 223 thousand nodes . . . . The benchmark executable advances the Mach 0.50 AGARD flow solution. A benchmark score is reported as a CFD cycle frequency in Hertz.

So the higher the score, the faster the computer. I understand the STARS Euler3D routines are both very floating-point intensive and oftentimes limited by memory bandwidth. Charles has updated the benchmark for us to enable control over the number of threads used. Here’s how our contenders handled the test with different thread counts.

The E6750’s slight but consistent advantage over the E6700 remains, well, consistent.

SiSoft Sandra Mandelbrot
Next up is SiSoft’s Sandra system diagnosis program, which includes a number of different benchmarks. The one of interest to us is the “multimedia” benchmark, intended to show off the benefits of “multimedia” extensions like MMX, SSE, and SSE2. According to SiSoft’s FAQ, the benchmark actually does a fractal computation:

This benchmark generates a picture (640×480) of the well-known Mandelbrot fractal, using 255 iterations for each data pixel, in 32 colours. It is a real-life benchmark rather than a synthetic benchmark, designed to show the improvements MMX/Enhanced, 3DNow!/Enhanced, SSE(2) bring to such an algorithm. The benchmark is multi-threaded for up to 64 CPUs maximum on SMP systems. This works by interlacing, i.e. each thread computes the next column not being worked on by other threads. Sandra creates as many threads as there are CPUs in the system and assignes [sic] each thread to a different CPU.

We’re using the 64-bit version of Sandra. The “Integer x16” version of this test uses integer numbers to simulate floating-point math. The floating-point version of the benchmark takes advantage of SSE2 to process up to eight Mandelbrot iterations in parallel.

As ever, the processors based on Intel’s Core microarchitecture excel here thanks to their ability to execute 128-bit SSE instructions in a single cycle. The E6750 doesn’t gain much from its faster bus, though.

Power consumption and efficiency
Our Extech 380803 power meter has the ability to log data, so we can capture power use over a span of time. The meter reads power use at the wall socket, so it incorporates power use from the entire system—the CPU, motherboard, memory, video card, hard drives, and anything else plugged into the power supply unit. (We plugged the computer monitor and speakers into a separate outlet, though.) We measured how each of our test systems used power during a roughly one-minute period, during which time we executed Cinebench’s multithreaded rendering test. All of the systems had their power management features (such as SpeedStep and Cool’n’Quiet) enabled during these tests.

You’ll notice that I’ve not included the Athlon 64 FX-72 here. That’s because our “simulated” FX-72 CPUs are underclocked versions of faster processors, and we’ve not been able to get Cool’n’Quiet power-saving tech to work when CPU multiplier control is in use. I have included test results for genuine Athlon 64 X2 4400+ and 5600+ chips, though. I’ve also included our simulated Core 2 Duo E6600 and E6700, because SpeedStep works fine on the D975XBX2 motherboard alongside underclocking. The simulated processors’ voltage may not be exactly the same as what you’d find on many retail E6600s and E6700s. However, voltage and power use can vary from one chip to the next, since Intel sets voltage individually on each chip at the factory.

The differences between the CPUs are immediately obvious by looking at these plots of the raw data. We can slice up the data in various ways in order to better understand them, though. We’ll start with a look at idle power, taken from the trailing edge of our test period, after all CPUs have completed the render.

Our Core 2 Duo E6750 test system is at something of a disadvantage here, because its Asus P5K Deluxe motherboard appears to consume more power at idle (and presumably also under load) than the most 975X boards, including the Intel D975XBX2 board on which we tested the other Core 2 processors. The E6750’s idle power draw is also higher because of a limitation of the processor. Like most Core 2 processors, the E6750’s minimum clock multiplier is 6X. In most Core 2 chips with a base FSB clock of 266MHz, that means the CPU’s lowest speed when throttled via SpeedStep or C1E halt is 1.6GHz. In the E6750, the 6X multiplier works out to a minimum idle clock of 2GHz. The E6750’s higher minimum clock will limit the effectiveness of power-saving schemes like SpeedStep.

Next, we can look at peak power draw by taking an average from the five-second span from 10 to 15 seconds into our test period, during which the processors were rendering.

The Core 2 Duo E6700’s power draw jumps by 45W when going from idle to rendering. The E6750’s delta between idle and load is only 27W—quite a bit less. Another way to gauge power efficiency is to look at total energy use over our time span. This method takes into account power use both during the render and during the idle time. We can express the result in terms of watt-seconds, also known as joules.

