CorkSport External Wastegate Housings: Design and Product Details

During our development of the new CST6, it became clear that an external wastegate (EWG) would be the only option for optimum performance and boost control — Read about the CST6 design here. Since we were already developing a housing for this turbo, we just HAD to make similar options for both the tried and true CST4 and the upcoming CST5. In today’s blog we will cover what makes the new CorkSport EWG Housings tick and go through how the systems are different from a typical internal wastegate (IWG)setup.

CorkSport External Wastegate for Mazdaspeed
Mazdaspeed External Wastegate Housing.

How The CorkSport EWG Housing Works

Starting with the basics, the CorkSport EWG housings replace the turbine or “hot” side of your CST4 or CST5 turbocharger. The CS housings eliminate the small “flapper valve” of the internal wastegate with a path to a standalone wastegate actuator. While the housings will not come with the actuator itself, they will include everything you need to mount up a Tial 44mm (or equivalent) EWG, with the option to grab a screamer pipe to go with it.

CorkSport MazdaSpeed3 external wastegate
Mazdaspeed 3 external wastegate.

EWG Housing Placement

The placement of an external wastegate is very important for proper boost control. We chose the location of it carefully as the offshoot from the scroll of the turbine provides an optimum path into the wastegate. As shown in the diagram below, a shallow angle into the wastegate offers the best exhaust flow path which results in optimum boost control.

Wastegate Placement Diagram
Mazdaspeed wastegate placement diagram.
Click to Expand

A Properly Sized Wastegate

Sizing is also important for boost control. Too large of a wastegate is hard to fit on the car and can be difficult to control boost as it does not need to open much to flow a lot of volume. Too small of a wastegate can cause over boosting from a lack of flow. For most Mazdaspeed applications, 38mm and 44mm sized wastegates are most commonly used. We chose the 44mm option for its better usability on the street. Since a street car is not using the turbocharger at peak efficiency 24/7 and is usually at a relatively low boost level, the larger 44mm wastegate will more effectively vent the excess exhaust gases.

EWG Tial for Mazdaspeed 6
External Wastegate Tial for Mazdaspeed 6.

Choosing An Internal or An External Waste Gate

Now on to a big question: why an external setup over and internal setup? While they both have their pros and cons, the biggest answer is better boost control. In general, an external wastegate setup will control boost more accurately and respond quicker to changes in boost than an internal one. While the CST4 and CST5 have been optimized to work with IWG, an EWG setup will almost always have better boost control. It should be noted that, generally speaking, an EWG will have higher peak power capabilities. First, back pressure in the exhaust system is reduced due to the removal of the exhaust gas that would usually cause turbulence as it exited the IWG right near the turbine exit. Second, an IWG setup can reach a limit in flow where literally no more gas can flow through the turbine & IWG port. Most EWG setups do not have this problem as the EWG port can be larger & the wastegate gases do not have to exit in the same location as the turbine gases.

Check out the diagram below, as it illustrates a common problem with internal wastegate setups. A boost spike as desired pressure is achieved then inconsistent boost as RPM increases. External setups (right diagram) typically do not have either of these issues and can achieve a near flat boost curve.

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

The CorkSport External Wastegate Design

Moving to an EWG design allowed us to tweak the design of the housing itself. The CorkSport external wastegate housings all use a ~0.82 A/R. This number is a ratio that is determined by the geometry of the scroll. I won’t go too much into it, but for more info be sure to check out Barett’s turbocharger white paper here. This is larger than the CorkSport internal wastegate setups. Essentially, the increase means you may sacrifice a little bit of spool time but gain peak power capabilities at high RPM.

In addition, the volume of the scroll itself was increased. This is referred to as “swallowing capacity”. By increasing this volume, the turbine housing will be able to air more efficiently at peak, again increasing your max power potential. This is especially important when you have a high flowing ported head or an upgraded exhaust manifold. Check out the CAD models below, you can really see the difference in the scroll size.

