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.

Testing – CorkSport External Wastegate Housing for Mazdaspeed

Turbo EWG

Why EWG?  (it’s just about awesome turbo noises)

We hear this alot as the Mazdaspeed platform continues to grow and the 450-500whp build becomes the status quo. Following up the EWG Housing Design & Details Blog about the new CorkSport EWG Housing, we want to share some testing data and differences we saw between an IWG (internal wastegate) and EWG (external wastegate) setups.  

Details about design, flow, placement, data, and feedback from our CST4 EWG Beta Tester.  

IWG vs EWG comparison on the CST4
IWG vs. EWG on the CST4

Let’s jump right in!  First up is a spring pressure comparison between the IWG and EWG housing on a CST4 turbocharger.  Let’s first define what “spring” pressure is: this is the resulting boost pressure with 0 added wastegate duty cycle.  AKA we are not trying to add boost pressure.

Immediately you can see some very obvious differences.   The IWG setup has a taper up boost curve that could be considered boost creep.  Some boost creep is ok, but an excessive amount may reach the capacity of the fuel system or other systems in the vehicle.  In this setup that is not the case, but it does show that the IWG is at its limits for boost control.

With the EWG setup you see a much different curve.  The boost builds a few hundred RPM later (due to the larger 0.82 A/R) then climbs right to the spring pressure and then settles to a consistent plateau; very predictable and controllable.  

CAD EWG and IWG Designs
CorkSport EWG and IWG Designs

Now let’s look at the design to better understand why.  On the left is the EWG turbine housing with a 0.82 A/R and on the right is the IWG turbine housing also with a 0.82 A/R (we don’t want the A/R to be a factor in this review).   

The EWG housing has a very efficient flow path for the exhaust gas to reach the EWG control valve along with a much larger path to flow.  Both of these features provide excellent flow and thus control of boost pressure.

The IWG housing uses a side port in the turbine scroll to exhaust gas.  In this setup, the exhaust gas must make an abrupt turn and pass through a much smaller port.  Both of these issues reduce boost control.

EWG and IWG Explained

Here is a diagram showing placement of an EWG in the exhaust pre-turbine.  Granted we are comparing a EWG and IWG, but the concept of flow is the same.  

Exhaust gases will always take the path of least resistance and if the turbine wheel is the easier path than the wastegate then boost control will be more difficult.  

Internal and External Wastegate performance chart
(Left) Internal Wastegate Setup | Common Issues
(Right) External Wastegate Setup | Optimized Setup
Click to Expand

This graph was shown in the last blog, but we want to show it again so you can directly compare it to the data graph below.  

Below is the boost curves for the CST5 in both IWG and EWG setup.  Alone each graph actually looks really good, but when overlaid you can see some interesting differences.  

CST5 Dyno testing with IWG and EQG setup

IWG vs. EWG on the CST5

The purple IWG graph has a crisp spool and then flat-lines at approximately 30psi with a slight fall off at 6500rpm.  The CST5 IWG setup does control boost really well, but holding the turbo back at spool up and not over-boosting or spiking was a small challenge.  An abrupt boost curve like this can make the car somewhat difficult to drive because the torque “hits” very hard and you lose traction.

The EWG setup was a bit more controllable.  Not only did the CST5 turbo spool a bit sooner, but we were able to better control the spool up boost curve so we could create a torque curve that was more friendly to the FWD traction.  This makes the car more fun to drive. Looking at the higher RPM range we were also able to hold boost more consistently to 7500rpm.

CorkSport External Wastegate

We hope you guys and gals are as excited for the EWG options for the CST4, CST5 and CST6.  They really are an awesome setup for any driving style and power goal.  

Thanks for tuning in with CorkSport Performance.

-Barett @ CorkSport

CST5 Spools!! Testing and Validation

We’re back on the new CorkSport turbocharger lineup again with today’s blog, this time focusing on the testing & validation of the “medium big” turbo, the CST5. Just in case you missed it, the CST4 (formerly known as the CorkSport 18G) is getting some company to go along with its new swanky name. Check out the full lineup here and the design behind the CST5 here. Now that you’ve read all that, let’s get into what you’re really here for, testing & dyno numbers.

We started with the internal wastegate option, to validate the CST5 for drop-in fitment. Since we’ve had great experience with the drop-in CST4, we knew how to design a turbo around the tight confines of the Mazdaspeed engine bay. The CST5 fit great in the OEM location with just a few minor revisions for proper fitment. It looks pretty good in there too if we do say so ourselves!

Next the car got put on the dyno for tuning and to push the new CST5 to its limits. With a little help from our friend Will at PD Tuning, the CST5 was soon putting down some impressive numbers. We started off with a “calm” boost level of ~25psi. This netted us 450WHP and spool time that surprised us, achieving 20psi by 3500-3600RPM. Turning up the boost and pushing the turbo to its limits, we achieved 519WHP at ~30-31psi on Barett’s built GEN1 MS3. Check out the dyno graph below.

Taking the car out on the street surprised us further at just how early the car was building boost for this size of turbo. Road logs showed that we were making 20psi slightly sooner than on the dyno (3400-3500RPM) but even more surprisingly the CST5 was making 30psi by 3700-3800RPM! Obviously this is an aggressive tune that would most likely kill a stock block, but, the CST5 can be tuned to be stock block friendly and still make good power.

