Im in the process of tuning my car and have a question for fellow tuners.

My setup consists of:
Large street port
GT35/74 Turbo
1680cc x2, 850cc x2 Injectors
Dual Walbro 255 Fuel Pumps T-ed into the stock feed line.
40psi base fuel pressure
20psi boost.
550cc water injection
92 Pump Fuel
45c Air Temp
85c Water temp



I am getting 85% duty cycle with 11.5 a/fs at 20psi at 8krpm. Is this normal? What should my duty cycle be for my setup at 20psi? I have been wanting to tune to 23psi but have been rethinking that because I am already at 85%. From previous readings and research, I thought the dual FP/ 5000cc fuel setup was enough for mid to high 20psi boost...Any thoughts?

 

Yes I think 5000cc fuel should be plenty for what you are wanting.

So you have both pumps feeding the stock line? If so, I would think that would be your limmiting factor. I had basically the same fuel setup but I ran parellel lines one to each fuel rail then back to the regulator and a single return line from there and was able to make 600whp at around 9?% duty cycle IIRC. Unless you have a bad pump, injector issues or dirty fuel filter.

 

Can you verify rail pressure while under full boost? If the lines are the limiting factor, you would be getting lower pressure than you should under full load. I'd say with the dual pumps, you could always bump your static pressure up to 45 to bring DC's down.

 

New filter, 2 new pumps, injectors are fine.

So yes, I would agree with you that that might be my issue. Anyone else have input?

 

No I cant because that gauge is mounted on my FPR. Yes, I guess I could do that but I do not want to retune my whole map...Ugh.

 

Yep looks like your going to have to turn up the base fuel pressure. Also make sure that both your pumps are getting equal voltage.

 

20 psi and a GT35 compressor is 60 pounds of air.

11.5 AFR... is 5.2174 pounds fuel X 85% duty = 6.138 pounds fuel

6.138/6.35 = .9666 GPM or 3658 CC/Min

you supposedly have 4900 CC/Min.

you either have a pump, line, FPR or filter problem.

fuel pressure is one of the 4 items i always log. it does little good to have a small gauge on your FPR so you can set idle pressure if you can't see it at 8000 rpm. buy a FJO 100 psi pressure sensor, screw it into the FPR and hook it up to your Datalogit.

don't bother changing the OEM fuel lines. they are good for over 600 rwhp.

good luck,

howard

 

Ive verified that my filter and FPR are fine, both pumps are new. The only thing left is the line size as they are both Ted together.

I will up the base FP to 45psi and check that both pumps are getting equal voltage and report back. I dont need much more fuel to get to 23psi from 20psi and I think that might just do the trick. Thanks for the input.

 

Got a pic of how the pumps are " T'd "?

 

Its not your base pressure. I run a single supra pump with less base fuel pressure then you and a bigger turbo (500R-SP) with smaller injector setup 750/1600cc's. I don't reach 85% @ 20PSi boost and 11.0 AFR. I agree with howard coleman, check your fuel pressure during a pull. What type of wiring do you have going to those pumps?

Also, are you using resistors and/or an injector driver? What lag settings are you using for the injectors etc.

thewird

 

Could it be the lag settings?

-.20 850cc front rotor
-.10 850cc rear rotor
.14 1680cc front rotor
.14 1680cc rear rotor

3 ohm injector resistors.

Each pump is wired with a 10 gauge ground and 2 16 gauge wires that connect into one 10 gauge wire that goes to the battery.

Each pump is getting 13.5v and the alternator is putting out 14v at idle.

 

Negative lag (or incorrect) settings will skew the injector duty cycle. How much, I couldn't tell you as I never really tested it. A proper injector driver like the FJO, will give you more fuel as well since your injector will open and close quicker.

thewird

 

Ah I see! Can anyone add to this? As to about how much it would skew the number?

 

Well a negative lag time would subtract from the pulse width rather than adding to it.. In reality the injectors need a positive lag time to flow predictably.

Tuning with lag times is bad! They should be measured and entered into the ECU so that it knows how to make proper compensations and ensure accurate fueling. I will try to get the math together to show you exactly what you are doing...

 

I think its the voltage going to the pumps when you are on boost. How are they wired? Whats your alternator voltage like?

 

"Tuning with lag times is bad! "

Aagree w Aaron

hc

 

This wasnt really a choice. I spent a week trying to get it to idle with 850ccs and zero lag to no avail. So I used negative lag as recomended by other tuners (Chuck Westbroke and Steve Kan use negative lag all the time to get 850ccs to idle if need be ).

Yes, if you could please put the math together for me that would be great. I want to know exactly how much more I can get out of these injectors and what the real duty cycle is.

 

Alternator voltage is constant 14v. I dont know the voltage to the pumps under boost because I am not able to hold a multimeter to the wires while boosting, but I am able to monitor the alternator voltage under boost and it stays at a constant 14v, so I dont know why the voltage to the pumps would change.

 

Seriously, it's pretty straight forward. If you have proper pressure at the rails under load, then that addresses all the problems mentioned, and you need to up the base pressure or get larger injectors. If you are getting lower than base + boost pressure under load, it will be one of the problems mentioned. You should get an electronic gauge on it and go for a ride, or at minimum an automotive fuel pressure tester set with an extended hose, tape it to the wipers and go for a ride.

 

You don't think that the negative lag settings are the moat plausible cause? After reading the above post about it skewing numbers, that makes the most sense.

 

What does lag have to do with duty cycle? if your injectors are at 85% duty cycle then they are at that duty cycle.

I reckon its your fuel pumps.

I have the same injector combo in a customer car and it is at 95% duty cycle at 30psi with a gt3582r but it has two raceonly 044s not walbro intank pumps.

 

Injector duty cycle in the PowerFC is a calculation based on values inputted. But like I said, I'm not sure how much of a difference that tiny bit of lag would make.

Watch your fuel pressure during a pull like everyone has mentioned.

thewird

 

Where is your pumps getting the power supply from? the stock setup with speed relay? or independent wiring from the battery? I had a similar issue when I was my pump from my stock power source for my pump (single denso supra pump) was out of fuel by about 15 psi..thinking it had something to do with fuel pump speed relay or something of the sort. Wired it straight the battery for a test and it solved the problem. So i just used to the stock pump setup to trigger a relay for the battery power straight the pump

 

I would say just redo your fuel lines. You already have 3 going forward to the front...just use the 2 largest ones for the pumps and use the small for you return. It's easy and doesn't take much to do...can't hurt at this point

 

I dont think the lag times would make enough difference that you would be close to running out of fuel at 20psi with your setup. When I was running similiar fuel setup I also was running the pfc with resistors. So I would still think that the problem is going to have something to do with the fuel setup.