Regulated fuel pressure

[Scottm]

The fpr watches manifold vacuum, and raises fuel pressure at low manifold vacuum. High vac means low throttle/cruise conditions, and low vacuum means more throttle, needing more fuel. But changing the fuel pressure doesn't make sense to me. The injectors will flow more of course, but the tuning data changes at the same time. So the ecm has to juggle pulse width with the changing pressure. Why not just leave fuel pressure constant, say 50 psi? Is the point of reduced pressure at cruise to save wear on the fuel pump?

 

[L31MaxExpress]

The fpr watches manifold vacuum, and raises fuel pressure at low manifold vacuum. High vac means low throttle/cruise conditions, and low vacuum means more throttle, needing more fuel. But changing the fuel pressure doesn't make sense to me. The injectors will flow more of course, but the tuning data changes at the same time. So the ecm has to juggle pulse width with the changing pressure. Why not just leave fuel pressure constant, say 50 psi? Is the point of reduced pressure at cruise to save wear on the fuel pump?

Actually having a vacuum reference fuel system keeps the injector flowing the same volume constantly. The fixed pressure systems are the one that have to have added programming to compensate for injector flow changes as vacuum changes.

What you are missing is the system is operating on pressure differential. The total operating pressure on that injector is fuel pressure plus manifold vacuum or in the case of a boosted engine fuel pressure minus boost pressure.

 

[Schurkey]

In other words, the injector is spraying into variable vacuum, or even into above-atmospheric pressure. Changing the fuel pressure compensates for the changing pressure in the manifold.

TBI on the other hand, sprays into atmospheric pressure minus whatever restriction the air cleaner/air filter presents--much more stable than a "port" injection system.

 

[Supercharged111]

So stated yet another way, with a constant 50psi not referenced to intake manifold pressure, if there's 50psi at the injector and 7psi of vacuum, there's a 57psi delta in pressure and, effectively, 57psi of fuel pressure going into the engine. If you have 50psi of fuel pressure spraying into a manifold with 7psi of boost, you now have 43psi of fuel pressure entering the manifold. In a perfect WOT NA scenario, you would see the actual 50psi fuel pressure. If you reference the fuel pressure to the intake pressure with a 1:1 regulator, you will have a constant 50psid(ifferential) no matter the vacuum or boost.

 

[Scottm]

OK I get it now, thanks!