M720 Ported Shroud Closer Look

Background:

The Mabotech M720 turbocharger installed on my GTI uses a ported-shroud compressor inlet. In the picture below, this difference is the open area between the compressor housing and the compressor blades.

The ported shroud serves two purposes: to mitigate compressor surge and to allow more air through the compressor when the throat leading to the compressor inducer becomes choked at high airflow. At high airflow, the side openings allow more air into the compressor, albeit near the exducer.

The downsides of a ported shroud are a reduction in compressor efficiency and more ‘noise’ from the compressor (perhaps not a downside to turbo enthusiasts).

Alternatives:

While discussing the M720’s use of the ported shroud with Mabotech, they mentioned that compressor surge should not be an issue for the turbo. That meant the increased airflow was the only benefit the ported shroud would provide, but I have no intention of exploring the turbocharger’s capacity to support 40-44 psi boost pressure. Thus, the additional airflow benefit of the ported shroud would also not be utilized.

This led me to wonder if the airflow to the compressor could be improved by ‘eliminating‘ the ported shroud, at least for the operating range in which I expect to use the turbo.

To investigate this question, I made a model of the compressor housing inlet to perform airflow testing.

Then, an insert was made to block off the ported shroud section and funnel air directly into the compressor wheel.

A couple of different adapters were developed to compare airflow rates.

A flow bench flow test showed that when the compressor wheel’s effects are removed, the insert’s addition can significantly improve airflow.

Of course, the turbocharger operates with a compressor wheel, but this result suggested that the use of the insert might be beneficial.

Elbow Testing:

Meanwhile, I also tested alternate inlet elbow designs with the compressor housing adapter. The goal of these elbow designs was to explore the differences that the bend centerline radius has on airflow rate. One model, the black elbow, has an expansion area just before the entry to the turbocharger compressor housing.

These elbows were connected to a bellmouth to limit the contribution to only the elbow.

The results of the elbow testing are shown below.

There’s not much difference in the alternative designs from the elbow that Mabotech supplied with the turbocharger.

While most of the alternative designs have slightly higher airflow, they also do not include a PCV port like the Mabotech TIP, which is likely to cause additional turbulence that could reduce airflow.

I then made another adapter incorporating the insert transition shape into the adapter.

This M720 adapter is what I now use for testing the elbows and intakes that are being considered for use with the M720 turbocharger.

An alternative insert that I am working on opens up the exterior channel for airflow while aiming to smooth air entry.

This version still needs some more development before I am confident of placing it in front of a spinning compressor wheel with high airflow passing over it.

This insert is now printed using ABS, a tough material with good heat resistance, and is installed inside the compressor housing for street testing.

Next:

In the next post, I will compare the results of using the compressor housing insert.

References:

• Mabotech