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Fuel pressure drop when in boost

 
Old 11-18-2018, 12:47 PM
  #1  
dopefedz
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Default Fuel pressure drop when in boost

So I tried putting my car on the dyno yesterday after swapping to a smaller pulley and going to 3" exhaust. On the first pull attempt we noticed the car breaking up top. Tuner checked some stuff out and had a buddy record my fuel pressure gauge. Same thing happened and the pull was aborted. At 5k rpm the car made 20lbs of boost. Im running the magnafuel 9950b regulator which is boost referenced with no boost the fuel gauge was showing about 44psi and during the pull it went up to 64psi which is inline with 1:1 ratio but then it dropped off right away back to about 44-45psi.

All parts of the fuel system are new. Everything has less than about 10 months of actually seeing any E85 through it. Easy thing will be to check and clean the stainless external filter and the harder one will be to look inside the tank at the filters on the pumps. Yesterday I ran each pump individually with no vacuum line attached to the regulator and cranked up the regulator till I saw 70psi on the gauge just to see if they would go that high and hold at that pressure which they both did just fine. Not sure if that proves anything or not.

At this point I'm trying to determine if a filter or filters are clogged or if maybe a pump is weak. Pumps are cheap enough to replace but a major pita to get to. When I initially finished the build and put the car on the dyno back around March we had no fuel issues at all.

This what my fuel system consists of I really don't think im running out of fuel but maybe the more experienced will chime in.
* twin walbro 450's Y'd in the tank to a -10 bulkhead
* -10 feed from fuel hat to Y block at the rails the legs of the Y go to -8 to each rail and -8 into the regulator
* - 8 return
* redhorse 6" filter 40 micron stainless element
* holley rails
* 1300cc fic injectors
* fore fc3 controller (wiring is 4 gauge from alternator to controller ground is also 4 gauge, pump pigtails from controller are 12 gauge and attach to wiring a bulkhead at the top of the tank which is 14 gauge to the pumps this about a 2 foot run of wire)

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Old 11-18-2018, 03:15 PM
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junk c5
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What are you using for the vacuum line on the regulator ? Years ago I used cheap vacuum line on a manual boost controller and under high boost the vacuum line would collapse. Now I use thick fuel system grade line or -4 braided.
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Old 11-18-2018, 04:25 PM
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ysb02
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The voltage isnít dropping as you near the top is it?
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Old 11-18-2018, 05:17 PM
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dopefedz
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Originally Posted by junk c5 View Post
What are you using for the vacuum line on the regulator ? Years ago I used cheap vacuum line on a manual boost controller and under high boost the vacuum line would collapse. Now I use thick fuel system grade line or -4 braided.
I thought about that but dont that line see positive/boost pressure when in boost so it wouldnt be able to collapse right? Unless im think a$$ backwards haha. Its currently a 1/4" fuel hose with a decent wall thickness.
Originally Posted by ysb02 View Post
The voltage isnít dropping as you near the top is it?
Not sure about this. Other than maybe looking at the gauge during the pull unless hptuner logs it? Ill have to ask.
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Old 11-18-2018, 05:35 PM
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junk c5
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If it’s fuel hose you should be good. I made the mistake of using rubbery flimsy thin hose.

HP tuner should log voltage.

Last edited by junk c5; 11-18-2018 at 05:35 PM.
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Old 11-18-2018, 06:01 PM
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Originally Posted by junk c5 View Post
If itís fuel hose you should be good. I made the mistake of using rubbery flimsy thin hose.

HP tuner should log voltage.
Yeah its a thicker hose. I did just ask him about voltage he did say its logged and will get back with me tomorrow.
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Old 11-18-2018, 09:25 PM
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So while waiting to see what the voltage looks like I tried something I saw a guy recommend someone else online. With the car off I turned on each pump individually. I set my fuel reg to 50psi and my air compressor reg to 20psi. I then applied the air to the boost reference fitting on the reg and the pressure went up to 70psi accordingly testing each pump individually.

The gentleman that posted this suggestion said if it made the pressure that it should then there is either a lack of pump or a flow issue. What Im not 100% sure of is if he means I need another pump because two cant keep up or if the pump is weak/deffective and as far as a flow issue is concerned I guess that would mean that maybe there is some filter clogging causing issues.
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Old 11-18-2018, 10:30 PM
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junk c5
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Taken into consideration your combo, is it possible that you’ve surpassed the maximum output for dual pumps ? I run my base fuel pressure at 58 lbs. I’ve got a Bosch 044 pump for running the car and a magnafuel 4303 pump on a Hobbs switch for boost on E85. My cars tuned at 17lbs.

Do you know your injector duty cycle ?

