Intel Coffee Lake-S: Core i5 8400/ Core i5 8600K review

Intel's Coffee Lake-S flagship, the Core i7 8700K is - in our view - the fastest gaming CPU that money can buy right now, but what if you don't have £350/$350 to spend on a new processor? The new i5 line looks almost as exciting, bringing six physical cores to the mid-range market for the first time. The 8600K continues Intel's tradition of cutting hyper-threading and reducing clocks while leaving overclocking as an option, shaving off £100/$100 in the process. However, just as intriguing is the i5 8400 - a £180/$190 hexacore offering that offers remarkable value.

Going into this review, we had plenty of questions. Bearing in mind how brutally powerful the 8700K is, to what extent is hyper-threading necessary? Is the i5 just as good once overclocking is factored into the mix? Just how fast is the more budget orientated 8400 and how does the new Coffee Lake-S stack compare to last year's Kaby Lake? If you're still holding onto your legacy 2500K - as many are - just how much of an upgrade are the six-core i5s? And to what extent can AMD's Ryzen 5 compete?

That's a lot of topics to cover, so this review will be somewhat on the lengthy side, so let's kick off with the core specs - and some degree of confusion. Intel's official base and boost clocks for Coffee Lake show a yawning chasm between the 8400 and the 8600K, but in actual fact, running flat out, the budget chip happily locks at 3.8GHz - a state of affairs confirmed across multiple motherboards, even remaining the case when we paired it with a stock Intel heat sink and fan. The boost clocks actually signify the fastest single-core speed - suggesting just a small step between 8400 and 8600K - yet real world testing does show a more noticeable gap.

Our test system, featuring the MSI Z370 Gaming Pro Carbon AC

We tested and benchmarked the Core i5 8600K on a Z370 board supplied by MSI, the Z370 Gaming Pro AC, using the Z370 chipset required for eighth-gen Coffee Lake processors. The LGA 1151 v2 socket is identical to Z170 and Z270 boards but unfortunately older chips will not run on this platform, while the new CPUs are also incompatible with prior gen boards.

This MSI offering features DDR4 support up to 4GHz and beyond, 'Mystic Light' RGB lihgting, 2x Turbo M.2 slots, Intel Optane compatibility and Intel gaming LAN and wireless support. Two-way Nvidia SLI and three-way AMD CrossFire is possible, while audio duties are taken care of by Audio Boost 4 technology with Nahimic 2+. Four USB 3.1 ports, HDMI 2.0 and Displayport complete the package.

We paired this board with four sticks of Corsair Vengeance LPX at 3000MHz, with 15-17-17-35 latency and ran everything from an OCZ Trion 100 SSD. Corsair's RM1000i supplied power while the firm's H100i GTX closed loop cooler sat atop the processor for the duration. The Core i7 7700K was benched using the same components, albeit on an MSI Gaming M5 Z270 board, while the 8700 was tested on an Asus Z370 Maximus 10 Hero. Meanwhile, the 3770K was tested on an MSI Z77A-G45 board paired with 16GB of Corsair Vengeance 2400MHz DDR3 (11-13-13-31 latency) while the same RAM was used to test the 4790K, running on an MSI Z87i Gaming AC mini ITX board. Please refer to our Ryzen 5 1600/ 1600X vs Core i5 7600K review for specific AMD system set-ups.

There has been some discussion on certain Z370 boards auto-overclocking Coffee Lake processors beyond their spec, but Cinebench numbers from MSI, Gigabyte and Asus boards resulted in just a three per cent variance, even with all-core turbo disabled. This option - enhanced turbo, as some call it - essentially allows Z-boards to run overclocked processors at their maximum single-core turbo speed, and it usually activates itself automatically if you activate the XMP profile on DDR4 memory. On the one hand, it can skew benchmark results, but on the other, this is the likely minimum speed you'll be running the processor at if you buy one. It's free performance out of the box. Our results are with all-core turbo disabled, and we'll be manually overclocking later.

Our focus is on gaming performance, but we ran some basic benchmarks to get an idea of what to expect from our test processors. Cinebench R15 is a rendering tool that calculates a complex scene using all the cores and threads available, and then re-runs the same test with just a single core active. It gives us a baseline for competitive comparisons.

There are few surprises here: in terms of single-core power, the Coffee Lake i5s aren't as quick as the 8700K, but bearing in mind the single-core turbo advantage the flagship K-chip has, we expected a wider differential on that particular result. The i5 results show the 8600K at stock speeds equalling the 7700K's single-core performance, while outperforming it in multi-thread throughput. It's a fascinating competition here between two extra physical cores vs hyper-threading - a theme that will crop up several times during this review. The 8400 is a touch slower with one core active, but is within touching distance of the 7700K when all six processing threads are active.

The takeaway is that this initial synthetic bench suggests that a stock 8600K offers last-gen i7 power, with the budget orientated 8400 not too far behind. Meanwhile, as expected, Ryzen 5's 12 threads hand in extra multi-thread performance, but single-core power is off-pace - and we should expect that to persist into gaming results.

