I decided to pick up some RS4 intake components and see how they performed on the flowbench.  I have on hand a set of RS4 bipipes, an RS4 metal throttle body boot and an RS4 throttle body (shown below).

The RS4 bipipes are an interesting looking set of inlet pipes, rather flat – I assume to better fit within the engine compartment, but also wide, giving them a rectangular cross section.  The 2.5″ diameter inlet and outlet are a nice step up from the 2″ diameter stock size that is used with the APR bipipe as well as the various alternatives such as 034, JHM, ARD, etc.  This gives the RS4 bipipe 61% more cross sectional area at the inlet.

Comparing the outside dimensions of RS4 bipipes to the APR bipipes I measured the following:

The lower portion of the APR bipipe just after the inlet is rectangular and measures: 50×66 mm

The RS4 bipipe measures: 41×84 mm

The upper portion of the APR bipipe is circular in cross section and measures: 58 mm

The upper RS4 bipipe is the same rectangular shape as the lower portion.

At the top the APR bipipe where it attaches to the throttle body the inside diameter is: 68.5 mm

At the top the RS4 throttle body boot measures approximately: 78 mm

The increase in cross sectional area of the RS4 bipipe compared to the APR bipipe is:

Inlet: 61%

Lower portion: 4%

Upper portion: 30%

Outlet: 30%

The question I had was if the shape of the APR bipipe (shown below), which seems smoother to me, would overcome the smaller cross sectional area.

Another question I had was how these RS4 intake components would compare against the 80 mm Hemi throttle and aftermarket intake pipes (shown below) that I had tested last fall.

Using the stock throttle body the APR bipipe had recorded the highest flow, and with the addition of the 80mm throttle the 2″ diameter aftermarket style (JHM) bipipes had flowed more than the APR setup.

The Results:

The results of the flow test, calculated out to a depression of 28″ of H2O are shown below.

Given the greater size of the RS4 throttle body it isn’t too surprising that the RS4 setup flowed more than the APR bipipe configuration.  What is a little surprising is by how much more the RS4 intake setup outflowed the 80mm Hemi throttle body, 034 throttle body boot, and aftermarket bipipes.  Despite the RS4 throttle body being smaller than the Hemi the other RS4 components were higher flowing in comparison to the aftermarket alternatives.

Notably, I currently do not have the inlet adapters for the RS4 bipipes, and as the chart above shows, the trend when adding the silicone adapter at the entrance to the bipipes is for the flow value to increase.  Therefore if the RS4 bipipes were equipped with inlet couplers it is likely that the flow results would increase slightly.

The results above indicate to me that if optimizing the intake setup is desired then use of the RS4 intake components is preferred unless a custom set of bipipes can be fabricated.  Using the large 80 mm Hemi throttle body in conjunction with 2″ diameter aftermarket bipipes doesn’t allow for increased airflow over the RS4 throttle body, throttle body boot, and bipipes.

Addendum:

A custom made set of bipipes were passing through so I put them on the flow bench to see how they compared to the other setups I’ve checked.

This design uses an RS4 throttle body, what looks to be the RS4 throttle body boot modified for the custom up-pipes, and the circular up pipes that are about 48mm in diameter on the inside at the inlet.

Running up to a true 28″ of depression they achieved 578 cfm.  Shown below is how they compare to other setups: