[bobcowan]

Simply adding a bigger injector will not give you more power. Unless the injectors can't feed the engine you have now. What does your dyno curve look like? Especially the AFR curve? If you have a good fuel curve all the way to red line, bigger injectors will be a waste of money, and may even worsen low end performance.

The problem with the 4.6l engine is that it's a 4.6l engine. Do the math. Ford has done an amazing job with 281 cubic inches, but there's only so much you can do. There's no replacement for displacement. You can't beat cubes. If you want more power, you're going to have to stuff more cubic inches of air in there.

You really should consider forced induction. Then you would need bigger injectors.