the 5th injector should come on at the end of the mfi's limit on fuel, my s2 5th injector came on at 12psi,and boosted to 22psi that was 230bhp.

I AM in the same boat mate, have a s1 and want to keep it standard......and need more fueling, but no-one wants to tell me how to ajust the fuel form the metering unit LOL