Then came the testing on the EWG variant of the CST5. We had developed fitment for the CST6 which meant the CST5 had no issues upon install on both MS3 and MS6. Next was a quick retune and some power runs. The larger swallowing capacity of the EWG housing meant some extra power at peak, yet spool was nearly unchanged. We made 525WHP at the same ~30-31psi.

Comparing the IWG and EWG turbine housings you can see a small variation in the graphs.  This variation is mainly due to the change from internally waste-gated and externally waste-gated.  The EWG setup provides more precise boost control through the RPM range. The EWG setup allows us to better tune the “torque spike” around 4200rpm vs the IWG setup.  For peak power the IWG and EWG housings are within the margin of error which makes since because they are both 0.82 A/R housings.

Further supporting the IWG and EWG setups, both options allow you to tune the spring pressure so you can better setup your CST5 and Speed for the fuel and boost levels you want and of course the most noticeable difference is what you hear. What’s an EWG without a screamer pipe!  

Wrapping up testing showed exactly what we were hoping for with the CST5: a great middle ground between the existing CST4 and the upcoming CST6 that can be used on both high powered stock block and fully built cars. Our testing continues as this blog is written as the CST5 is being beta tested by a close friend of CS with a freshly built Dankai 2.

There’s more to come from the new CorkSport turbo lineup so stay tuned for more info on the CST5, CST6, and EWG housings.

-Daniel @ CorkSport

CST6 – The CorkSport Stock Flange Record Turbo

A few weeks ago we discussed some of the design intent behind the CST5 turbocharger for the Mazdaspeed platform.  Today, we want to follow up with the CST6. The CST5 and the CST6 both were a result of CorkSport’s desire to develop a new stock flange turbocharger that goes beyond the power limits of our FANTASTIC  CST4 Turbo.  

During development of a higher power stock flange performance turbo we found that we were asking too much of the CST4 Design.  The result of our efforts are the CST5 Turbo which you can see here and the CST6 Turbo which we are about to dig into.  

In this blog we’ll dig into the wheel sizing, the CHRA, and some of the challenges we faced in the development and testing.  

The compressor wheel utilized on the CST6 the well-known and trusted GEN1 GTX76.  The GTX76 compressor is rated for 64 lb/min and is capable of boost pressures that will require a 4bar MAP sensor upgrade.  Like the CST5, the compressor housing is a 4inch inlet with anti-surge porting.

Unlike the CST4 and CST5, the CST6 uses a completely different CHRA and bearing system, and for good reason.  As the turbocharger wheel sizes increase so do the weight and potential boost pressure. This results in higher loads on the wheels, turbine shaft, and bearing system. To increase durability and performance of the CST6, we opted to move from a conventional journal bearing design for a more modern and robust ball bearing design.  

The ball bearing system improves durability and stability for high horsepower/high boost operation along with improved spool and transient response.  Changing the CHRA did pose some new challenges however. Ease of installation has always been a key feature with CorkSport products and that’s not lost with the CST6.  The CHRA has been modified to support use of the OE oil drain line and all necessary oil feed components and coolant components are included for seamless installation.

Like the CST5 Turbo, we’ve put focus on the wheel size ratio and have validated it’s performance. The CST6 Performance Turbo uses the Gen1 GTX76 compressor wheel paired with the Garrett GT35 turbine wheel…aka GTX3576r.  This wheel combination provides us with a ratio of 1.12 which falls well within the rule of thumb discussed the in the past CST5 blog.

In testing, we found that increasing the size of the turbine wheel from a GT30 to a GT35 with the same GTX76 compressor wheel resulted in more top-end power and no penalty in spool time.  This combo also provided a good power delta from the CST5 to better provide an optimal power option for the community. Since then the CST6 has proven power at 600+whp at ~34-35psi and testing will continue past 40psi.  

The initial testing of the CST6 started with an internally wastegated turbine housing as that was the original goal with the CST5 and CST6.  However, it quick became obvious that a turbocharger of this size and power potential could not be safely controlled with an internal wastegate.  The amount the wastegate port and “exhaust” or flow was not nearly adequate for proper boost control.

Boost would creep to nearly 26psi with no signs of tapering off.  Nevertheless we continue testing knowing that auxiliary fueling was necessary.  Once the CST6 power and durability was validated we moved to design a turbine housing that could provide the necessary boost control and power potential.

Above is the removed CST6 internally waste-gated housing.  In our testing we pushed the turbo to nearly 600whp with 40gph of methanol auxiliary fueling.  This amount of heat combined with a turbine housing that was literally being pushed to its limits resulted in a great learning experience.  As you can see, the turbine housing was cracking! The GT35 turbine wheel and power was just too much.

From this discovery and analysis we developed the EWG turbine housing with the CST6 in mind.  The scroll size was increased, wall thickness increased in critical areas and the 44mm EWG port added.  

With the use of the EWG turbine housing, boost control is now spot on and can easily controlled from spring pressure to an excess of 35+psi.  Stick around as we continue to push the limits of the CST6 as we continue testing and validation of the CorkSport V2 Intake Manifold w/Port Injection.  

Thanks for tuning in with CorkSport Mazda Performance.

-Barett @ CS

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