Continuing an investigation of the affect that downpipes have on the engine performance an ARM Motorsport catted downpipe for the Mk7 GTI is subject of a flow test. Flow testing will not necessarily correlate to engine performance, but when trying to select among different downpipe options to install on the GTI the flow test is a good method of determining which products are worth the effort to swap onto the car.
For the flow test the ARM Motorsport downpipe is configured with a supplied adapter for connection to a 3″ exhaust. My GTI uses the AWE Track exhaust and this adapter is the proper size to maintain the same pipe diameter from the downpipe to exhaust.
The downpipe is attached to the flow bench using a silicone coupler and a polylactic acid (PLA) 3D printed adapter that has a diverging nozzle opening into the flow bench upper chamber. At the inlet of the downpipe a 3D printed bellmouth is fit to assist with smoothing the entry of air into the pipe, shown below.
The flow bench is operated at a depression of 16″ of H2O and at this pressure differential the airflow through the downpipe is recorded.
The ARM Motorsports catted downpipe flowed 385 CFM at 16″ of H2O. This is compared with other downpipes in the chart below.
The ARM Motorsport catted downpipe flows roughly 216 CFM (128%) more than the stock GTI downpipe at 16″ of H2O. This is not an unexpected result given the two catalytic converters that the stock GTI downpipe is equipped with compared to one on the ARM downpipe, along with the smaller diameter piping of the stock downpipe.
The ARM downpipe is a 3″ downpipe and interestingly it flows approximately 22 CFM (6%) more than the 3.5″ CTS downpipe. The CTS downpipe is 3.5″ at the inlet and necks down to 3″ after the first 180 degree bend coming off the turbine housing. This leads me to hypothesize that the construction of the catalytic converter is potentially the most dominant factor affecting the flow performance through downpipes that have an exit diameter of 3″, although not the only factor affecting airflow.