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
Today we’re taking another dive into OEM Mazda parts to better understand how they function. Specifically, OEM suspension springs, since there are CorkSport Lowering Springs coming soon for the 2018+ Mazda 6 2.5T. While a simple concept, springs are very important to the handling, appearance, and comfort of your vehicle.
The new Mazda6 Turbo uses a lot of the same components as the GEN3 Mazda3 and Mazda6, however the suspension has been optimized for the new “premium” feel and to deal with the extra weight that comes when adding a turbo. The SkyActiv chassis has largely remained the same though, with the same MacPherson strut front suspension and multi-link rear suspension shown below.
Now, onto the springs themselves; both the front and rear suspension of the Mazda 6 use standard compression springs. The springs job is to support the weight of the vehicle when at rest and adsorb impacts when hitting bumps or going quickly around a corner. That’s it. Seems simple enough right? Since the springs are the parts of the suspension that “suspends” the vehicle though, their characteristics and how they interact with the rest of the suspension system are critical.
There are two main characteristics that define a spring: rate and free length. Both are pretty easy to understand. Free length is simply the length of the spring with no weight or force acting on it. So set a spring by itself on a table, measure how tall it is, there’s your free length.
Spring rate is a little more complex, as it is the measure of how much weight it takes to compress a spring a given distance. So, if you have the same weight and put it on two different springs the one with the higher rate will compress less. The rate is usually measured in kg/mm (often shortened to K) or lbs/in.
For example, if you had a 2K spring and a 4K spring and applied 100kg to each, the 2K would compress 50mm and the 4K would only compress 25mm.
What do these measures mean for your car though? If we keep the rate the same but only change free length, the shorter the spring, the lower the car. For a given car, a spring can be too short, causing poor ride (sitting on the bump stops all the time), or the risk of a spring coming out of place, causing noises or at worst, the spring falling out of the vehicle.
If we change the spring rate and leave the free length the same, things are a little more complicated. The higher the rate, the stiffer the ride is, plus your ride height will increase. Since the weight of the car is not changing, the higher rate spring will now compress less when the car sits on it, meaning your car sits higher at rest. Too large of a rate and your OEM shocks cannot keep up causing a bouncy ride, and vice-versa if too soft you are hitting
bump stops over the smallest bump. Obviously there is a balancing act to get the spring rate and free length correct for the application for the best in appearance, handling, and comfort.
Now that the basics are covered, let’s look specifically at the Mazda 6 2.5T. The OEM springs give a good ride as to be expected (likely very soft spring rates) as this can be a huge issue for potential customers if the car ride quality is harsh. Handling is decent overall but has a few quirks. When going around a corner quickly, the car rolls over onto the rear springs excessively before settling, and getting through the corner. When at the limit of traction, the car understeers severely, like most cars sold today.
Finally the ride height is pretty high, likely to prevent any issue with driveways saying hello to the new front fascia. Interestingly, the MZ6T sits a little higher in the rear; we think to ensure enough suspension travel in case there’s a full load of passengers and a full trunk.
For further analysis we also had the OEM springs tested for rate and ended up with the following: 3.05K front, 5.05K rear. While these numbers are fairly arbitrary right now, they are a necessary data point to have when designing lowering springs. These rates also contradict a very common misconception. Many people think that because there is less weight in the rear of a front wheel drive car, the spring rates must be softer in the rear for a good ride & handling. This is simply not true in most cases, after all why would Mazda do the opposite? Due to the design of the rear suspension, the spring is basically being pushed on by a lever. This means the spring needs to be stiffer in order to support the same amount of weight as if the lever wasn’t there.
So overall, the OEM springs are good, but have plenty of room for improvement. I just touched the surface of suspension design and as we go through more of this project we’ll get into dampers, natural frequency, and much more. Stay tuned for more info and if you have any questions, don’t be afraid to ask!
-Daniel @ CorkSport
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