Background:
The turbo muffler (TM) is a part that is meant to quiet the sound produced by the turbocharger compressor discharged air.

Several vendors offer turbo muffler products that eliminate slots in the muffler. Some have larger internal diameters than the stock part. Claimed benefits vary, but these from APR are not uncommon.
- Faster spool
- Better power delivery
Previous tests:
Testing the airflow through the compressor outlet using a stock TM and aftermarket turbo muffler delete (TMD) showed no significant difference.
Then, a street test was conducted, and the stock and aftermarket parts were installed on an Mk7 GTI with an IS38 turbocharger. Again, no significant difference in performance was observed.
The GTI now has a Mabotech M520 hybrid turbocharger installed, which produces a peak wheel horsepower near 500 HP. This presented a good opportunity to see if the TMD would register a change at this higher power level.
Test process:
The GTI is accelerated from approximately 2,000 RPM to 6,500 RPM at full throttle in fourth (4th) gear while engine and vehicle data is recorded.
With the aFe Power TMD installed on the GTI, baseline data was recorded. Then, the stock GTI turbo muffler is installed.

Test results:
The chart below shows the turbocharger wastegate duty cycle (WGDC) using the aFe Power (Orange) and stock GTI (Green & Purple lines) turbo mufflers.

The WGDC is clearly higher in the stock TMD data, becoming significantly different above 5,500 RPM.
The substantial difference between the two pulls using the stock TMD is concerning. If the part is making a difference, the effect should be consistent, and it is not consistent.
The following chart shows the boost curve:

Again, a difference is shown, with an anomaly in the boost onset of the first pull with the stock TMD—the light purple line. It takes longer for the boost to build during this pull.
Analysis:
Assessing what other factors might contribute to the difference led me to suspect the Diverter Valve.
When swapping out the TMDs, I also replaced the GFB T9831 DV+ with a stock DV. This was done because, simultaneously, I was working on a turbo inlet elbow and needed to use the low-profile GFB to provide clearance for the elbow.

I needed to keep the DV+ off the car to use for the TIP project, so I decided to install a Forge Motorsport BOV to see if my suspicion that the culprit was the stock DV was correct.

Another data collection session was conducted with the BOV installed in place of the DV.

The Dark Orange line is the WGDC using the Forge BOV with the stock TMD. This result is very similar to that when using the aFe Power TMD, confirming the suspicion that the stock DV was causing the difference in WGDC, not the TMD.
For completeness, a comparison of the peak WHP values using Virtual Dyno was made:

Unsurprisingly, given the similarity in WGDC, the peak WHP values using the stock TM and aFe power TMD were very similar.
Conclusions:
Consistent with previous street tests using an IS38 turbocharger and flow bench testing, an aftermarket turbo muffler did not significantly improve performance compared with the stock GTI turbo muffler at a 500 whp level using a hybrid turbocharger.
Differences in WGDC observed during the first part of this comparison test were attributed to the stock DV, which, upon post-test inspection, showed signs of air leakage from the ring surrounding the piston.

Have you ever set up a rig to test diverter valves for their capacity to hold seal?
No. It isn’t something I’ve had an interest in looking into.
Do you have bigger discharge piping from the outlet of the TMD to the intercooler? Wondering if that combined with the bigger tmd would have any measurable difference.
Yes, I have the complete aFe Power Bladerunner Charge Pipe kit. When I swapped to the stock TMD I attached it to the aFe charge pipes using the coupler hose that aFe Power supplies with the kit.