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
We are proud to release the https://corksport.com/2018-mazda-6-2.5l-turbo-boost-tubes.htmlCorkSport Upgraded Boost Tube for 2018+ Mazda 6 2.5T and 2016+ CX-9 2.5T. The CorkSport boost tube is larger, stronger, more reliable, and of course better looking than the OEM rubber tube. Increase throttle response down low, hit boost targets easier and future proof your ride for mods down the road with a simple 1-hour install. Read on for full details and be sure to check out the R&D blogs here and here for the backstory.
In case you haven’t read the previous blog installments, the CorkSport Boost Tube improves on the OEM boost tube by first strengthening the tube. Instead of using rubber with one reinforcement layer, the CS boost tube use silicone with 5 layers of reinforcement. Aside from the extra layers of reinforcement, silicone stays strong at high engine bay temperatures that may cause rubber to flex excessively. In addition, silicone lasts longer and will better resist cracking as your Mazda 6 Turbo ages. The OEM boost tube is made from materials very similar to the OEM Mazdaspeed 3 boost tubes that showed signs from aging extremely quickly, especially when subjected to higher than OEM boost levels. Cracking or splitting of the OEM tubes results in boost leaks and a poorly running car, definitely not what you want from your brand new SkyActiv 2.5T.
The added strength prevents the CorkSport Upgraded Boost Tube from expanding excessively when subjected to pressure. When pressure tested at 20psi (the largest pressure we have seen at the intercooler outlet), the OEM tube was shown to expand 12% at the internal cross-sectional area. The CS tube tested under the same conditions expanded 3x LESS. This difference would get even larger when subjected to the same pressure at a higher temperature. What does this mean for performance though? When you get on the gas, the boosted air will have to expand the tube before it can enter your engine. The less the tube expands, the easier it is to hit boost targets, and the better throttle response you have, especially down low in the RPM range.
The CS Boost Tube also is a larger inside diameter than your OEM tube. It is 3” through the middle vs. the OEM ~2.44”. Since this area of the charge piping system is directly ahead of the throttle body, this large volume of air has the same effect as it does with our GEN2 Mazdaspeed3 FMIC kit, reducing boost lag and increasing throttle response. For full info on why this happens, check out the release blog for that kit here. As a basic overview, the large volume of air right before the throttle body fools the engine into thinking it has a larger intake manifold plenum than it really does. While not as severe of an effect with just changing this boost tube, try it for yourself and see what you think!
Installing the boost tube is a little tricky due to where it is located, but we include high quality installation instructions to make it easier. Even so, it can be installed in an hour or less in most cases. We also include polished stainless steel T-bolt clamps to ensure a complete seal and add a subtle visual boost.
Be sure to check out the product listing for more pictures, the install instructions, and a detailed product video. Let us know if you have any questions, we’ll be sure to help you any way we can!
Lastly, if any of you are looking for a more serious upgrade, stay patient, our FMIC upgrade & full piping upgrade kit are coming soon!
If you’ve been paying attention to the CorkSport channels, you may have seen rumors here and there of a race header for the GEN3 Mazda 3 and Mazda 6 2.5L. While I can’t say too much on that just yet, I can give you a breakdown of the OEM exhaust header that’s hiding in the back of your engine bay. Analyzing the OEM part is usually our first step in creating a new performance part and I wanted to bring you all along for the ride. It’s surprisingly complex for an OEM manifold/header and some serious engineering went into it.
Excuse the dirty part, this OEM header has had a hard life! I imagine many of you have not seen the stock header as it’s in the back of your engine bay covered in heat shields. Taking the heat shields off gives us a glimpse of the craziness that is the stock header. Mazda has gone with a true 4-2-1 design (also known as tri-y) with an integrated catalytic converter and what appears to be equal length runners. Stay with me, I’ll explain what all that means.
The image above hopefully helps you visualize the 4-2-1 design. Starting at the engine, there are four exhaust ports from the head. Each exhaust port gets its own pipe, known as a “primary”. The primaries then pair together to form two “secondaries”. Finally, the two secondaries combine into one collector pipe, in this case heading directly into the catalytic converter. The three unions or “y’s” are where the tri-y name comes from. The 4-2-1 design was chosen by Mazda for a very specific reason. Check out the image below and Mazda’s explanation HERE.
Essentially, using a very high compression ratio causes very high exhaust gas temperatures. If too much of this exhaust gas is leftover in the cylinders for the next combustion cycle, knocking can occur. In addition, if you have a short 4-1 header or a log-style manifold you can suck exhaust gas into a cylinder before combustion as one cylinder can be on an intake stroke while another is on an exhaust stroke (see the upper image in Mazda’s diagram).
The 4-2-1 has two benefits to fight this. First, the long length means the exhaust gas takes longer to traverse the pipes, so one cylinder sucking in another’s exhaust is drastically reduced. Second, the cylinders are paired correctly to one another (1 with 4 and 2 with 3). Since the firing order is 1-3-4-2, each secondary is receiving an exhaust pulse at a regular interval. If you paired 1 with 3 for example, you would receive two pulses quickly, and then a large gap as the other two cylinders fired. This helps with exhaust scavenging as the pulse from one cylinder helps “pull” the leftover exhaust from the cylinder it’s paired with. These benefits can also be present on a long tube 4-1 if designed well but, there is a good reason why Mazda did not choose this option.
Typically a well-designed 4-2-1 will make more power and torque in the midrange while a well-designed 4-1 will make more power way up at the top of the RPM range. Since normal driving does not involve being at the top of the RPM range all the time, it makes sense that Mazda went with the 4-2-1. We will likely do the same as we want to retain the low knock characteristics of the 4-2-1, high midrange power & torque, and because the SkyActive 2.5L is a fairly low revving engine.
It appears that Mazda also went with close to equal length runners. This means that each primary section is the same length and each secondary is also the same length. This ensures the exhaust pulses are arriving to the collector or Y at uniform intervals. The easiest way to explain why this is a good thing is to visual the entrance ramp to a highway. When the cars entering the highway follow the “zipper” method for merging, the cars currently on the highway do not need to slow down. The high and entrance ramp merge and flow in a smooth and consistent rate.
However, if a surge of cars come down the entrance ramp to merge onto the highway you will get a back-up of cars on the entrance ramp and will disrupt the flow of cars on the highway. If the cars are exhaust gases and the highway is the exhaust pipe, you can understand why equal length can help. Again, we will adopt this strategy with the CS race header.
So far so good then, as Mazda has put a lot of thought into making a high quality stock header. However, as usual there are a few areas we can improve on. That’s coming in a later blog though so you’ll have to stay tuned for more details! Let us know if you have any questions or thoughts down below.
-Daniel @ CorkSport