The following stock Mk7 GTI parts have been identified by aftermarket parts vendors as being a “restriction“:
Air Filter:
“Typically your engine is equipped with a cheap, mass-produced and restrictive paper filter system“
APR
Intake:
“these true performance designs remove the restrictive factory intake systems“
Integrated Engineering
“the factory air filter, upper airbox, and turbo inlet tube were deemed too restrictive..”
034 Motorsports
Turbo Inlet Hose:
“replaces the entire restrictive rear factory plastic ‘bellows’ pipe…”
Racingline
“replacement for the unattractive and restrictive OEM corrugated plastic hose.”
034 Motorsports
Turbo Inlet Elbow:
“the necked-down design becomes a major flow restriction“
Integrated Engineering
“eliminate upstream airflow restrictions…”
Unitronic
“replace the small and restrictive plastic factory inlet.”
Racingline
Turbo Muffler:
“replaces the restrictive, power-robbing compressor outlet…”
Racingline
“The OEM turbo muffler is just that, a wasteful restrictive part…”
Leyo Motorsport
Charge Pipes:
“eliminates restrictions in the boost (charge) piping..”
Unitronic
“plastic and rubber turbo outlet pipe is a small and restrictive..”
APR
Intake Manifold:
“addresses all the concerns of the restrictive stock manifold”
Galano Engineering
Intake Manifold Flaps:
“allows for the removal of the restrictive intake flappers …”
CTS Turbo
Downpipe:
“replace the restrictive factory downpipe..”
034 Motorsports
“replaces restrictive 60mm factory tubing”
Integrated Engineering
Exhaust
“removing restriction and reducing back pressure.”
Unitronic
With all of those restrictions, it’s a wonder the stock Mk7 GTI can operate.
To be fair, your own testing indicates some of those claims to be accurate, at least in a few cases. The of the TIP is a pretty clear example. The downpipe as well, if you are willing to remove the catalytic converters and go to 3″. But I do agree that most of the “restrictive” claims made by most aftermarket manufacturers are exaggerated or false.
I get a chuckle from the non-descriptive use of the term restrictive. Any part is going to create restrictions, including all of those aftermarket parts. Saying that a part is restrictive is stating the obvious in a vague way that is done to imply superiority of the alternative being offered for sale without providing the consumer with useful information to decide if the purchase is justified.