The ARM Motorsport catted downpipe has been on my GTI for a while now and with the arrival of a Baun Performance mid-pipe I was ready to move onto installing a Trackslag catted downpipe.
Note: The GTI has been using an AWE mid-pipe with Vibrant resonator which is the same diameter as the Baun mid-pipe, in fact the Baun mid-pipe is their “AWE Fitment” version.
While both downpipes were off the GTI I took the opportunity to measure the outside circumference of the catalytic converters.
- ARM Motorsport: 327 mm
- Trackslag: 420 mm
That works out to a cross sectional area of 13.2 sq. in. for the ARM and 21.8 sq. in. for the Trackslag.
Previously both downpipes have been flow tested and the Trackslag measured higher than the ARM downpipe.
With the swap underway the next comparison between these two is of the boost onset.
Measuring boost onset is done by placing the GTI in third gear and fully depressing the accelerator when the engine RPM is within the range from 1500-3000.
The GTI is equipped with a Shuenk IS38+ turbocharger.
The time for the boost pressure to build from 1 psi to 20 psi is measured and plotted versus the engine speed when the boost pressure was at 1 psi.
The time is plotted against the engine speed since engine speed factors into the airflow through the engine which in turn drives the turbine wheel, further explained here.
This test should be conducted under the same ambient temperature conditions since the outside air temperature influences how quickly the boost pressure is generated.
For this comparison the ambient air temperature was NOT the same. This was the first drive with the Trackslag downpipe installed and the air temperature was 7 degrees Fahrenheit cooler than when comparison data with the ARM Motorsport downpipe was recorded.
Despite the slightly cooler air temperature I was still interested to see how the Trackslag compared to the ARM downpipe in terms of the boost onset.
The Trackslag boost onset time averages approximately 0.1 seconds faster than the ARM Motorsport downpipe.
Warmer ambient air temperature should slow the rate of boost onset, and therefore the two curves should be closer if the outside air temperature is closer between the test cases.
Oddly that turned out not to be the case when additional data was recorded using the Trackslag downpipe. (Chart below)
A possible explanation for the unexpected result is that the pre-turbo intake air temperature was lower when the Trackslag was measured. As the intake (airbox/accordion/TIP) warms up the air temperature going into the turbo also rises. If the ARM pre-turbo air temperature was higher that could have affected the boost onset time.
In a more familiar format. Three pulls are shown using each downpipe where 1 psi was reached between 1900-2100 rpm.
The ARM Motorsport catted downpipe and Trackslag catted downpipe were evaluated to compare boost onset times.
Based upon the similarity of the boost onset times under the test conditions, and the effect that air temperature plays, it is likely that the boost onset performance will seem very similar to a driver operating with either of these downpipes.