Not a step design. Step design is where you go from say 1.5" to 1.75" to 2.0" pipe in the header(s). If you take a 1.5" pipe and bend it using a mandrel, the inside may or may not still be 1.5" in the bend, but for the gas passing through it, the pipe will 'act' like it's smaller than 1.5". I don't know a better way to describe it, but a bent pipe that's 1.5" ID is smaller than a pipe of 1.5" to the gases passing through it.

0.5" is a bit of a jump in size though.

I was asking because I want to design a 4-1 header that has ~40" long primaries (1-5/8" ID) heading into a custom collector that chokes down to 1-5/8" then has a cone expanding up to 2.5". But there will have to be atleast two 45* bends, and if I do them in the same 1-5/8" ID as the rest of the header, they'll be more like 1-3/5" (made up number) because it's a curved pipe.

Hope that makes sense.

I asked this on a few forums, and the answer I got is not to worry about stepping up the diameter in the bends (beauty of mandrel bends, ID stays the same). Kinda found the answer to my own question.