CTS vs ARM Downpipe Boost Onset

CTS vs ARM Downpipe – Boost Onset


As part of researching how different downpipes perform in terms of airflow and affect on the performance of the GTI a comparison is being made between the CTS Turbo catted downpipe and ARM Motorsports catted downpipe.

Previously the CTS and ARM downpipes were flow tested. For this comparison boost onset data recorded while operating the GTI with each downpipe will be evaluated.

Test Subjects:

Notable differences between the products as they relate to this test are the dimensions of the downpipes and location of the catalytic converters. The CTS downpipe is a diameter of 3.5″ off the turbocharger turbine housing and after the initial 180 degree turn the DP tapers to a 3″ diameter. The CTS DP catalytic converter is located after the mounting hanger and just before the midpipe.

CTS Downpipe

The ARM Motorsport downpipe is a uniform 3″ pipe, aside from the catalytic converter and flex joint. The catalytic converter on the ARM downpipe is located immediately after the first bend off of the turbine housing.

ARM Motorsport downpipe flow test
ARM Motorsport downpipe flow test

Two claims that will be evaluated by this test are 1) that a larger diameter downpipe just off the turbocharger improves boost onset/spool up, and 2) locating the catalytic converter further back along the exhaust path improves boost onset/spool up. A third contributing factor to boost onset that likely has a significant contribution is the catalytic converter design which is not being evaluated directly, but results of the CTS and ARM downpipe flow testing suggest the ARM catalytic converter flows slightly greater than the CTS catalytic converter.

Test Procedure:

With the respective downpipes installed on the GTI the transmission is placed in third gear and the engine operated without generating boost pressure. With the engine speed between ~1500 to ~2500 rpm the accelerator pedal is fully depressed and held down until the boost pressure increases past 20 psi. The accelerator is then let off and the GTI slowed to repeat the process.

This process was performed on multiple days with slightly changing ambient air temperature.

The GTI is equipped with a Shuenk IS38+ turbocharger and has a Stratified customer E30 tune that produces the following boost curve:

Typical Boost Profile

Boost onset data is analyzed to determine the amount time that it takes for the boost pressure to increase from 1 psi to 20 psi. The engine RPM when the boost pressure equals 1 psi is recorded so that the boost rise time can be plotted versus the starting engine RPM.

Ambient/outside air temperature during the testing sessions is also recorded because the outside air temperature affects boost onset. Temperature readings were reasonably consistent across testing sessions as shown in the chart below that compares the CTS and ARM outside air temperature readings.

Test Session Outside Air Temperatures

Test Results:

Data recorded with the ARM and CTS downpipes is shown on the chart below. Each data point includes a label that shows the ambient/outside air temperature during the testing session. Nineteen (19) data points were recorded with the ARM downpipe and sixteen (16) data points were recorded with the CTS downpipe.

CTS vs ARM Downpipe Boost Onset
CTS vs ARM Downpipe Boost Onset

A second order polynomial curve is fit to each of the data series to help identify trends between the downpipes. There is no significant difference in the boost onset time between the CTS and ARM downpipes recorded following the process described above.


There are no significant differences in the boost onset time measured following the process and under the conditions described in this post.

A hypothesis that boost onset is improved by a larger diameter downpipe in the area of the turbine outlet is not supported by the results of this comparison test.

A hypothesis that boost onset is improved by locating the catalytic converter well downstream from the turbine outlet is not supported by the results of this test.

The contribution of catalytic converter design on boost onset cannot be determined from the results of this test.

The multiple factors mentioned above are likely to affect boost onset to some degree. It was show during flow testing that the ARM downpipe has a slightly higher airflow than the CTS downpipe, likely attributable to the ARM catalytic converter. Assuming this to be the case, the contribution of the catalytic converter to boost onset time appears as significant as the combined contributions of the downpipe post-turbine diameter and catalytic converter location for the product designs used in this test.


ARM Motorsport Downpipe product page

CTS Turbo Downpipe product page