HMF Engineering Utv Perf Dual Full Brshed The Performance Series uses a 2 1/8" non-restrictive core for maximum flow. Depending on the model, you can expect a 4-6 dB increase over stock. The exhaust shell is made of T6061 Aluminum that weighs substantially less than stock. USFS Approved: All HMF off road exhausts come with a USFS approved spark arrester. Two Year Warranty: HMF Engineering Inc. warranties all exhaust systems for two years from the date of purchase for all manufacturing defects.