Fox fuel pump upgrades

hawkinsdrag · Aug 17, 2023

At what point is a upgrade from stock fuel pump required?, My engine is a new 306 , 10:1 comp , TFS170 heads, 1.6 roller rockers,Comp XE274hr-12 cam, 93 cobra intake , FMS 24# injectors, PRO -M 75mm mass air, Accufab 70mm tb, BBK shorty headers and off road H pipe .It Idles perfect,cruises great just seems to stop pulling much above 60 mph . It really needs a gear change as still has stock 2.73 rear . Im going to install fuel pressure gauge so i can monitor it to see what its doing while driving. Thanks in advance for any help or advice

AeroCoupe · Aug 17, 2023

I would at least change the fuel filter if you have not done that but with your mods I would say a 155 or 190 lph pump would be a good idea.

Mustang5L5 · Aug 17, 2023

I run a 190 pump with my similar TFS170 headed combo. I'm fairly convinced I could have gotten away with a 155.

I'm against oversizing the fuel pump with stock fuel lines. When you increase flow through lines without increasing diameter, you increase pressure as well. The fuel pressure regulator drops pressure in the fuel rail, but up to that point you've increased the pressure in the main feed line. Your pressure test port (and injectors) is on the downstream side from the regulator, so we don't really have an accurate picture of what a bigger pump does to pressure in the stock feed line prior to the regulator. However, i work in an industry that tests flow in piping and see what cramming more flow through a tiny tube does. Every time i see a post of someone's fox going up in flames, I can't help but wonder if this was the cause. The lines at the tank and the rubber connection on the pass side lower bay are where i would be concerned. Some of these are 30+ years old.

AeroCoupe · Aug 17, 2023

If I remember correctly the 155 and 190 lph pumps have a maximum output pressure of 50 psi. The 190 lph high pressure and the 255 lph pumps have a maximum pressure of 87 psi. You will not see more pressure than that from these pumps.

With liquid being incompressible the pressure in a non-flowing piece of pipe is the same in that entire piece of pipe. With flow the pressure will be higher where the pump discharges versus the end of the pipe where the fluid leaves the system.

With the fuel rails we have and it being a return style system the pressure difference at any point on it will not drop unless the fuel rails are incapable of supplying the required amount of volume for the demand or the return system is not able to return enough volume at a given pressure.

Again, from my understanding the stock fuel rails are good up to 42 lb/hr injectors and anything larger than that you start to get uneven delivery to the injectors as there is not enough volume in the rails to support the demand and the pump cannot move any more volume due to the diameter of the rails restricts the flow. Generally you see 500 rwhp at the limit for a NA motor on pump gas. The SN95 rails are supposed to be good for around 600 rwhp on pump gas. I have seen numbers all over the place but in general those seem to be pretty consistant.

With all that being said you can run a 255 lph pump but that is on the ragged edge of the stock Fox fuel system so really a 190 lph high pressure pump is about all the stock fuel system can support. In all honesty the problem is the return line being too small in low flow conditions and not allowing enough fuel to get back to the tank. In high demand the supply line starts to be a problem in that it restricts the pump from providing enough fuel. Then you get into the fuel pump hanger discharge line and return line being issues. So really once you need a 255 lph pump the stock system is not really adequate to take full advantage of that pump.

hawkinsdrag · Aug 17, 2023

AeroCoupe said:
I would at least change the fuel filter if you have not done that but with your mods I would say a 155 or 190 lph pump would be a good idea.

Thanks for reply, it has new filter and new stock pump

hawkinsdrag · Aug 17, 2023

Mustang5L5 said:
I run a 190 pump with my similar TFS170 headed combo. I'm fairly convinced I could have gotten away with a 155.

I'm against oversizing the fuel pump with stock fuel lines. When you increase flow through lines without increasing diameter, you increase pressure as well. The fuel pressure regulator drops pressure in the fuel rail, but up to that point you've increased the pressure in the main feed line. Your pressure test port (and injectors) is on the downstream side from the regulator, so we don't really have an accurate picture of what a bigger pump does to pressure in the stock feed line prior to the regulator. However, i work in an industry that tests flow in piping and see what cramming more flow through a tiny tube does. Every time i see a post of someone's fox going up in flames, I can't help but wonder if this was the cause. The lines at the tank and the rubber connection on the pass side lower bay are where i would be concerned. Some of these are 30+ years old.

