Bigger than the GT28RS, ATP's GT2871R has slower spool, but can handle around 400 horsepower. 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's range of bolt-on Garret turbos are designed to provide the performance of a dual ball-bearing turbocharger with the convenience of a stock-location turbo. The ATP GT2871R 08+ STI turbo easily installs in place of the stock journal bearing turbo, due to the high-flowing GT-inspired ATP STI exhaust housing and a series of custom engineered components built to suit the installation.
The ATP GT2871R drop-in turbo consists of a Garrett factory assembled and high speed balanced dual ball bearing CHRA which is then assembled into a set of CNC machine ported housings that have been optimized in every aspect for maximum flow and efficiency and designed to attach to the STI uppipe. This turbo includes all the necessary oil & coolant supply hardware to seamlessly attach the turbo to your STI. ATP's focus with this turbos isn't peak power numbers; they're designed to make more torque under the curve and give you more accessible power.
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.
In addition, ATP reccomends a higher flowing 3" turbo inlet, like the PERRIN, Agency Power or AMR. If you must use a stock-sized inlet, you will need to source a 2.4" to 3" reduction coupler, as the ATP turbos have a 3" inlet. We recommend using a matching-sized 3" inlet for minimal restriction, since GT turbos thrive with minimal inlet obstruction. ATP's tests have shown that these turbos, when used with the stock sized 2.4" inlet, produce significantly less power than when used with the 3" inlet. When coupled with a 3" downpipe, the 3" inlet promotes faster spool, and higher power numbers with less pressure at the manifold.