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My guess is less backpressure. Mackey always told me our cars were engineered to have a certain amount of backpressure.
I definitely lost horsepower going by the numbers at the track. Don't know about a torque loss, but the loss was worth the better sound and since I only lost 1-1.5 mph in the 1/4 I can live with it since I'm still turning the same times.
Hard to say I think. Removing my main cat definitely gave me a power boost. Sure it isn't the mufflers, but it is/was a main flow restraint in the system.