for PEUGEOT horizontal inner diameter: 21mm Product description Installation of a tuning manifold is a must for whoever wants to improve performance. Basically, manifolds are responsible for the flow of the gas-air mixture to your carburetor. Your original manifold’ s flow channel is usually smaller than your carburetor – creating a throttle effect, resulting in a hearty power reduction. A tuning manifold corrects this imbalance with a larger diameter, giving you more flow and more power – even with your original carburetor. Installing a tuning carburetor without an open manifold doesn’ t make sense. In case you want to de-throttle your motor and already have an aluminum manifold, the aluminum manifold can be bored out to increase flow. All open manifolds have two things in common: an optimized diameter as well as a shorter and better induction path. Irregardless which sport manifold you choose, your scooter will reward you with a remarkably higher performance. Get some fresh air in