Fuel Pressure

[DanC]

Two questions: (1) How important is it to maintain a constant or consistant fuel pressue on EFI, (2) is there any advantage in running a higher fuel pressure? Specifics: Kawasaki 650 twin (bored and stroked to 750), stock throttle bodies, and stock in-tank fuel pump. Holds 44 - 46 PSI at low RPM and drops to around 30 PSI under load measured at fuel log, 

[maj]

constant fuel pressure is one of the basics of fuel injection,
you probably have a restricted in tank filter , they can be back flushed and made better for a while but the real fix is replacment
trouble is the regulator is after the filter .
And initially as the filter blocked slowly the pump worked harder and harder to get your regulator pressure untill eventually when you start noticing a drop off in performance  the pump has probably used up the best of its life

Best test to check pump and filter is a flow test measuring the fuel supplied at the tank outlet , you will need to externally power the pump or bypass the ecu grounded trigger wire on the relay to do this , service manual will give you the cc or ml/min   

[RansomT]

I would put my money on a bad fuel pump ... heats up and looses it efficiency.

[Rex Schimmer]

As the Maj said a consistent fuel pressure is the very basis of electronic fuel injection. You need to maintain a constant differential pressure across the injector this assures that the fuel flow will be consistent. This makes the Maj's other point that the fuel filter, and you better use one, needs to be after the pump and before the pressure regulator. If your fuel pressure control valve happens to have a pilot port that will allow you to sense the manifold pressure and then adjust the fuel pressure to maintain a constant differential pressure across the injector I would highly suggest doing this, it is mandatory for super charged or turboed motors but works well on normally aspirated engines also.

If you increase the fuel pressure you will flow more fuel through the injector but it is not a linear relationship, i.e. you double the pressure you will not get twice the fuel. The formula for figuring the flow through the injector with pressure different than you have been running is:

New flow rate= (original flow rate) x the square root of ((new pressure)/original pressure)

Example: original pressure = 40 psia, new pressure 80 psia and let's say the original  flow rate is 1. Then the new flow rate would be: 1 x square root of ((80 /40)= 1.414 times as much fuel for a doubling of the fuel pressure.

Rex

[NathanStewart]

Is there a problem you're trying to fix?  Does the bike run poorly?  While having consistent fuel pressure is generally the rule, it's not absolute.  Motorcycle EFI systems can be very, very basic and not have all the same features and functions that a full fledged car ECU would have.  A lot of motorcycles have returnless fuel system and have a pressure regulator that's mated with the pump in the tank.  It's not completely inconceivable that you'd have high fuel pressure when injector duty is low at idle and then have fuel pressure drop some as more fuel is delivered through the injectors.  While some might immediately think this is a problem, if the fuel map was mapped for this change in fuel pressure, then there is no problem and AFRs should be acceptable.  This is exactly what you have to do when you tune an engine that doesn't have a fuel pressure regulator that references manifold pressure to maintain a constant "injection pressure".  As fuel pressure drops, large injector pulse widths values would be needed to maintain the same AFR. 

If there isn't a problem and the bike seems to run ok then I'd say that this is possible how the fuel system is supposed to work.

[johnneilson]

IMO, Nathan is right, the system sounds like it is functioning correctly.

If your measured pressures are accurate, when the pressure drops to 30psi you have only about 80% the fuel.
This should cause a pretty lean condition especially under load. Get a wideband on the goes-outta pipe and measure.

I have had a few cars where the injectors were large and would not idle down cleanly because the inj pulse was too short.
In these cases, a manifold referenced pressure regulator would drop the inj pressure down and allow for a longer inj pulse.

If you really start to modify the motor and need more fuel, a simple fix might be to increase pump capacity to maintain the higher pressure.

John

[firemanjim]

Don't think I agree with Nathan, I do alot of FI bikes on dyno and most have regulator in the tank with a relief that acts like a return and all of those maintain steady fuel pressure. If not the bike runs like crap under heavy load.

[stay`tee]

check your batterys voltage under load,,,

[RansomT]

Don't think I agree with Nathan, I do alot of FI bikes on dyno and most have regulator in the tank with a relief that acts like a return and all of those maintain steady fuel pressure. If not the bike runs like crap under heavy load.

Ditto!   Typically, the fuel pressure is dead on at 43.5 psi.  When the fuel pump craps out, typical on raced bikes when owners leave oxygenated race fuel in the tank and/or run very low levels of fuel, the fuel pumps start eating themselves.  During heavy load, AFR  goes crazy lean.  When you put a fuel pressure gauge on those bikes they will be stable at low loads, but during WOT pressure will drop to near nothing.

[NathanStewart]

Don't think I agree with Nathan, I do alot of FI bikes on dyno and most have regulator in the tank with a relief that acts like a return and all of those maintain steady fuel pressure. If not the bike runs like crap under heavy load.

I think the Power Commander motorcycle tuning world is just a small piece of a pretty big pie that comprises all "EFI" usage and tuning so in those circles where you're still having to work around the OEM ECU, having a consistent fuel pressure is probably important.  In the full stand alone EMS world, you can tune to varying fuel pressures.  In a way you're proving my point - on the bikes you tune if fuel pressure isn't steady the bike runs bad under load.  Obviously this would fall into a situation where fuel pressure is supposed and is expected to be consistent.  If an engine has a fuel system that changes fuel pressure under load, the ECU could and would need to be mapped to account for this change.  The only way to know if there is really a problem would be to look at other indicates like AFR.  Looking at fuel pressure alone doesn't really mean anything if other conditions aren't considered. 

It'd be nice to get a follow up from DanC about this.  If the bikes runs bad and has high AFRs then yeah, there's probably a fuel supply problem.