Originally Posted by Stu @ M Developments

The amount of fuel we can inject is heavily reliant on one main factor, and that factor is the "Induction Stroke Time Base" This, in more english terms is simply the amount of time we have to inject the fuel whilst the inlet valve is actually OPEN. So imagine we have, on a theoretical engine, an intake valve opening time of 16MS at 7000rpm. You are revlimited to 7100, so we dont need to concern ourselves with higher rpm. OK? This 16MS is also subject to camshaft duration normally, a longer cam will give a longer opening time. So, given this info, we also know our peak power requirement is going to be 300bhp. Our chosen fuel pressure for the system, based on all requirements (See recent essay. lol) is 3.5bar. So we need to now select an injector, that can supply enough fuel in 16MS for its share of 300bhp (4 cylinders = 75bhp each) with a pressure of 3.5bar but pushing them no harder than 70% for injector reliability.

So cool, weve chosen our injector, the engine is dynod or mapped and weve got total success, 300bhp at 7000rpm, the fuel system and the engine is totally within tolerances. Game Over, home in time for tea. Or is it..

Our intake valve thats only open for 16ms (pure example) at 7000rpm is in fact open for 32MS at 3500rpm isnt it? (7000/2 so 16x2)) The same injector that can only sustain the fuel for 2bar of boost and 300bhp at 7000rpm is PERFECTLY capable of supplying the fuel for a "Theoretical" 3bar+ of boost at 3500rpm as the fuel has twice as long to get into the engine so we can hold the injector open for twice as long if need be or possible.

In reality, we would only open the injector in this theoretical engine spec for 25% longer to keep within the absolute max constraints of 95% injector duty cycle and even then, 95% is only for peaks such as described and NEVER used at high rpm. But im sure your now getting the picture as to why engines CAN run a nice big peak, but its not safe to hold the boost that high.