We are asked often why we choose to retain the factory GT22 CHRA and Exhaust wheel, and not use a GT28 CHRA and wheel. In mid-2012 we explored both options while testing. We found the GT22 in its proper designed housing to outperform its GT28 counterpart in the stock Mercedes exhaust housing.
The GT22 family is the newest and most advanced turbine design introduced to the Garrett gasoline turbocharger catalog. This high flow design flows almost equivalent rates as its older sibling, the 53.8mm NS111 in the GT28, yet is more compact and has lower inertia.
It is slightly smaller, but more efficient, and proof that bigger is not always better. The process of adapting the GT28 into the factory Mercedes exhaust housing requires extensive machining of the volute profile, an increase in radius and flow width to install the larger, older exhaust wheel. This reduces the A/R of the already small housing, as well as reducing the yield of how much exhaust gas actually drives the exhaust wheel forward via the volute.
A/R is the inclination of the flow as it reaches the wheel. Larger AR = less restriction
A/R Ratio= AREA/RADIUS
Example: .82 AR flows more than .63 AR housings.
The larger the A/R, the more flow capability of the turbine wheel in the housing. Turbine efficiency is also negatively affected by smaller ARs. This is why some larger AR turbine housings spool just as quickly as a smaller variant.
Machining to fit a larger wheel into the same exhaust housings effectively reduces the AR even further and with it, the flow potential and turbine efficiency of the GT28. The result is operation at an even lower level of efficiency than the OEM configuration at multiple points. Although the wheel is larger, less exhaust gas will flow through the turbine. This is reducing the efficiency of the system. In the performance world, efficiency is power, drivability, time to torque, response, back pressure and again Power!
The next problem with switching over to the GT28 is the mess of water and oil lines needed for the conversion. All lines need custom adapter fittings that increase the possibility of having leaks and problems. Also, since our turbo upgrade is identical on the outside to the OEM turbochargers, it is unlikely that anyone will know that the turbochargers have been upgraded unless they are told; this includes technicians at Mercedes Dealers.
The Power level, refinement, and OEM fitment our customers expect from our offerings is not in line with taking shortcuts with turbo upgrades. Placing a larger turbine in an already optimized turbine housing and living with the consequences of such a compromised design and engineering mismatch is not something we would ever offer to our customers.
After countless hours of design engineering and testing we have developed the best option for our customers. Our 5 axis CNC Impeller and compressor cover contour is the right option for the M157 platform based on a stock exhaust housing. With stealth installation and proven performance, there is no reason to sacrifice.