When upgrading a 6.7 cummins turbo charger is it important to understand that you can create problems. In summary, the larger turbo is increasing the amount of airflow being pressed into the engine, as well as being removed from the engine. However downshifting with combination of using the exhaust brake can cause compressor surge. Compressor surge is when the turbo is trying to compressor more air into the engine than the engine is capable of disposing of out the exhaust system. The reason this is such a major issue with VGT model turbo chargers is because the VGT system is used as an exhaust brake. When the VGT System is used as an exhaust brake, it spools the turbo to high rpm, because it is closing the Vane for the VGT system to work as an exhaust brake which is also the same way the VGT System is used to spool up the turbo charger.
There is a fix for this issue. A blow off valve kit can be used to relieve the pressure between the turbo and the intercooler, but you need a kit designed for diesel. XDP has the best kit that i have seen, that is available here. Compressor surge becomes a major problem when the exhaust brake is on and the driver is down shifting into lower gears which is increasing the output boost from the turbo charger, but at the same time restricting the pressure from escaping turbo. I didn’t have a problem for years with the modified turbo chargers, because the problems of the upgraded turbo chargers for the 6.7 cummins is dependent on the driver. It depends on how the driver operates the vehicle. If the truck is has an automatic the transmission is controlling the gear selection when driving, the driver can also select tow mod, which will engage engine braking. With a manual transmission, the driver controls how low of a gear they want to select, in which the driver can control the amount of strain on the turbo.
The stock turbo failing from engine braking with the exhaust brake is not as common, but it is common for the strain of turbo to be heard, which is commonly referred to as turbo bark. Turbo bark term used to describe the sound of compressor surge. It is much more common for an upgraded turbo to break in compressor surge conditions because of the increased airflow. The t51r version of the turbo is much more likely to have problems because the anti-surge, doesn’t allow as much relieve as a stock compressor housing without the t51r.
If you plan to use an upgraded turbo, but you don’t plan on towing, you likely wont need the blow off valve, but if you plan on towing then you need the blow off valve to protect your turbo from ripping the blades off due to compressor surge.