Last edited by junk c5; 11-18-2018 at 10:38 PM.
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Old 11-19-2018, 12:59 PM
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As pressure increases, flow rate decreases. So it's possible you can hold 70+psi, but the actual volumetric flow of your pumps is reduced so much that you are flowing much less than 450gph * 2. Then when your engine is drinking e85 quick enough to keep up with 20lb of boost it causes the pressure to drop off. You can see what I mean about increased pressure decreasing volumetric flow at this link: https://walbrofuelpumps.com/walbro-f...-fuel-pump-e85

Last edited by nickalltogether; 11-19-2018 at 12:59 PM.
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Old 11-19-2018, 01:11 PM
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dopefedz
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Originally Posted by junk c5 View Post
Taken into consideration your combo, is it possible that youíve surpassed the maximum output for dual pumps ? I run my base fuel pressure at 58 lbs. Iíve got a Bosch 044 pump for running the car and a magnafuel 4303 pump on a Hobbs switch for boost on E85. My cars tuned at 17lbs.

Do you know your injector duty cycle ?
Thought about that also. Seen others make 1100 even 1200 on two 450s so i thought i wouls be ok. I did find out the voltage was at 13.2.
Originally Posted by nickalltogether View Post
As pressure increases, flow rate decreases. So it's possible you can hold 70+psi, but the actual volumetric flow of your pumps is reduced so much that you are flowing much less than 450gph * 2. Then when your engine is drinking e85 quick enough to keep up with 20lb of boost it causes the pressure to drop off. You can see what I mean about increased pressure decreasing volumetric flow at this link: https://walbrofuelpumps.com/walbro-f...-fuel-pump-e85
I appreciate the reply I was reading up on that too last night while trying some things and was surprised how much they drop at higher pressures
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Old 11-19-2018, 01:24 PM
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Last night me and a buddy conducted a few more experiments lol that lead me to believe the pumps are ok. I couls be wrong but maybe you guys can tell me what you think.

First test was running each pump individually thru the external filter also and comparing how much fuel they could push into a bucket in 10 seconds running off just battery power. I was hoping one wouls be drastically less maybe proving it was either weak, failing or maybe its fuel sock was clogged. Well they both filles the bucket to the same mark.

Second tests my buddys cousin did some calculating and told us the pump should be able to fill a one gallon jug in 30 seconds. Tried this with each pump individually and they both filled a jug within 30 seconds.

So at this time both pumps can make the pressure and from the two half assed test appear to be moving the same volume of fuel.

i was hoping the voltage logged would show maybe a drastic drop but it holding at 13.2 volts according to my tuner.

The last thing I noticed yesterday while pressuring up the reg with air was that although the vacuum hose to it was not collapsed or split it was not anywhere near as tight on the nipple I installed it over on the side of the intake and my dumbself didnt put a clamp over it. Maybe just maybe pressure was leaking by at this connection causing the reg to drop the pressure. Its the only thing that is starting to make more sense now.
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Old 11-19-2018, 03:07 PM
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What pump hanger are you using? What wiring bulkhead is feeding power to the pumps? The OEM bulkhead is rated for a ridiculously low current and could be causing issues at higher RPMs where the pumps are demanding more current than the bulkhead can deliver.
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Old 11-19-2018, 10:09 PM
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Im running this Holley bulkhead thru the top of my tank down to the pumps. The bulkhead is 14 gauge wire just like the leads on the pumps. From the bulkhead wiring to my fore controller it is 12 gauge.


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Old 11-19-2018, 10:41 PM
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silver408z
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I am by no means a fuel system expert but when it comes to any pump, they could be failing as the pressure rises from the boost reference. I was told also that these pumps have a built in by-pass reached at certain pressure. Not sure what that pressure is but under boost you might be opening a bypass on a pump.
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Old 11-20-2018, 12:16 AM
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Yeah I read it starts to open at 110psi and fully open at 135psi.
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Old 11-20-2018, 03:45 PM
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Got the car back together and atttempted a quick pull to see how the fuel pressure would respond. i have my base fuel pressure set to 58. I got in the throttle and immediately let off when I heard some popping/backfiring out the exhaust. my boost gauge showed a peak of 21lbs and my fuel pressure only went up to 63psi. Something is definitely up. Im pretty sure if I add the 21lbs of boost to base fuel pressure I should be seeing something like 79lbs of fuel.

I cant figure out if my pumps are weak or I dont have enough pump for what the car needs or maybe the regulator is acting up. The difference between the car now and when it was dynoed back in march/april is bigger exhaust and a smaller pulley. Peak boost previously was 19lbs now im at 21lbs pretty quick.
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Old 11-20-2018, 11:25 PM
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junk c5
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Do both pumps run at the same time or is 1 triggered by a Hobbs switch ?

Last edited by junk c5; 11-20-2018 at 11:26 PM.
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Old 11-21-2018, 12:07 AM
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I turn on the second pump with a toggle switch. Both pumps come on. Ive tested them individually and together.
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Old 11-21-2018, 12:25 AM
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Gotcha. Just making sure both pumps are activated during big boost. I’ve got a 5lb Hobbs switch kicking my big pump on.
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Old 11-21-2018, 03:34 AM
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Might be time to stop d!cking around with these walbros and run an external magnafuel.
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