Our Handbrake encoding tests are based on the 4K processing we carry out for our own video encoding we use for YouTube and our download site, digitalfoundry.net. Those physical cores clearly matter here, with both h.264 and the super-heavy HEVC encoding seeing the i5s beat the 7700K. Ryzen can keep up in h.264, but the x265 encoder used for HEVC files utilises AVX instructions heavily, and AMD is off-pace here.

Gaming analysis begins with an Intel showdown - an attempt to place the new i5s within the current-gen and last-gen Core hierarchy. We're a little uncomfortable with reviewing the Core i5 8400 using a Z370 overclocking board, so we've limited its memory bandwidth to 2666MHz (the limit on upcoming cheaper boards) and used a standard Intel cooler. In addition to Coffee Lake and Kaby Lake processors, we've also included Skylake's locked Core i5 6500 in the benchmark line-up. Its performance should be relatively close to the 7400, the last-gen locked processor that Intel's new entry-level i5 replaces.

As always with our gaming tests, we try to gauge relative CPU performance by removing the graphics card as a bottleneck, using an overclocked Titan X Pascal running at 1080p resolution. The CPU's task is to run game logic and prepare instructions for the GPU, so we run all titles at max settings or close to it in order to ensure the toughest gaming workloads. Ashes of the Singularity's CPU test is like a synthetic bench within a game engine and reveals that hyper-threading remains highly potent. The i7 8700K has a commanding lead, while the last-gen 7700K is still a great performer, with four cores and eight threads beating the i5 8600K's hexacore set-up.

On the flip-side, Rise of the Tomb Raider and The Witcher 3 reverse the trend, with the 8600K besting the 7700K. Otherwise, results between 8600K and 7700K are similar and from our perspective, the takeaway is clear. If you bought into the old Kaby Lake line, you still have access to excellent gaming performance. The 7700K was - and still is - a great performer, so if you're running a less capable chip on the old platform, you still have a good upgrade option available.

There are some great results from the Core i5 8400 too. The generational leap from prior locked i5s (presented here by the i5 6500) is simply tremendous, and while not quite on par with a stock 8600K or 7700K, the results are very close overall. The Far Cry engine is heavily reliant on single-core power and memory bandwidth, so the 8400's result shows relative weakness here, but it's still a great result compared to older-gen locked i5 performance. Elsewhere, most results show it comparing well with the 7700K - not bad for an entry-level i5.

Hand on heart, we have a real soft spot for AMD's Ryzen 5 1600 and 1600X - the former in particular, owing to its decent overclocking and excellent bundled cooler. In fact, given the choice between a last-gen i5 K chip, and the Ryzen 1600, our recommendation goes to the red team, owing to stronger gaming performance under the most demanding multi-threaded games. But Coffee Lake's i5 change the game.

We've got stock performance numbers below ranging from our entry-level Core i5 8400 up to the most expensive eight-core Ryzen 7 1700 with those excellent Ryzen 5s in the middle. Far Cry Primal dramatically demonstrates Intel's advantage in games that favour a single thread, while The Witcher 3 and Rise of the Tomb Raider also show commanding leads in game engines that are heavily threaded (and the latter was tested with the Ryzen optimisation patch installed).

Coffee Lake-S is clearly faster overall and by extension, we'd rate the new i5s as more future-proof - especially as there's a clear upgrade path to i7, while Ryzen 7 (even when overclocked) only offers a small boost over the 1600 and 1600X. However, forensic analysis of the Crysis 3 stress test shows Ryzen can actually outperform Coffee Lake in the most intensive areas of the benchmark. Meanwhile, some scenes in AC Unity reveal occasional microstutter on the i5 that we don't see on Ryzen.

Given a straight choice, we'd take Coffee Lake over Ryzen for gaming, but well worth a mention is the cost of the surrounding system. Right now, there are some very cheap B350 boards out there that work beautifully with Ryzen 5. Meanwhile, Coffee Lake is only served by expensive, overclock-centric Z370 boards. We've used MSI and Asus boards extensively in our testing and they're excellent offerings, as you might expect, but there's a good argument that OC-centric mainboards are overkill for the locked Core i5 8400.

As a locked processor, the 8400 is omitted from this next test, which centres on overclocking. Our 8600K matched the 8700K in hitting 5.0GHz with a simple multiplier and voltage tweak in the BIOS (1.3v did the trick). In fact, we suspect that our i5 8600K sample may actually go higher as temperatures were significantly lower than our i7 running at the same clocks. We've stacked up both 5.0GHz Coffee Lakes up against Kaby Lake and Ryzen, all operating at the fastest speeds we could extract from them.

The delta between the stock 8600K and the 5.0GHz overclock ranged from nothing to 10 per cent in Crysis 3. It's clear that even with an overclocked Titan X Pascal running at 1080p resolution, CPU technology is so powerful that we can even hit GPU limits here and by extension, we do wonder how much more the Coffee Lake i7 may have in the tank. It's fascinating to see the Core i7 7700K remain competitive with the hexacore i5, despite the fact that it's two cores and 200MHz down against the 8600K.

