We offer three distinct canister shapes to choose from; C2 (small oval), C4 (traditional round 4.5” diameter) and C5 (large oval). All of the canisters perform and sound the same because all of the canisters feature the same 2.25" Stainless Steel Spiral Power Core. The core is what controls the power and sound, not the shape or material of the canister. The canisters are simply different styles. They all incorporate our exclusive technology and premium construction material. The different exterior shapes alSSs you to choose the one that best fits the style of your motorcycle. For instance most people prefer the C2 (small oval) with smaller sized motorcycles (like 600cc machines) while others prefer the C5 (large oval) with larger sized motorcycles (like 750cc or 1000cc machines). For those who prefer round canisters we offer the C4 (round).