1. Woody's on the money - injectors are measured at 3barg in and 0barg out (atmospheric), if you apply 1bar back pressure flow will be reduced.

2. "In general" flow increases proportionally to the square root of the pressure applied....So... doubling the pressure increases the flow by approx. sqrt(2) = 140%.


Applying these 2 facts....
for a stock 450cc (@3bar) injector running off the in filter 3.9bar FPR and against a WOT manifold pressure of 1bar (14.7psig) the injector would see a total drop of 3.9-1 = 2.9bar - lets round that to 3bar......so its going to be able to deliver 450cc/min.
Now the design should have about 30% fat in it to account for a max duty cycle of 80% and some wear and tear/blockages etc. So you'd be aiming for 315cc/min.....In stock condition.

The final equation to add is "how much more fuel do I need if I up boost from 1bar (stock) to 1.5bar (stacked).
Additional air = [1atm+1.5]/[1atm+1] = +25%
So you should need to provide at least 25% more fuel - probably a little more for cooling, but then there's a little less air due to intake restrictions and inefficiencies.
+25% is hopefully going to +25% more hp...;>)
315cc +25% = ~400cc/min (rounded up)

So a 450cc injector "could" provide this - but you've significantly cut into your 30% head room. "Feel lucky punk!"
If you apply the 30% head room you'd need 520cc/min.

To get 520cc/min out of a 450cc (3bar) injector you need 520^2/450^2 x 3bar = 4bar
Because your mani prs is 1.5bar - you need to add that to your 4bar because that's the drop across the injector = 5.5bar from the FPR.....
......or you could just change the injectors out for 550cc (3bar) injectors and then hope your pump and FPR can deliver the fuel....I'll save that for the next post.