Mk7 GTI stock parts that I have tested generally have shown room for improvement by the aftermarket vendors. The stock catback exhaust is a part that seems to be a good candidate for performance enhancement.
Looking through some Mk7 GTI discussion boards for evidence of how the stock catback performs versus an aftermarket exhaust did not turn up any comparison data.
I found some people advocating that the stock exhaust leaves little room for improvement:
“Stock exhaust flows well enough, a catback won’t even get you gains.” – golfmk7.com forum member
“Cat back exhaust- Dual tips, quad tips, ten tips! It doesn’t matter, the stock catback is not a restriction even at 450+whp.” – golfmk7.com forum member
My GTI was purchased used and already had an aftermarket exhaust which meant I would be waiting for an opportunity to find a stock exhaust before I could test one.
Another GTI owner local to me was moving and had the stock exhaust that they were looking to get rid of, score!
I can confirm that the stock GTI exhaust will fit inside the GTI for transport.
To test the exhaust it is being joined with the Trackslag downpipe and the pair are then tested using a flow bench.
To make a comparison the Trackslag downpipe is then joined to my aftermarket exhaust to measure the aftermarket exhaust flow data.
My exhaust consists of a Baun Performance mid-pipe with Vibrant resonator that is sized to fit with the Trackslag DP and the AWE Track Edition exhaust muffler section.
The AWE Track Edition exhaust muffler section was modified with a Vibrant Ultra Quiet resonator, shown below, and then later the resonator was replaced by a Magnaflow muffler.
Both the stock GTI exhaust and the aftermarket exhaust are tested at 16″ of H2O with the bench blowing air through the exhaust system.
The Trackslag combined with the stock GTI exhaust flow 222 CFM @ 16″ of H2O.
The Trackslag and aftermarket exhaust flow 388 CFM @ 16″ of H2O. This is approximately 75% more airflow at the test depression than the stock exhaust.
These results along with the Trackslag downpipe-only flow rate are shown in the chart:
The stock Mk7 GTI exhaust was flow tested and compared with an aftermarket exhaust. The stock exhaust flows 222 CFM @ 16″ of H2O and the aftermarket exhaust flows 388 CFM @ 16″ of H2O.
The aftermarket exhaust adds some restriction to the downpipe, leading to an approximately 10% decrease in airflow at the test depression. The addition of the stock exhaust with the downpipe led to an approximately 50% decrease in airflow.
The GTI stock exhaust flows significantly less than an aftermarket exhaust when tested as described above.