CorkSport CST4 vs. OEM K04

For the last four years, we’ve been asked countless times if the CST4 is a direct replacement for the OEM K04. With the number of cars experiencing their 2nd, 3rd or even 4th owner, this question is being asked more frequently.  While CorkSport’s intent is to freely share information across the Mazda community, we cannot be everywhere all at once, and we will most likely miss the exact moment the debate explodes on your favorite Facebook page. For that reason…

Today, we’re setting the record straight: The CST4 is a drop-in UPGRADE from the ground up, and by no means is it an apples to apples comparison with the asthmatic K04.

How Does It Work?

Before we dive into the dirty details, let’s go over how a turbo works. A turbo is comprised of three major sections: the turbine, the center housing rotating assembly (CHRA), and the compressor. The compressor brings in clean air, and as the name implies, compresses the air before sending it through the CHRA and into the cylinder intake. With extra air in the cylinder, the engine is able to burn fuel at a faster rate. This process creates exhaust, which leaves the cylinder and is sent past the turbine, causing the turbine to spin. The turbine and the compressor are placed on the same shaft, such that when the turbine spins, the compressor will also spin. By burning fuel at a quicker rate, we are able to create more power in the engine, which eventually finds its way down to your tires, propelling you forward. If you’re craving more details on how turbos work, make sure to check out our white paper on the CST4. Now let’s get back to what this means for the CST4 and K04.

CST4

Is The CST4 Reliable?

The CST4 continues to prove itself as one of the best bolt-on options coupled with the needed reliability to withstand the additional demands of chasing 400 WHP  – something the KO4 cannot accomplish. This is due to an upgraded center housing rotating assembly (CHRA) which has a larger center shaft and larger bearings than the OEM turbo. The CorkSport turbo also sports a performance journal bearing with a full 360° thrust collar, which is what allows the turbine shaft and compressor to spin freely. The OEM K04 turbo comes standard with a limited 270° thrust collar.

Does The CST4 Have Increased Airflow Over The K04?

CST4 and K04 compressor comparison

Next comes wheels, which is where the CST4 really shines. Shown in the image above, we have the CST4 on the left and the OEM K04 on the right. The CST4 is 12% larger on the compressor inducer, and 21% larger on the exducer than the OEM K04. Combined with the use of a taller wheel (green line), every revolution of the CST4 not only brings in a greater quantity of air into the compressor, but has a higher airflow capacity, thus moving a greater volume of air. For those of you that need a few more key specific numbers; The OEM K04 uses a 45mm inducer; 56.25mm exducer cast compressor wheel, while the CST4 uses a 50.5mm inducer and 68.1mm exducer forged billet compressor wheel.

On the turbine side, the OEM K04 uses a 50.1mm inducer and a 44.5mm exducer 12-blade cast wheel. The CST4 uses a 56.2mm inducer and 49mm exducer high-flow 9-blade design. Again, the CST4 outshines the K04 with the turbine inducer being 12% larger and the exducer 10% larger. The 9-blade design has two key benefits: more peak exhaust flow as there is less material in the way of flow, and 21% lighter for a faster spool time.

CST4 Side View

The final component is the compressor and turbine housings. The K04 uses restrictive housings that cannot keep up at higher RPMs, and especially at higher boost levels. You can feel this as your stock turbo “runs out of steam” up above ~5200RPM. The CST4 housings may fit exactly in the OEM locations and use the OEM hook up points but that is where the similarities end.

Both the compressor and turbine housings were increased in size, increased in A/R, and optimized for the stock inlet and outlet sizes to provide better top end capabilities. The compressor ended up at a 0.53 A/R while the turbine ended up at a 0.66 A/R. This combo of housing and wheels keeps power all the way to redline, and in initial testing showed a 50WHP gain at the same boost pressure.

How Much Power Does The CST4 Make?

So what does all of this mean in terms of power? We’ve seen the OEM K04 pushed way out of its comfort zone and make in the 350-360WHP range with the right supporting mods. This is far out of the efficiency range of the little K04, and it’s a ticking time bomb when running at this power level. The CST4, on the other hand, is perfectly happy running in the 400WHP range all day, again, with the right supporting mods. We’ve even seen it pushed to its limit in the 450-460 range.

For those of you more interested in boost pressures, the K04 can hit a max of ~24-25psi in the midrange before it’s out of its efficiency range and starts producing just heat.  At redline, the K04 is typically at a max of about 17-18psi. What you feel as your car seems to stop accelerating after ~5200RPM on the stock turbo. The CST4 does a lot better, hitting a max of ~29-30psi in the midrange but carries the high pressure into higher RPMs, with peak boost pressure at redline of ~26-27psi. This keeps you pushed into your seat with a smile on your face!

