Mabotech M720 First Drive

Background:

My interest in a turbocharger that could be used for testing the MGM7 intakes led to an offer by Mabotech to test out a prototype they are developing. I would work on fitting my intake inside the engine compartment, and they would gather data to refine their design.

The M720 prototype was recently installed, as previously discussed.

For the time being, I am using a Racingline R600 intake with the MST Performance inlet hose. Luckily, the MST hose has some stretch to it and can fit onto the Mabotech’s 3.5″ inlet elbow.

Tune:

I haven’t been excited about the prospect of more ECU software tuning. At the power level that the Mabotech M520 operates at, which I have been using up until this swap, and what this larger turbo is capable of, recording data for tune revisions needs to be made in fourth (4th) gear to have good traction.

There is only one stretch of road nearby that is adequate for collecting this data, with limited periods of infrequent traffic. To avoid more tuning, I decided to test the M720’s performance using the M520 tune.

The tune for the M520 is from Russell Road & Racing and holds around 34 psi, tapering to 32 psi. Fueling is with MPI using an RS3 LPFP on E50.

The compressor wheel of the M520 is 51/70, and the M720 is 58/76.

Results:

A comparison of boost pressure during the 4th gear pull is shown below. The green line is the M520, and the Orange/Tan line is the M720.

It’s no surprise that the smaller M520 has a quicker onset of boost. The M720 is not helped by the fact that the M520 tune momentarily levels off the boost onset, which adds delay to the M720 boost curve.

Another result is that the M720 produces more boost at higher RPMs.

The following chart compares the wastegate duty cycle (WGDC) from each turbocharger.

This difference surprised me. I was not expecting the M720 to be operating at a higher WGDC while at approximately the same boost pressure. This stems from the significantly larger turbine of the M720.

The M720’s larger turbine wheel can flow more exhaust gas with less back pressure than the M520; hence, it is well-suited to operate at higher boost pressure than the M520.

This lower back pressure of the M720 coincides with a reduction in the turbine inlet pressure (TIP). Lowering turbine inlet pressure while the outlet pressure downstream of the turbine remains unchanged, since the DP and exhaust are not changed, results in a lower expansion ratio across the turbine.

The expansion ratio relates to the energy being extracted from the exhaust. With less energy being extracted but the same boost target needing to be met, the ECU must close the wastegate to direct more exhaust gas through the turbine – ergo, the M720 has a higher WGDC than the M520 at this boost pressure.

Ported Shroud:

One aspect of the M720 design that I am interested in exploring some more is the ported shroud. That will be the subject of a future post.

Looking further down the road, I’ve been in discussions with Shuenk about testing a ported shroud version of their IS48 turbocharger.

References:

• Mabotech
• Russell Road & Racing
• Shuenk IS48 product page