Re: performance mods?
Most of the "modifications" can be done if your motor is within a family of motors based on a similar block configuration. As an example, if you have a mid 1980s 90hp crossflow V4 it is possible to machine the block to the intake and exhaust porting specs of the 115 (or older rated 140) and add on the 115/140 exhaust tuner and carburetors to achieve a bit of a boost. Would be easier to start with the block of the 115/140. Similar things can be done with some of the V6s, etc. If your motor is already at the high end of its block family, then your only real choice is to go to a bigger motor.
Sure, you can tweak a lot of things - porting, decking the heads, opening up exhaust, etc. that might gain you a little IF you know what you're doing, but in general most backyard modifications succeed in ruining perfectly good motors....
