Maxing Injector Duty Cycle

[madjak]

On my race car, I have a Walbro 255, Sard FPR running at 60 PSI and 430cc injectors. According to the tune, the injectors are close to running at 85% duty cycle at high RPM and we're close to leaning out. ECU is a Haltech PS1000.

That is a lot of fuel going into a N/A engine! Could there be something wrong with the injectors or do I just need larger injectors as this engine is just hungry for fuel? Can you have issues with the stock fuel rail supplying enough fuel but maintaining pressure?

On the dyno the fuel pressure is not dropping and sitting at 60PSI so I don't think there is an issue with fuel supply. What else can I test? Is there any way to know what rate an injector is flowing at?

[Magpie]

I do not know my duty cycle, however fuel pressure is 43 PSI, ID700 injectors (I think) and same fuel pump. Engine is NA as well.

[NitroDann]

Something is wrong. Need to be making 350 odd rwhp to do that to those injectors on that fuel system.

Dann

[madjak]

Something is wrong. Need to be making 350 odd rwhp to do that to those injectors on that fuel system.

Dann

I don't know much about injectors but it doesn't sound right to me. I don't know what the injectors actually are as they were supplied with the engine. The engine builder gave me the specs and said they had no issues running them.

Can a mismatched impedance of the injector be a problem. I assumed a 'high' impedance but it could be something else.

Fuel pressure doesn't drop so that means there is fuel getting out of the rail as the FPR is on the return. I have bought a new fuel filter as the one on the car is old, but still I don't think fuel supply is the issue.

The only other thing I can think of is the injectors aren't 430cc, or they are clogged or something. The engine runs great, though it does use a lot of fuel, like 10-12 liters per 15 mins of running on a track.

[NitroDann]

They are probably stock.

Dann

[Dan]

A few things.

How much power are you running? That would tell if that IDC is normal. Also have you checked voltage getting to the pump?

Also why are you running the pump at 60psi? Flow reduces significantly on the 255 with higher pressure, you can google flow tests of it to see the difference.

The 255 also has a pressure relief valve that cracks at higher pressure, not sure if that may be cracking prematurely either (you can mod it not to release).

They are probably stock.

Dann

Makes a lot of sense, raise fuel pressure to get more flow out of the injectors since the fuel pump is pretty big

[madjak]

A few things.

How much power are you running? That would tell if that IDC is normal. Also have you checked voltage getting to the pump?

Also why are you running the pump at 60psi? Flow reduces significantly on the 255 with higher pressure, you can google flow tests of it to see the difference.

The 255 also has a pressure relief valve that cracks at higher pressure, not sure if that may be cracking prematurely either (you can mod it not to release).

They are probably stock.

Dann

Makes a lot of sense, raise fuel pressure to get more flow out of the injectors since the fuel pump is pretty big

Thanks for you help guys...

Power is around 170hp at the wheels.
Fuel pressure was raised whilst on the dyno to get a bit more buffer in the duty cycle on the injectors. I'll find out the part number on the injectors to see what make and flow rate they are.

I am using stock wiring to the pump, maybe I should upgrade it to larger power wire. But given I'm getting 60PSI at the fuel rail throughout the dyno run I don't think fuel supply is the issue.

[Magpie]

http://www.autospeed.com/cms/article.html?&A=107986
Pros/cons of upping fuel pressure instead of replacing injectors.

[madjak]

Yes I can get bigger injectors and will if I need to but if they are 430cc injectors the problem must lie elsewhere. I wouldn't have thought I'd hit the limit of 430cc injectors with N/A and the posts above confirm that. I'll check the model number tonight and confirm.

For now I'm running higher pressure to stop the engine leaning out at revs, but it's not really a long term solution. Once I upgrade my air intake I'll need more fuel again so I do need to resolve the issue.

[Magpie]

I'm making about the same power and NA as well. However I'm running bigger injectors and less fuel pressure.

[sailaholic]

Ok can you confirm why you think they are at high duty cycle?
Is it based on the haltec calc?
Engine leaning out and adding more fuel in the table has no effect?
Looking at the pulse width and doing the calc?

Sorry if you've covered this but I'm not clear.

I'm also voting something wrong. That size injector should run any NA.

Wait, unleaded or e85?

I know plohls old haltec used to tell him 110% duty cycle or something but doing the calc showed 80% or something and the car never leaned out.


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[Dan]

Thanks for you help guys...

Power is around 170hp at the wheels.
Fuel pressure was raised whilst on the dyno to get a bit more buffer in the duty cycle on the injectors. I'll find out the part number on the injectors to see what make and flow rate they are.

I am using stock wiring to the pump, maybe I should upgrade it to larger power wire. But given I'm getting 60PSI at the fuel rail throughout the dyno run I don't think fuel supply is the issue.

I agree, if you've already checked and there is no pressure dip on the dyno fuel supply isn't the issue here, if you needed more pressure to create a buffer on Duty Cycle then the Injectors are almost certainly the problem and you probably don’t have as large Injectors as you think you do as Dann suggested or you are running E85 and that IDC doesn't sound too unrealistic.

To try to simply explain the relationship between Injectors, pressure and fuel pump flow in your situation, if the Injectors were large enough and you had a problem with fuel flow then you would run less pressure which would increase flow from the pump but your tuner did the opposite and increased pressure to create headroom on the injectors which means the Injector size is the restriction and not the fuel pump.

If your objective of all of this is to run lower fuel pressure then the only real way to solve it is to get some bigger Injectors and re-tune for them, or if you are happy with 60psi Fuel Pressure then 85% IDC is ok. Your Tuner will be able to make a suggestion on what to do.

I know plohls old haltec used to tell him 110% duty cycle or something but doing the calc showed 80% or something and the car never leaned out.

IDC is just a calculated number so you can often go over 100% before the injector is open 100% of the time, it’s just the closer to full open your injectors are it becomes less controllable in the same way you don’t want overly large injectors as it's difficult to control.

[gslender]

No. If IDC is over 100% then you've got the deadtimes wrong.

[project.r.racing]

another vote for smaller than expected injectors.

[Dan]

No. If IDC is over 100% then you've got the deadtimes wrong.

No, IDC is only a calculated number which uses IPW and RPM as inputs and that is why it can go over 100%.

To try to explain it simply, the Injectors only have a limited window to deliver the fuel the engine needs in each cycle (2 revolutions) and this windows lessens with increasing RPM. If the ECU calculates that the amount of fuel the engine needs necessitates the Injector be open (IPW) for more than the window theoretically allows then this is demonstrated with an IDC which is over 100%.