I have the Penland Series and it is a great pipe, I have it in conjunction with the HMF programmer.
Here is the specs on the HMF Penland Series:
Mike Penland teamed up with HMF to produce “The most winning utility ATV muffler on the planet," the Penland Pro Series. This is the premier exhaust for the utility rider/racer looking for maximum power, durability, and appearance. Exclusive to the Ballance and Penland Pro Series, the torque range modifier broadens the torque range, allowing better hook-up in rough terrain. And with the best muffler packing in the industry, you can spend more time riding and less time repacking.
System: Slip-On System Can: Aluminum Shape: Round Forging Kit: No Core Diameter: 2.125 in Shell Diameter: 4in Quiet Core: Yes Forrestry Approved: Yes Torque Range Modifier: Yes Core Design: Straight
Here is the Specs on the HMF Utility pipe:
The HMF Utility Series has become the most popular aftermarket exhaust system in ATV racing. Featuring a 2-inch, non-restricted core, the HMF Utility Series delivers the power, torque and quality that racers demand. Match your machine with GNCC and BAJA tested powder coated exhausts and end caps for a fully customized look. And with the best muffler packing in the industry, you can spend more time riding and less time repacking.
System: Slip-On System Can: Aluminum Shape: Round Forging Kit: No Core Diameter: 2.125 in Shell Diameter: 4in Quiet Core: Yes Forrestry Approved: Yes Torque Range Modifier: No Core Design: Straight