Direct Port on Mazda RX-7
 

I have a 13B-RE (Cosmo) engine with 4 X 1000cc injectors. Since I'm running them at 75PSI base pressure, the flow is actually 1200cc each, per the manufacturer's data. I'm planning to run 3 injectors -- one at the outlet of the intercooler, and one in a runner to each of the two rotors. If you look at the picture, the two outboard runners are the secondaries; inboard are primaries.

I plan on using an HFS-3, so my initial thought was to scale the injection off the secondary injector duty cycle and inject into the secondary (outboard) runners. The concern I came across was that the injection will be immediately upstream of the vacuum port for my brakes.

Q1: Is there any risk of water being forced into the brake booster while under boost? I know there's a check valve in the line, but . . .

Due to my fear of brake booster water ingestion, I considered injecting into the primary runners instead.

Q2: For staged runners like mine, does primary or secondary make a difference?

In either location, the water will be injected just above the fuel injectors. I realize the just after the IC location is also before the injectors, but a larger portion of that water will have a chance to evaporate before it reaches the fuel injectors.

Q3: Will water injection just before the fuel injector cause a problem?

Q4: What size nozzles do you recommend?