Fuel Pressure reg

[jakobsladderz]

That post does look confusing now that I look at it a second time - thanks for making the point clearer ProZac...

[mrb1]

ProZac,
hey I was waiting for somebody to say something sensible good one. This is to maintain the SAME fuel pressure across the injector no matter what the manifold pressure is, boost or vacuum.
This is so when you set eg 2.0mS injector time it will give the same amount of fuel at all times.
Consider this if you had 35psi fuel pressure and 35psi boost NO FUEL WOULD COME OUT of the injector!
As far as rising rate regulators go they all started back when nobody had any way to re-program the ECU and when big injectors were hard to come by. It was a way to increase the amount of fuel. You have to be very careful with rising rate regulators because the EFI pump capacity DECREASES WITH PRESSURE. So your pump might flow say 160 litre/hr at 3.0 bar but only 120 litre/hr at 5.0 bar.
You have to make sure your pump capacity is always greater than your injector capacity otherwise you will lean out.

[StarionChef]

Thanks for that input guys

[ProZac]

jakobsladderz is the one with the brain here, i interpreted what he wrote and remembered some things from a class when i was training as a mechanic, (didnt last long). Interesting point was that the fuel pressure also changes with vacuum on an MPI setup (but not on that starion because the injectors are above the throttle plate). I wonder how many custom setups take this into account, could make it easier to set idle duty cycles with large injectors.

[StarionChef]

Yeah, I'll have to speak to someone in the know.

[jakobsladderz]

I have always seen the hose connected from the regulator to the manifold on MPI setups. It will help to maintain better idle mixtures to have the pipe connected to the manifold, and there is nothing to lose.

Sequential EFI helps a bit with getting a better idle from bigger injectors because instead of two short injection pulses per intake, you have one bigger one.

Injectors take around 0.8-1.5 milliseconds to open and 0.3-0.6 milliseconds to close. because of this, fuel delivery becomes a bit erratic when injector times are less that 2-2.5 milliseconds. Essentially as opening times get shorter the fuel flow depends more and more on the opening and closing characteristics of the injector. Big injectors mean shorter opening times and more erratic fuel delivery at idle.

That is also why staged injection setups are used. have small injectors for idle-midrange, then a set of other injectors (maybe the same, maybe bigger) to pour fuel in at higher throttle settings. much easier to get good idle, cheaper for two smaller injectors/cylinder (eg 8 magna 2.6 injectors = $80 vs 4 520cc RX7 injectors = $400+) but you need the right computer to control them.

[NXTIME]

Many large HP rotaries use a staged set up just as jakobz has described for exactly these reasons. They use some thing like 880cc primaries and 1700cc secondaries (2 x each).

Thanks for the helpful input.

Also, has anyone priced a new SARD fuel pressure regulator?