APEXi - GT Frontpipes, Nissan Skyline GTSt (HCR32) 89-94 (145-N002)GT Downpipe 1989 1994 Nissan Skyline [GTS t] The Apex Down Pipe is the pipe on turbo vehicles that connects the turbine exhaust housing to the catalytic converter. Although short in length, the Apex downpipes provide incredible amounts of power because the piping diameter is enlarged so early on the exhaust system. This enlargement in diameter results in a dramatic increase in exhaust flow capacity as well as a significant reduction of back pressure.