I am very skeptical these inlets yield a performance improvement. On the contrary, I think there is a very real possibility that they degrade performance and shorten turbocharger lifetime.
TL;DR
90˚ elbow turbo inlets are an engine packaging compromise that introduce pressure differentials into the air flow resulting from the distance between the inner curve and outer curve of the bend. The pressure difference creates air flow separation and turbulence (as I understand it) in the airflow - reducing the turbo’s efficiency. The pressure difference also has adverse effects on the compressor blades and turbocharger bearings - shortening the turbo’s life.
One of the simplest methods of reducing the differential on the curve is to reduce the distance between the inner and outer curves of the bend. That is what the stock inlet shape looks like it’s doing to me. The aftermarket inlets do the exact opposite by creating a much larger distance between the inner and outer curves of the bend - increasing air flow separation.
Wall of text:
I am not a mechanical engineer or fluid dynamics expert but I did some googling, so I’m dangerous.
I have a GTI. My curiosity was piqued when I first stumbled on these aftermarket turbo inlets. It seemed like such a simple ‘no brainer’ mod that the original design puzzled me. Surely, the turbo engineers must have had a reason, right?
Maybe it was a market segmentation ploy. Lower performance GTI gets the air restricting inlet while higher performance models get a bigger inlet. But the Golf R and Audi S3 have the same inlet design. So that’s not it. In itself, the fact that the Golf R shares the same part as the GTI implies the inlet is more than adequate for the GTI with its smaller turbo.
I started searching for academic papers on turbo inlet designs. Interestingly, turbocharger inlet design appears to be a flourishing field of academic research. It has seen a resurgence in the last decade or so because of the move to smaller turbocharged engines to meet emissions standards. There is nothing settled about inlet design and new designs are being researched using computer simulation and lab experimentation. Unfortunately, much of the research is not publicly available due to the restrictive nature of academic publishers, like Elsevier. I was able to find some though.
One of the earliest papers on turbo inlet designs is from 1946. It is about airplane superchargers at higher air velocities but the principles are the same.
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930081827.pdf
The paper lays out the problem:
And then proposes a fix:
Another interesting item I was able to find is a patent application:
https://www.google.com/patents/US20100221107?cl=en
The patent goes over various inlet designs and explains their limitations. In particular figure 2 shows what looks a lot like the aftermarket turbo inlets in this thread. The application has this to say about it:
In summary, the plain elbow is not efficient at best and can lead to turbo failure at worst.
The application’s invention is drawn in figures 7-9 and appears, to me, to be more similar to (not exactly like) the GTI’s stock inlet design. There is quite a bit of discussion on the merits of this design in sections 0039-0052.
There is a lot of a research out there on the topic and much of it is done using computer simulations (so called CFD or Computational Fluid Dynamics). Given the tiny bit of academic research I was able to read online, I think the GTI’s (and R’s) particular inlet design is not a mistake or oversight. It’s likely the result of extensive computer simulation and experimental verification around air flow optimization. I don’t think the same can be said about these aftermarket designs which appear to be obsolete since 1946.
