ARM Motorsport Catless Downpipe Boost Onset


The ARM Motorsport catless downpipe was installed on my GTI and boost onset was measured.

Boost onset is measured by first switching the DSG to manual mode so that the transmission does not change gears during the test. The transmission is shifted into third gear.

The accelerator pedal is fully depressed at an engine speed between roughly 1500-3000 rpm and the boost pressure is allowed to build past 20 psi.

Once boost pressure builds up past 20 psi the accelerator is released and the vehicle slowed. The process is then repeated several times.

The time that it takes for boost pressure to increase from 1 psi to 20 psi is calculated and plotted against the engine speed when 1 psi of boost pressure was reached.

The result of this test is shown in the chart, compared with the results of the same test performed on the catted version of the ARM Motorsport downpipe.

ARM Catless and ARM Catted DPs
ARM Catless and ARM Catted DPs

A second comparison is shown with the Trackslag catted downpipe.

ARM Catless and Trackslag Catted DPs
ARM Catless and Trackslag Catted DPs

There is no significant difference in the boost onset time between these downpipes. In fact, there is more variation within runs than between runs.

This suggests there are other factors contributing to the boost onset time other than the products under test. This is especially noteworthy since all of the data points for each product test case were recorded during a session that takes about 30 minutes to complete.


Boost onset has been compared between an ARM Motorsport catless downpipe and ARM Motorsport catted downpipe. The ARM catless was also compared to a Trackslag catted downpipe. No significant difference in boost onset is observed.

Both the ARM and Trackslag catted downpipes have previously been operated under various conditions with different accessory components; such as intakes, inlet elbows, and intercoolers.

In some cases, there is a consistent difference in boost onset time, typically around 0.1 seconds. In other cases, as shown above, there is almost no difference. From this result, my conclusion is that other factors are affecting the boost onset time more than the different downpipes are.

2 thoughts on “ARM Motorsport Catless Downpipe Boost Onset”

  1. Hey Jeff, I believe the PID based boost controller will skew your data.
    The greater the error between boost measured and boost requested will cause the the controller to add more duty cycle to your wastegate bleed valve, therefore reaching the target boost level faster than if the system were to do it without the added help from the wastegate.
    Turning the system off in the calibration would remove this variable, something I hope to do to compliment your work.

    1. Thanks for the feedback Jake!

      I purposefully selected 20 psi as the end point of the range to measure because it is sufficiently below the requested boost level, around 28 psi, that the wastegate has not started to open. By keeping the wastegate shut during the evaluation range, 100% WGDC, there should not be any contribution from the wastegate controller.
      Boost onset and WGDC

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