Fuel Pressure dropping at 5500 RPM? - Ford Mustang Forum
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post #1 of 10 (permalink) Old 07-02-2019 Thread Starter
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Fuel Pressure dropping at 5500 RPM?

I recently had a positive displacement supercharger installed on my 2013 Mustang GT and there seems to be an issue with my fuel system we are trying to figure out. I am not complaining about the kit or the installer. Just trying to help figure out how to solve the issues while having it dyno tuned with an HP Tuner.

It is a DOB phase III kit with a VMP Gen II supercharger that came with 52 lb injectors. I also had a JLT 123 mm Big CAI upgrade, VMP boost a pump, and FRPP twin 65mm cobra jet throttle body installed.

When tuning I was told that the fuel pressure was good but the engine was running out of injectors so I had 72 lb injectors installed. When tuning with new injectors the fuel pressure was dropping about 15 psi beginning at approximately 5500 RPM so I had another OEM fuel pump installed and the issue still exists. All the fuel lines have been checked for obstruction and voltage checked at the pump is 18 Volts.

Any suggestions on what might solve the problem or what to check next?

Thanks in advance.


2013 GT, Track Pack, Recaros, Kooks Axlebacks, Airaid CAI, BMR Rear Adj. LCAs, BMR Relocation Brackets, JLT Oil Separator(s), Barton 2 Post Shifter Bracket, Steeda Motor Mounts, Barton Shifter, JHR Clutch line, BMR adj. UCA and Mount, Kooks 1 3/4 headers/catted H, DSS Al Shaft, BMR Loop, Vogtland Sport Springs front/BMR rear, Strange Adj. Struts/Shocks, Extended ball joints, Steeda Bumpsteer Kit, Whiteline Tranny Mount Bushing, Borla Overaxles, BMR Watts, Vorshlag 3" Ducts
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post #2 of 10 (permalink) Old 07-02-2019
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I really don't know, but I'm suspecting the Boost-A-Pump is not adequate, maybe you need a real high volume fuel pump system . . . I think DOB has one that they recommend but I don't remember the specifics; I think I'd ask Jason or post on the question on their FB group.

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post #3 of 10 (permalink) Old 07-02-2019
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Need to run the calcs on the required fuel needed....my gut is saying not enough GPM/Flow/PSI...here is the formula...




Fuel Injector Sizing- Lb/Hr Maximum HP Times BSFC (.5)
Multiplythe number of injectors by .8


BSFC= Brake Specific Fuel Consumption

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post #4 of 10 (permalink) Old 07-03-2019 Thread Starter
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I asked Jason and he said that I have all the parts I need to make at least 650 to 700 hp on 93 octane. The only thing he could think of being the issue is the alternator might be weak and not supplying enough amps for the fuel pump to run hard. I will post on the DOB owners FB page and see what fuel pressure they are getting to the fuel rails under 10 psi of boost and upper RPMs.
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2013 GT, Track Pack, Recaros, Kooks Axlebacks, Airaid CAI, BMR Rear Adj. LCAs, BMR Relocation Brackets, JLT Oil Separator(s), Barton 2 Post Shifter Bracket, Steeda Motor Mounts, Barton Shifter, JHR Clutch line, BMR adj. UCA and Mount, Kooks 1 3/4 headers/catted H, DSS Al Shaft, BMR Loop, Vogtland Sport Springs front/BMR rear, Strange Adj. Struts/Shocks, Extended ball joints, Steeda Bumpsteer Kit, Whiteline Tranny Mount Bushing, Borla Overaxles, BMR Watts, Vorshlag 3" Ducts
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post #5 of 10 (permalink) Old 07-03-2019 Thread Starter
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Quote:
Originally Posted by Beechkid View Post
Need to run the calcs on the required fuel needed....my gut is saying not enough GPM/Flow/PSI...here is the formula...




Fuel Injector Sizing- Lb/Hr Maximum HP Times BSFC (.5)
Multiplythe number of injectors by .8


BSFC= Brake Specific Fuel Consumption
I calculate if I want 650 rwhp>>> ((650 hp)(BSFC = .65)/(8 cylinders)(.85 Duty Cycle) = 62lb/hour injectors. The 72 lb injectors are supposed to be good for 753 hp with .65 BSFC @ 40 psi. And (72lbs/hour)(.85 Duty Cycle)(8 cylinders) = 489.6 lbs/hour

I also calculate based on Kenne Bell data that a stock coyote pump at 17.5 volts working against 60 psi of pressure can pump 435 liters per hour = 114.9 gallons/hour = 706.6 lbs/hour. So the stock pump should be good if I did the calcs correctly.

Maybe the drop in fuel pressure is normal? coyote pump limited to 58 psi - (7.5 psi losses)-(10 psi of boost) = 10.5 psi

2013 GT, Track Pack, Recaros, Kooks Axlebacks, Airaid CAI, BMR Rear Adj. LCAs, BMR Relocation Brackets, JLT Oil Separator(s), Barton 2 Post Shifter Bracket, Steeda Motor Mounts, Barton Shifter, JHR Clutch line, BMR adj. UCA and Mount, Kooks 1 3/4 headers/catted H, DSS Al Shaft, BMR Loop, Vogtland Sport Springs front/BMR rear, Strange Adj. Struts/Shocks, Extended ball joints, Steeda Bumpsteer Kit, Whiteline Tranny Mount Bushing, Borla Overaxles, BMR Watts, Vorshlag 3" Ducts
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post #6 of 10 (permalink) Old 07-04-2019
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Quote:
Originally Posted by PMDmustang13 View Post
I calculate if I want 650 rwhp>>> ((650 hp)(BSFC = .65)/(8 cylinders)(.85 Duty Cycle) = 62lb/hour injectors. The 72 lb injectors are supposed to be good for 753 hp with .65 BSFC @ 40 psi. And (72lbs/hour)(.85 Duty Cycle)(8 cylinders) = 489.6 lbs/hour

I also calculate based on Kenne Bell data that a stock coyote pump at 17.5 volts working against 60 psi of pressure can pump 435 liters per hour = 114.9 gallons/hour = 706.6 lbs/hour. So the stock pump should be good if I did the calcs correctly.

