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
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
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.
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.
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
4 years ago, we released the 60.5mm Exhaust kit for the 3rd Gen Mazda 3 (both in
axle back and full cat back flavors). It’s certainly been a hit, but there has been a few of you longing for more noise. Today we are proud to announce the 80mm variant of our cat back exhaust for the 2014-2018 Mazda 3 Hatchback and Sedan! At this time, just the hatchback version is available, but we will have the Sedan version ready in just a few short months.
Now I know what you’re thinking, an 80mm exhaust seems excessively large for a naturally aspirated car making less than 200whp. But, hear me out because I think you’ll like what’s coming.
80mm piping allows for some unique & louder tones its smaller little brother can’t offer, but it wasn’t as easy as just using the old design and making the pipes larger. We had to do quite a bit of resonator experimentation and NVH analysis to get to the finished result with as little drone as possible. I’ll be upfront with you guys though, this is loud. It’s a good loud with tons of fun noises, but if you’re looking for something subtler, then our 60.5mm cat back or axle back may be a better fit. We do a good job of capturing the audio for you though so you have a good understanding of what you’re getting. Be sure to check out the product video to hear it.
For those wanting this more aggressive exhaust note, sound isn’t the only bonus. We thought about the appearance, and how we could take advantage of this time to tinker with the design. The 80mm does a nice job of not only filling the exhaust tunnel under the car, but the axle back portion is a bit more prominent when you catch a glance.
On the Hatchbacks, the exhaust tips got a nice size increase up to 100mm and they are slant cut to help follow the profile of the bumper. Sedans have also been upsized to 100mm tips, which were lowered slightly to ensure your bumper doesn’t melt with the large piping. This has the added bonus of making the tips a little more visible from the rear and side of the car. In both cases, the way the upsized exhaust accents the rear of the car provides an aftermarket look, that’s classy and somehow the way it always should have been from factory.