Flow Designs
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The best Flow Designs parts for a 2015-2021 WRX or WRX STI are the two pieces in this collection: the Front Splitter Extensions, listed for the 15-21 WRX/STI under part code VAFEX, and the Flow-Lock Rear Diffuser, listed for the Subaru VA1/VA2 WRX and STI. Together they cover the front and rear undersides of the VA-chassis car, so choose by which end you are finishing first.
Splitter extensions and rear diffusers work with the air moving under and around the car rather than over it. A front splitter extension adds to the leading edge at the front bumper, while a rear diffuser shapes the panel under the rear bumper where airflow exits. Matching the piece to the exact chassis and bumper matters, because the look depends on it. Both products here are listed specifically for the VA WRX and WRX STI, and the wider aero range sits inside Exterior Trim and Side Skirts, so check your year and model before ordering.
Flow Designs is the specialist behind both pieces, and its catalog here is focused rather than broad: front splitter extensions and a Flow-Lock rear diffuser, each named for the VA WRX and STI application instead of a universal fit. That application-specific approach matters, because a diffuser or splitter extension only fits right when it was shaped for that bumper.
Four related brands put this pair in context. AeroFlowDynamics is known for splitters, diffusers, and exterior aero pieces aimed at sport-compact platforms including the WRX and STI. Verus Engineering approaches aero from the engineering side, with splitters, diffusers, and underbody components developed around measured aerodynamic function. Rexpeed is best known for carbon-fiber exterior pieces such as lips, spoilers, and trim for Japanese performance cars. Extreme Online Store offers a broad range of exterior and aero-style parts for owners finishing a full body look. For the base fitment range, see 2015+ WRX/STI, and read our guide to aerodynamics for the STI.
Will these Flow Designs parts fit my WRX or WRX STI?
The splitter extensions are listed for 15-21 WRX/STI and the rear diffuser for the Subaru VA1/VA2 WRX and STI. Cars outside the VA generation use different bumpers, so check your year and model carefully before ordering.
What is the difference between the splitter extensions and the rear diffuser?
The splitter extensions work at the front bumper, adding to the front splitter area, while the Flow-Lock diffuser is a rear piece fitted under the rear bumper. They address opposite ends of the car and can be fitted separately or as a matched front-and-rear set.
Are splitter extensions and diffusers hard to install?
These are bumper-mounted bodywork pieces, so the job calls for careful alignment before fastening and is easier with the car raised. Follow the product instructions, and consider professional fitting if you are not comfortable aligning bumper parts.
Should I add side pieces at the same time?
Only if you want the finished look in one step. Many owners fit front and rear first, then decide whether side skirts are needed to tie the look together.