Maybe the drop in fuel pressure is normal? coyote pump limited to 58 psi - (7.5 psi losses)-(10 psi of boost) = 10.5 psi
Ok, so in theory you seem to be good......I would definitely check the voltage/amps and also check the fuel filter....it might have gotten a few particulates in it that might be restricting the flow as well...Ö.then call the supercharger guys.....it might also be a software issue or just completely normal as the system is being maxed out but is operating within range.

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post #7 of 10 (permalink) Old 07-08-2019 Thread Starter
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Originally Posted by Beechkid View Post
Ok, so in theory you seem to be good......I would definitely check the voltage/amps and also check the fuel filter....it might have gotten a few particulates in it that might be restricting the flow as well...Ö.then call the supercharger guys.....it might also be a software issue or just completely normal as the system is being maxed out but is operating within range.
My fuel filter is part of the fuel pump assembly without an external fuel filter. Thus the new OEM fuel pump assembly should have a clean filter. The guy tuning my car said I had 2 options. Leave the OEM pump in there that is dropping pressure and live with 595 rwhp or get an aftermarket higher capacity fuel pump. I did not like the idea of only 595 rwhp and the tune not being perfect with a fuel pump that is losing pressure. The stock pump has about 281 liter per hour capacity with 13.5 volts at 55 psi fuel pressure and good for up to 753 rwhp according to Kenne Bell article. The boost a pump should increase stock pump to 463 liters per hour with 17.5 volts and 50 psi fuel pressure or 1.65 times the stock pump. It makes no sense why the stock pump is not sufficient.

Anyway I opted to go with a DeatschWerks In-Tank Fuel Pump w/ Install Kit - 415 LPH (11-14 GT, V6) so with the boost a pump I estimate I should be able to flow at least 680 liters per hour. It should be delivered today and hopefully by tomorrow I will find out the results.

2013 GT, Track Pack, Recaros, Kooks Axlebacks, Airaid CAI, BMR Rear Adj. LCAs, BMR Relocation Brackets, JLT Oil Separator(s), Barton 2 Post Shifter Bracket, Steeda Motor Mounts, Barton Shifter, JHR Clutch line, BMR adj. UCA and Mount, Kooks 1 3/4 headers/catted H, DSS Al Shaft, BMR Loop, Vogtland Sport Springs front/BMR rear, Strange Adj. Struts/Shocks, Extended ball joints, Steeda Bumpsteer Kit, Whiteline Tranny Mount Bushing, Borla Overaxles, BMR Watts, Vorshlag 3" Ducts
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post #8 of 10 (permalink) Old 07-09-2019
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Originally Posted by PMDmustang13 View Post
It should be delivered today and hopefully by tomorrow I will find out the results.
Let us know if this is your magic bullet. If so, I guess someoneís math is off somewhere, unless the original pump simply isnít delivering at rated volume.
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post #9 of 10 (permalink) Old 07-18-2019 Thread Starter
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Let us know if this is your magic bullet. If so, I guess someoneís math is off somewhere, unless the original pump simply isnít delivering at rated volume.
The DW400 fuel pump did the trick for fuel pressure. I have gotten my car back as of Friday and there is still an issue with driveability with the tune that the plan is to bring the car back in about 6 weeks to get the tune finalized. The extra power from the SC has been fun to experience. I have to baby the car in 1st and get up to about 4500 rpm in 2nd gear to avoid surging while getting into boost. The surge is less as I go up in the gears. I think it is running pretty rich for safety purposes but that is just my guess.
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2013 GT, Track Pack, Recaros, Kooks Axlebacks, Airaid CAI, BMR Rear Adj. LCAs, BMR Relocation Brackets, JLT Oil Separator(s), Barton 2 Post Shifter Bracket, Steeda Motor Mounts, Barton Shifter, JHR Clutch line, BMR adj. UCA and Mount, Kooks 1 3/4 headers/catted H, DSS Al Shaft, BMR Loop, Vogtland Sport Springs front/BMR rear, Strange Adj. Struts/Shocks, Extended ball joints, Steeda Bumpsteer Kit, Whiteline Tranny Mount Bushing, Borla Overaxles, BMR Watts, Vorshlag 3" Ducts
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Originally Posted by PMDmustang13 View Post
The DW400 fuel pump did the trick for fuel pressure.
So, it seems the old pump didn't deliver it's rated volume (as I suspected). I'm glad you got it worked out, and appreciate the followup. Let us know how the final tune cleanup works out...

Try starting out in second gear when not facing an incline. I don't have an SC, but I do this all the time and it makes for a smoother start.

The 3.73's are touchy when in 1st and cold. I can only imagine with a PD power-adder the effect is amplified. I skip-shift for a very smooth (automatic trans like ride) from 2 -> 4 -> 5-> 6 (if you are at a speed that calls for 6), when not hot-footing it. Practice that a bit and you'll be surprised how you can smooth her out, and still be very quick...


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