I am running the 3.2l m104. I have the old 1.6l from the autorotor design. I would probably get a 2.3l if I were to do it again. The 1.6 is maxed out right now. I might be able to get a couple more psi out of it but that is it. I know the new ones are supposed to put out more than the old designs though. When I built this setup I looked at the 2.3l dimensions to make sure it would fit in the same area.