So I shouldn't see much of a performance drop in my 2.25" catless downtubes? This is a pic of them clicky Would I see a performance gain at all? it's a direct swap to where the Y pipe was, going back to two Thrush Turbo mufflers.Well removing the cats and putting straight pipe, you usually won't feel any difference. The big drop in low-end torque comes from when people just punch out the cats. The reason for this is that the cats have a honeycomb structure, and when that is in there in terms of resistance to flow, it behaves pretty close to a straight pipe of the same diameter as the pipe going into and out of it, probably just a little bit more restrictive. If you punch out that honeycomb structure though, the exhaust is going along through a 2.25" pipe, then all of a sudden it hits what is effectively like maybe a 4" diameter pipe, and then it goes back down to 2.25". Doing that means the exhaust velocity drops like a rock as soon as it hits the opened area inside the cat. Also just doing that doesn't get you any gain up top because you are still going through the stock pipe diameter.