Originally Posted by Sikoriko
Wouldn't you have to direct the fuel flow almost into the back side of the intake valve? The closer the better?
Not necessarily so. We were plumbing in before the throttle body. Here's a PDF of the GReddy Rebic IV we were using back in the day on my RX-7.
The PDF tells you how it works and shows components that would be in a sub-injector kit.
The way things are now, the need for a sub-injector system have pretty much gone away with better injector, fuel pump and fuel management technology, BUT there's still a niche for this sort of system. Apparently, some people are still using them.
A rising rate fuel pressure regulator is the way to go if one is experiencing lean-out in the higher RPM band. Tuning it on a dyno is essential.