CorkSport Turbine Housing
CorkSport Turbine Housing for Mazdaspeed 3.

I almost forgot the best part! An external wastegate setup gives you the option to run a screamer pipe which sounds amazing while under boost. While screamer pipes are sold for race use only, they add far more excitement to a wide open throttle pull! Stay tuned for more, next time we will be sharing testing information & some more teaser shots.

-Daniel @ 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

Inside look: CorkSport Turbo Design

The development and evolution of the CorkSport Performance CST5 and CST6 turbochargers are uniquely intertwined.   We’ll be honest, we started with the goal of a single larger turbo than the CST4 in mind, but as development progressed we were not getting the exact results we wanted. We wanted fast spool & transient response, huge power, and to retain the internally wastegated system.  Something had to give…we realized that we were asking too much from a single turbocharger, thus we redefined what we wanted and realized that two separate and focused turbochargers for the Mazdaspeed platform was the ideal choice.

Today we will focus on the design around the glorious CST5, specifically the theory and design around the wheel selection for the CST5 and why it works.  

The compressor wheel utilized on the CST5 is the well-known and trusted GEN1 GTX71.  Compact and efficient, this compressor is rated for 56 lbs/min flow rate with a relatively high-pressure ratio threshold.  Paired with a 4-inch anti-surge compressor housing and we have a very versatile and responsive compressor setup.

Now here is where the design begins to deviate from the standard path.  The turbine wheel is a MHI TF06 design that is designed for high performance applications.  The TF06 turbine wheel is the key to the performance of the CST5. Let’s see how and why below.

If you are unsure of the turbine wheel size don’t worry, that will get covered shortly.  For comparison, the MHI TF06 is very similar in size to the well-known GT30, but there are a few very specific differences that affect performance.  

The first and most obvious difference is the number of turbine blades; this difference has a couple benefits. First, less weight; even a small difference is weight can make a significant difference in the spool and transient response characteristics of the turbocharger.  Second, reduce flow restriction; with one less blade the “open” area through the turbine wheel exducer is increased which increases the peak flow potential for top-end power.

Next are the less obvious differences.  The GT30 has a 60mm inducer and 55mm exducer which equates to a 84trim turbine wheel vs the TF06 with a 61.5mm inducer and 54mm exducer which equates to a 77trim turbine wheel.   

There are two key values to pull from this:  First, the turbine wheel inducer directly relates to the peak flow of the wheel and the overall wheel size balance which we will cover next.  Second, the turbine wheel trim affects the spool and response characteristics of the turbocharger. The smaller the wheels trim the faster the spool and response.  

Alright here is the most important and commonly overlooked aspect of a turbocharger.  There is a rule of thumb when sizing the compressor and turbine wheels for a turbocharger.  

If the turbine is too large then the turbocharger will be very “lazy” and have trouble building boost.  

If the turbine is too small then the compressor may be overpowering the turbine wheel causing excessive exhaust gas buildup that can rob power even though you may be running a very high boost pressure.  

So what is the right balance?  From our experience in turbocharger design, development and validation along with industry professionals we have consulted there is a rule of thumb we have found when sizing the compressor and turbine wheels.  The exducer of the compressor wheel should be 10-15% larger than the inducer of the turbine wheel as shown in the image above.

So why does this work?  Well let’s look back a bit first.  Many think you can just install a larger and/or higher flowing compressor wheel onto the turbocharger to make more power.  Now that is true to a point, but quickly the approach becomes very inefficient for the engine. Forcing more air into the engine without improving the flow out of the engine can only go so far.  

Everything that goes into the engine must come out right?  Increased A/R sizing and turbine wheel sizing is the key to exhausting all the gases from the engine efficiently, and efficiency is key to making power.

With both the CST5 and CST6 development we focused on the overall performance of the engine, not just the development of a high performance turbocharger.  

Thanks for tuning in with CorkSport Mazda Performance, more to come…

-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