CST4 Mounting Point

How Quickly Does The CST4 Spool?

We get a lot of questions on how fast this turbo spools, so let’s take a moment to discuss both. The OEM K04 spools very quickly since its housings and wheels are so small. If tuned incorrectly it can spool almost instantly and kill blocks with an extremely low-RPM torque spike. The CST4 also spools quick, making full boost by approximately 3300RPM on most cars. The big difference is that the CST4 carries power out to redline instead of falling off as the K04 does. To be clear, you still have to be careful with the CST4 as it too can kill a stock block with too aggressive of a tune.

Is The CST4 A Drop-In?

So bringing things full circle, the “drop-in” aspect of the CST4 means you can run it with almost no other supporting parts, only a HPFP upgrade, access port, and a tune are required. It also means that it hooks up directly to the OEM inlet and outlet flanges so that there is no excessive modification required to make the turbo fit. We even include new studs, lock nuts, gaskets, a custom upper coolant line, new coolant and oil crush washers, and the correct oil feed banjo bolt so there is no hassle of finding replacement hardware, gaskets, or lines to make your turbo function. We do strongly recommend picking up a CorkSport EBCS to best control boost on your CST4. We have also found that the stock intake size will be maxed out at around 18-19psi on the CST4. To get more power from there, a 3 inch or 3.5-inch intake will be needed.

By no means is the CST4 a K04 though as it’s larger and has much higher horsepower capabilities. For those of you more familiar with Garett turbos, the CST4 is just a smidge bigger than a GTX2867.

If you want even more info on what makes the CST4 tick be sure to check out the white paper on the subject HERE. As a final afterthought, remember that the CST4 is getting an EWG housing option in the coming months, for added features (and noise!) that just don’t come with the K04. Stay tuned for that, and be sure to ask any questions you may have.

Mazdaspeed Turbo – Choose Your Boost

May of 2015, CorkSport launched its first high performance drop-in turbocharger for the Mazdaspeed platform.  Fast-forward almost 4 years and CorkSport again is about to redefine what a stock flange turbocharger for the Mazdaspeed platform can truly be.  

The original “CS Turbo” is now the CST4 to follow the turbo line-up that is soon to launch.  The CST4 took a fresh approach to “big turbo” with all the included hardware, gaskets, and of course direct drop-in fitment.  It removed the guess work for a quick and easy installation, but the benefits didn’t stop there. This “little big turbo” packs a punch for its compact TD05H-18G wheels.  

With the CST5 and CST6 just around the horizon it would be easy to forget about the tried and true CST4, but don’t worry this Mazdaspeed Drop-In Turbo got some new love also.  You will now have a EWG housing option for the CST4. You can pick it up in EWG setup from the start or if you already have a CST4 that you love, you can get the EWG housing kit to do the upgrade yourself.

Moving onto the CST5 & CST6 the possibilities for the MZR DISI have moved up significantly.  What started as a single “bigger big turbo” has morphed into two “bigger big turbos” that, we feel, better provide for the various power goals of the community.  

We present to you the CST5

The CST5 bridges the gap between drop-in performance and big turbo power.  The journal bearing CHRA uses a hybrid TF06-GTX71 wheel setup that provides more top-end than the CST4 with minimal spool and response penalty.  Upping the big turbo feel is a 4in anti-surge compressor inlet which will require an up-sized intake system.

Unlike the CST6, the CST5 will be offered in both internally waste-gated and externally waste-gated setups.  This provides you with the flexibility to setup your Mazdaspeed just how you see fit and both have been proven 520+whp on our in-house dyno and tuning courtesy of Will Dawson @ Purple Drank Tuning.

Now… We present to you the Stock Flange Record holder…the CST6

Image: Mazdaspeed-6-big-turbo

The CST6 redefines what the community thought was possible from the stock turbine housing flange, but first some details.  The ceramic ball bearing CHRA uses a GTX3576r wheel setup that clearly out powers the CST4 & CST5, but that’s point remember?  

The CST6 is a legit big turbo, spool will be later, but still sub 3900rpm for full boost, however a turbo setup like the CST6 is not intended for low-end response.  If top-end power is your goal, the CST6 will deliver. In-house testing has pushed the CST6 to 633whp at a fuel limited ~33psi and 7900rpm redline.

Unlike the CST4 & CST5, the CST6 will only be offered in EWG setup.

In the coming months, we will be sharing more information about the CorkSport Turbo Line-Up; the design, the testing, and validation of each.  For more information about the CST5 & CST6 along with the new EWG turbine housing option, check out these sneak peek pages.  

Thanks for tuning in with CorkSport Mazda Performance.

-Barett @ CS