Split Downpipes vs. Open-Mouth Downpipes

Split Downpipes are also known as “divorced wastegate downpipes” in other areas of tuning. The reason this type of downpipe works better is because the exhaust coming off the turbo wheel (turbine) is spinning extremely fast and whipping in a circle. By keeping the exhaust in a pipe similar to the size of the exhaust wheel and separating the wastegate you dramatically speed up the velocity of the exhaust gases causing the turbos to spool about 500 rpm sooner than in open-mouth downpipe design.

If the exhaust comes off the turbo wheel and enters a much larger, colder area, it becomes more turbulent, slowing the exhaust velocity. In addition, when the wastegate opens, the two turbulent exhaust gases fight each other causing more turbulence and slower exhaust gas exit speeds. This is the case in open-mouth downpipe designs which currently exist in the market.

Please keep in mind that even though the downpipe is split & separates the flow into 2 separate pipes, the overall diameter of the opening at the turbo is 3 inches. The downpipe then merges the flow into a 2.5 inch “collector section” downstream and continues at 2.5 inches into the catalytic converter or test pipe.

For your reference, we have decided to include a forum post from www.impreza.net. This post was made by Jay Kavanaugh, a turbocharger development engineer at Garrett Engine Boosting Systems. These statements support the use of split or divorced wastegate downpipe applications. The entire post can be found here.