Background:
I acquired my Mk7 GTI as a used car from an owner who had made significant modifications. These modifications included the entire exhaust system.
I’ve had different goals for the exhaust system than the prior owner, which has led to piecemeal component replacements over the years. Recently, I decided to replace the entire system and contacted CC Design to fabricate a full turbo-back exhaust to my specifications.
Downpipe Spec:
Up first is the downpipe (DP):
I opted for a 4″ catless downpipe as the initial configuration.
Unlike some other downpipes I have evaluated that are advertised as 4″ (Trackslag) or 4.5″ (ARM Motorsport), but neck down to 3″ by the outlet of the downpipe, this CC Design downpipe will maintain a 4″ diameter up to and including the first v-band clamp past the subframe.
After the v-band clamp, the midpipe will contain a transition down to 3.5″ diameter.
I decided to opt for the 4″ downpipe instead of the 3.5″ on the principle that the larger diameter will lower exhaust gas velocity by approximately 23% compared to the 3.5″ pipe. With an exponential relationship between exhaust gas velocity and pressure drop, the larger diameter is a “better” performance choice. Whether there’s a practical difference is another matter, and given the relatively small displacement of the GTI engine, probably not.
Another reason for choosing the 4″ DP is that both sizes were available, and when it came to picking one, I decided I might as well go with the larger DP.
The catless version is for testing, but I have ensured the midpipe will support a catted downpipe in the future.
Next:
The CC Design resonated midpipe.