ATP Turbo Bolt-On GT2871R Turbo Subaru 2002-2007 WRX / 2004-2019 STI
Bigger than the GT28RS, ATP's GT2871R has slower spool, but can handle around 400 horsepower. This is a good turbo for built-engine 2.0L WRXs that don't mind adding some lag for some more top-end performance. This turbo is still pretty small for a 2.5L, but will have rapid spool if you decide to go that route. The GT2871R uses a 53.85mm 76 trim turbine wheel and 71mm, 56 trim compressor wheel.
ATP recommends that you use a bellmouth downpipe with their bolt-on Garret WRX & STi turbos for optimal performance. Stock-type flat plate WRX & STi downpipes (HKS, DC Sports, etc.) and divorced wastegate downpipes (Perrin, Invidia Divorced) do not allow enough clearance for ATP's custom wide-swinging wastegate door. This can result in boost "creep," "spiking," and inaccurate boost control. The Turbo XS & Invidia Bellmouth downpipes would be good choices for use with this turbo.