just some fuel system questions

[CalicoDreams]

okay I drive a 1990 olds delta 88 with a 3800 3.8l V6. I just have a few questions about fuel systems, I have been doing alot of recent studying up on forced induction, and fuel systems. okay, I have an EFI engine. Now the computer in the car only controls the length of time the injector stays open for right? so if you were running lean or need more fuel for forced induction... you could just increase the PSI of the incomming fuel and the computer wouldn't lessen the open time of the injectors? one more question... how does a fuel pressure regulator that is boost sensitve, sense when it needs to supply more fuel?

[SaabJohan]

Basicly the ECU controls the time the fuel injectors are open, which never exceeds about 85% of the time since that migh overheat the injectors. But the ECU can also control ignition, idle air and so on. In newer cars they often control the throttle.

With forced induction the fuel pressure regulator is connected to the intake manifold so the difference between intake pressure and fuel pressure is always constant, therefore the flow from the fuel injectors are independant on boost.

The fuel flow is basicly controlled by a map, which can be based on different methods. One that is common in production cars is a system based on a air mass meter. In that case the air will be measured by the air mass sensor, the throttle position will then indicate load and the fuel flow needed will be calculated. With forced induction it is intead common to use the manifold absolute pressure as a load indicator.

Another system which is more common on high performance cars is what is called "speed density", with that system the fuel flow will be calculated from load (usually MAP) and engine speed, the air temperature is then used to correct the value.

Then information about engine temperature, lambda and so on are used to correct the fuel flow. Lambda can usually only handle small changes.

So if you for example are adding forced induction to an NA engine the fuel injection system must usually be reprogrammed/modified or replaced. Fuel pressure can't just be increased as this will give a too rich mixture since it increases the fuel flow under all conditions.

[pro_am]

If you have a mass air flow sensor it will detect that there is more air coming in and compensate with fuel.
Jake