If you have been following our series on building the Turbo Nissan MR20DD engine, you can read about our analysis of the engine in the first chapter, the engine internals in the second chapter, and what we are doing to the engine to make it more suitable for turbocharging in the third segment. There are a lot of small details involved in turbocharging an engine designed for fuel economy, emissions, response and not necessarily for power and we touched on all of it. There are also many variables to consider when doing this to an engine with virtually no aftermarket support. For our choice in turbo we went with Garrett Advancing Motion’s wonderful GT25-550 turbo. Which is the turbo we consider to be the ultimate small-frame turbocharger.
The G25-550 is a small frame turbo that is a direct replacement for the older T2, T25, and T28 series turbos. The G25 takes advantage of the latest Garrett OEM R&D and has unheard-of efficiency and surge margin, numbers that were previously unheard of in a small-frame turbo. Usually, small turbos have lower efficiency due to proportionally larger wheel gaps to the housings, but Garrett has somehow gotten great numbers out of a very small package.
In our anticipated power range the G25-550 is going to be rocking at a crazy 80%! High compressor efficiency means much cooler charge air temps, less lag, and less power recovery needed from the turbine. Which means lower backpressure and better VE. It is a big win all around! If you remember how awesome the famous Disco Potato turbo was, think of this turbo as a Disco Potato on mega steroids!
Not only is this turbo tiny and fast spooling, but it also has the potential to make close to 500 hp!
The turbine side of the G25-550 is no slouch either. With a smallish, fast spooling 0.72 A/R turbine housing, it still flows an amazing 20 lbs per hour. To put that in perspective, that is slightly more than the T3/T4 hybrid turbos we used to rock on high-power imports back in the day!
The compressor housing is a ported design. A ported housing gives more surge margin. Surge is caused when the turbo can produce more boost than the engine is capable of taking at a given rpm. It is the cause of the chirping noise that many people like. Like it or not, it is harmful to the turbo and can also cause driveability issues. Having a good surge margin is especially important for a fast-spooling turbo like the G25. Normally having a ported shroud housing sacrifices a few points of compressor efficiency, but somehow Garrett got around that with this turbo.