We have received many questions about turbochargers. One being about the groove in the compressor housing (also called the MWE groove, MAP WIDTH enhancement groove), the purpose and effect of changing it.
This groove also called the Surge Groove. This groove is placed in a turbo to move the surge line on the compressor map to the left and widen the map. The larger the groove the better surge characteristics, but at the sacrifice of top end air flow. In other words the larger the groove the less surging a turbo will do, but at a slight sacrifice to max air flow.
A groove that is properly sized and positioned will allow a larger compressor wheel to be used, which would normally surge. So while a surge groove causes a small sacrifice to top end, it allows a much higher flowing compressor wheel to be used. This ultimately flows much more air than a smaller wheel with a small or no surge groove. (This is exactly what we do on our D-Tech 64mm, a huge flowing wheel with lowered surge line.)
What is surge? Surge is when boost pressure exceeds the driving force of the turbine/compressor wheel, ultimately stalling or reversing the direction of air flow and the spinning wheels.
How does the surge groove help with this? A surge groove is positioned just after air begins to pressurize in the compressor housing. This allows some of the pressurized air to escape and return into the intake increasing the pressure in the intake at low turbine speeds. This makes the pressure differential between boosted air and intake air not quite as different. This is why a turbo surges less at low altitude then high altitude, because of the higher pressure at low altitude. At high turbine speeds the air is moving so fast that a lower percentage of air escapes through the groove so the groove has very minimal effect on max air flow compared to the benefits (but does have some).
Our DTech 64 turbo has a specially engineered surge groove to allow a smaller, quick-spooling turbine wheel to be used with a very high flowing compressor wheel (1190 CFM). With comparatively great surge characteristics.