We've also factored in both Ryzen 5 1600X and the Ryzen 7 1700 at 4.0GHz here (1700X and 1800X tend to have the same limits and perform identically in our tests). Crysis 3 remains Ryzen's friend, but the results demonstrate that even with our current GPU limit, Intel has much more overhead - and again, we believe that it offers a more future-proof system overall. This is not to say that Ryzen is not a great product - an overclocked 1600 or 1700 offers fantastic value, but the recommendation there would be to deploy Ryzen in a system not primarily targeted for gaming, where the additional threads can make a difference in productivity tasks, for example.

Overall, the results here continue to confirm our theory that Intel's hyper-threading technology allows its CPUs to punch above their weight - it's the key to the i7 8700K's gaming dominance, it's keeping the 7700K in contention as a superb performer, and looking further down the range, it's also the reason why the bargain basement Pentium G4560 offers such exceptional value.

If there's one regret we have with Coffee Lake's launch, it's that only premium Z370 boards are available - and this presents problems for prospective owners of the locked Core i5 8400. Unless you're looking to upgrade to the i7 K chip further on down the line, there's little point pairing a more budget-orientated processor with a richly equipped, costly motherboard. We've tried to simulate 'real world' conditions for the 8400 in this review by using a stock cooler and 2666MHz RAM - which will be the processor's bandwidth limit on the upcoming cheaper boards.

However, you can run the 8400 in a Z board and in theory, you can then extract extra performance from the processor by using faster memory - a state of affairs we've seen time and time again in both Intel and AMD processor reviews. In these final tests, we compare gaming performance in three benchmarks that have previously shown some dramatic improvements by scaling DDR4 bandwidth.

Both Core i5 8400 and i5 8600K at stock frequencies are tested below, and there are two takeaways. Firstly, running a Coffee Lake-S i5 with 2133MHz is not recommended - there is performance boost ranging from eight to 12 per cent in opting for 2666MHz RAM instead. However, the jump from 2666MHz memory to 3000MHz memory is much lower - generally just three per cent or thereabouts.

Memory bandwidth scaling is the only reason you'd want to put a locked chip like the i5 8400 into a Z370 board and as such, the meagre bump to performance from more expensive modules only serves to strengthen the case for waiting for the cheaper boards to appear. The Core i5 8400 is a value play, best suited to a value board - assuming it performs similarly, of course.

Intel Coffee Lake-S: Core i5 8400/ Core i5 8600K - the Digital Foundry verdict

When news first leaked of Intel's jump from four to six cores with the new Coffee Lake processors, the biggest question we had concerned the relationship between the last-gen i7 and the next-gen i5. Surely six physical cores could trump four, even if hyper-threading was enabled? The truth is that both i5 and older i7 have their own strengths and weaknesses, but by and large, the Core i5 8600K is a touch faster than the stock 7700K while the i5 8400 is slightly off the pace.

Bearing in mind that the Core i5 prices are lower - especially so with the i5 8400 - we're seeing a significant increase in value for the user, and in terms of price vs performance, there's no doubt that the locked Intel processor is the new mid-range gaming king. It's fast enough to take the challenge to the more expensive Ryzen 5 processors, and even with 5.0GHz overclocking in play, there are relatively few examples of the 8600K comprehensively outperforming the 8400 in a game-changing way, bearing in mind the frequency and price differentials.

With a good idea of how the i5 and i7 lines stack up now, our recommendations are pretty straightforward - the i5 8400 is a no-brainer in terms of what it offers and how it compares to the competition, but it should be paired with one of the upcoming, cheaper motherboards (or else the absolute cheapest Z370 model you can find). The i5 8600K effectively replaces the last-gen i7 - and for those willing to invest now to get a more lastable system in the longer term, that's a good bet. For Kaby Lake i5 K-chip owners feeling aggrieved by the lack of eight-gen compatibility, you can at least take comfort from the fact that the i7 7700K will be dropping in price like a stone now and will offer great performance on your system as a potential future upgrade.

For those doggedly holding onto their Core i5 2500K or 3570K chips, both of the new i5s offer substantial increases in performance - anything up to a doubling in frame-rate, even with a 4.2GHz overclock on the older chip taken into account. However, in the case of both, a cheaper route forward to refreshing your system is available with a 3770K upgrade paired with 2400MHz DDR3 - a process we've described in the past. Paired with a meaty overclock, your old platform can be brute-forced to its limits and will hand in some decent results, but you'll miss out on the additional performance and power efficiency offered by the new Coffee Lake processors.

In terms of where the Core i5 offerings leave the i7 8700K, well, it's pretty much the same old story. The i7 has always been there for best-in-class performance - but the eighth generation takes that to the next level, and the 8700K is a hugely impressive piece of kit. For those running 120Hz or 144Hz high frequency displays, the 8700K keeps your frame-rates higher and your frame-times lower, making it a great companion for a cutting-edge screen. However, if you're still in standard 60Hz display territory, the good news is that the new Core i5 is better than ever.

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