Largest overbore posted by toykoz32 on January 07, 2008 at 06:17 AM
Message
I did extensive research on this when I rebuilt my motor. Our VG motor has relatively thin walls to start with from the factory. Looking at the top of the motor down the cylinder bore is deceiving because the walls look thicker than they are. If you look closely inside the water jacket you will see it rolls in and the cylinder wall thins out considerable. In any case the walls are thin but the iron is very dense and of good quality. For a twin turbo motor I would not got more than 89mm, this would give you 3.1 litters, about a .14 increase or a 4.7% increase in displacement. Not too shabby! If you wanted to go more than 89mm, you will need to source a few blocks, measure the wall thickness and pick the best candidate, then sonic testing would be recommended to make sure there is no porosity. For my own motor I went 88mm to be safe. I didn't have the time or any additional blocks available to me. You have to factor the risk/reward of going too thin when there are so many other power adders available for the TT. If I had an NA I'd go 92mm or 3.3l and hope for the best, there isn't any other meaningful way to add 10% more power to a NA, nitrous not withstanding.
