more backpressure usually means higher torque and lower horsepower, less backpressure means less torque and higher horsepower, so no you are not losing horsepower, but you are losing torque
more backpressure usually means higher torque and lower horsepower, less backpressure means less torque and higher horsepower, so no you are not losing horsepower, but you are losing torque
Actually 5.0 and Superfords did a test on like 15 mufflers I think. They dynoed them all on the same car in the same day. Every muffler made close to 5hp more and about the same amount of torque than with no muffler. Plus open headers on a street car sounds nasty