The E6750 system’s higher idle power draw hurts it here, as expected. We can quantify efficiency even better by considering the amount of energy used to render the scene. Since the different systems completed the render at different speeds, we’ve isolated the render period for each system. We’ve chosen to identify the end of the render as the point where power use begins to drop from its steady peak. There seems to be some disk paging going on after that, but we don’t want to include that more variable activity in our render period.

We’ve computed the amount of energy used by each system to render the scene. This method should account for both power use and, to some degree, performance, because shorter render times may lead to less energy consumption.

Even with the handicaps, the E6750 remains relatively efficient, well head of any dual-core Athlon 64 and miles apart from what may be its most direct competitor, the Athlon 64 X2 6000+.

Overclocking
I started my overclocking exploits with an attempt to see how far I could push the E6750’s front-side bus speed, not its core clock. In our review of the P35 Express chipset, Geoff found that he hit a wall at about 490MHz (or 1960MHz, quad-pumped) and wondered whether it was his CPU or the chipset limiting the clock speed. That is, of course, a nice boost over the base FSB clock of 333MHz regardless, but I wanted to find out if I could take things further with the E6750. Turns out I hit the exact same wall. I lowered the E6750’s multiplier to 6X and was able to achieve a 490MHz bus speed with all-stock voltages. The system would not POST, though, with a 500MHz FSB. I tried raising the CPU, north bridge, and FSB termination voltages in the P5K Deluxe’s BIOS, but nothing helped. This motherboard—and perhaps the P35 north bridge—seems to have a ceiling of about 490MHz.

Those of you paying close attention may have realized that a CPU at a 6X multiplier on a 490MHz bus will be doing a very healthy 2.94GHz—and as I said, our E6750 hit that speed at its stock voltage. That was only the beginning, though. After an epic, trial-and-error iterative process, I finally decided that the max stable clock speed for this E6750 is 3.64GHz, nearly a full gigahertz higher than stock. That’s with a core voltage of 1.3875V and nothing fancier than regular old air cooling.

Throwing more voltage at the problem didn’t produce any higher stable clock frequencies. I was able to get the CPU to POST and boot into Windows at up to 3.76GHz with 1.425V, but the system would crash pretty quickly after I kicked off any sort of CPU-intensive task. In fact, I had to switch from a stock Intel cooler to slightly beefier model in order to extract the last 40MHz from the E6750. With the stock cooler, the system wasn’t 100% stable unless you dropped down to 3.6GHz.

But those are niggling details. We still got a near-1GHz overclock out of this thing, and we didn’t have to resort to crazy-high voltages or heroic cooling measures in order to make it happen. That’s a heckuva nice overclock from a relatively high speed grade CPU, and happily, its higher base FSB speed didn’t get in the way; we still had a little bit of room left before hitting the motherboard’s apparent bus speed ceiling.

Here’s a quick look at performance with the E6750 clocked up to 3.64GHz.

At this speed, the E6750 is the fastest dual-core processor we’ve ever seen.

Conclusions
Our tests results have shown us, pretty well conclusively, that the Core 2 Duo E6750’s faster front-side bus only offers minor, incremental performance gains over its E6700 predecessor. That’s not entirely bad news. The E6750 is still much faster than any dual-core CPU from AMD, and we’ve learned that current Core 2 Duo processors aren’t really hitting a bus bottleneck. That revelation may seem counterintuitive to those of us who watched the Pentium 4 post great gains in performance from nearly every bus speed bump it received, but the Core 2 is much more efficient in its use of bus and memory bandwidth than the Pentium 4 and its offshoots were. That’s one reason Intel can get away with packaging two Core 2 chips together in a single socket for quad-core action—there’s room on the bus for both of ’em. That’s not to say the 1333MHz front-side bus won’t have its uses. The most obvious candidates to benefit from the 1333MHz bus are those very same quad-core processors. In fact, Xeons have been using a 1333MHz bus for quite some time now, with great success. Also, our E6750’s ample overclocking headroom suggests Intel could introduce much higher speed grades of the Core 2 Duo if so desired, and those chips could likely use the extra bandwidth—especially with fast DDR3 memory coming into its own. Further down the road, Intel plans to pack 6MB of L2 cache into its 45nm processors, which works out to 12MB of total L2 cache on a dual-chip package. Those caches will have to be fed.

Unfortunately, most of those uses don’t apply to the E6750. Then again, there’s no major penalty to adopting this new bus speed baseline, either. The E6750’s one weakness here is its inability to drop its multiplier below 6X. That’s a bit of a step backward on the idle power efficiency front. I’d like to see Intel move to more aggressive lower multipliers like AMD has with Cool’n’Quiet, if possible. That’s a minor concern, though, in the grand scheme, and the move to a faster bus only solidifies the Core 2’s grip on the CPU performance crown.

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