With the spill port open stroke is limited by the distance from the bottom of the spill port to the bottom of the barrel. The stock injector does not use this full stroke. You machine them to get full use out of this stroke. There are two ways to increase stroke past this point, one is to plug the spill port. The other is to machine a spacer the will fit on the bottom of the barrel to allow more stroke to the bottom. This will leave your spill ports open and increase stroke. With spill ports open and no shims, the injector limit is basically 190cc. It is math, area of a cylinder. I can get you the dimensions if you want.