I was considering replacing my stock exhaust manifold on my Si with the stainless Type R header but wasn't sure what all I needed. My understanding is that no fabrication work has to be done, but rather it's a direct bolt-on. However, I thought that the header is a 4-1 type whereas my manifold is a 4-2-1 type. Does that mean that I just need the header and the downpipe, or are there fitting issues as well?

 

itr has a 2.5" collector and the length is probably different