I've heard that if you get a PI head swap that you will need to start running 89 octane gas instead of 87 is this true? I heard it was because of the higher compression. How I understand it the PI head swap is mainly 3 things, the intake, cams, and heads. If you do need to run 89 octane with the PI head swap what part of the swap increases your compression so that you do need to run the higher octane?
When you switch out the heads the combustion chamber is tighter giving you a higher compression ratio. The 96-98 GT has different style of pistons compared to the 99 and newer. I would also use no less than 91 octane with the headswap.
