aFe 15-19 Subaru Outback H4 2.5L Silver Bullet Throttle Body SpaceraFe 15 19 Subaru Outback H4 2. 5L Silver Bullet Throttle Body Spacer The Serrated Helix design creates air turbulence in the intake manifold. This turbulence increases the air pressure in the combustion chamber resulting in increased horsepower, torque, and improved throttle response. The Serrated Helix combination design also reduces intake whistle that occurs with other helix only designs. All Silver Bullet Throttle Body Spacers are precision