Thanks for the reply, will def keep this in mind , dont need any fires

Mustang5L5 · Aug 17, 2023

AeroCoupe said:
so really a 190 lph high pressure pump is about all the stock fuel system can support. In all honesty the problem is the return line being too small in low flow conditions and not allowing enough fuel to get back to the tank. In high demand the supply line starts to be a problem in that it restricts the pump from providing enough fuel. Then you get into the fuel pump hanger discharge line and return line being issues. So really once you need a 255 lph pump the stock system is not really adequate to take full advantage of that pump.

That's the problem i ran into when i did my 190 on a stock engine. without touching the FRP setting, my initial pressure was much higher. I had to reduce the FPR to get back to 39psi vac off and even at the lowest setting the best I could do was 40-42psi. I felt that even the 190 pump was too much for the stock system with stock engine.

Why i tend to suggest to pick an appropriate pump vs drop a 255 in for a stock 5.0 system.

AeroCoupe · Aug 17, 2023

I ran and continue to run a 255 lph pump and never realized the problems I was causing for myself both with the 306 combos and the current 331.

This last time around I changed the pump hanger to the Pro-M unit, ran 3/8” supply and return lines, in-line filter with 3/8” ends, ran braided AN6 from the body lines up to the fuel rails, and rewired the pump. Pump is quieter (no more cavitation at idle), pressure is more consistent, and the flow rate is higher (time to pump 1 gallon of gas before and after).

Again, anything more than a 190 lph on the stock system is not gaining a person much and it hurts the delivery at idle.

Dan02gt · Aug 18, 2023

I have a similar setup and run a 155 lph. Seems to do just fine. I'm also in the camp of not putting a 255 lph on stock fuel system because of the issues mentioned above.

hawkinsdrag · Aug 18, 2023

Dan02gt said:
I have a similar setup and run a 155 lph. Seems to do just fine. I'm also in the camp of not putting a 255 lph on stock fuel system because of the issues mentioned above.

Thanks for the reply , leaning towards a 155 lph pump

AeroCoupe · Aug 18, 2023

155 or a 190 just get what is available and buy from a reputable vendor. Looooots of counterfeits out there. Also, if you plan on putting boost on it later down the road get the high pressure version as you will have to overcome the added manifold pressure. So if you need 39 psi of fuel pressure on a NA car and you put 10 psi of boost on the motor you will need 49 psi of fuel pressure on the inlet of the injectors which will net you 39 psi at the outlet of the injector as you have to overcome the 10 psi of manifold pressure from the blower or turbo.

hawkinsdrag · Aug 18, 2023

AeroCoupe said:
155 or a 190 just get what is available and buy from a reputable vendor. Looooots of counterfeits out there. Also, if you plan on putting boost on it later down the road get the high pressure version as you will have to overcome the added manifold pressure. So if you need 39 psi of fuel pressure on a NA car and you put 10 psi of boost on the motor you will need 49 psi of fuel pressure on the inlet of the injectors which will net you 39 psi at the outlet of the injector as you have to overcome the 10 psi of manifold pressure from the blower or turbo.

Thanks for info , no plans to boost it as of now but never know when plans could change

Noobz347 · Aug 18, 2023

AeroCoupe said:
155 or a 190 just get what is available and buy from a reputable vendor. Looooots of counterfeits out there. Also, if you plan on putting boost on it later down the road get the high pressure version as you will have to overcome the added manifold pressure. So if you need 39 psi of fuel pressure on a NA car and you put 10 psi of boost on the motor you will need 49 psi of fuel pressure on the inlet of the injectors which will net you 39 psi at the outlet of the injector as you have to overcome the 10 psi of manifold pressure from the blower or turbo.

hawkinsdrag said:
Thanks for info , no plans to boost it as of now but never know when plans could change

...and just to add to this thought process, the 155 and 190 low press versions have an internal pressure pop-off that tends to make boosted applications go "Boom" when they do their job.

nickyb · Aug 18, 2023

To those worrying about too big a pump I say forget about it.
Twelve years ago I replace my original pump with a 255 and have daily driven it everyday with zero issues.

AeroCoupe · Aug 19, 2023

Same here and never affected my car until I started getting up on the hp limits of the stock fuel system and that is when the problems started. The 255 lph pump also is at the limits of the stock wiring in providing it enough amperage to operate correctly. Again, I would say that if you do not need a 255 then just get a 190 or 155 and go on down the road but its obvious from nickvb's and my experience with them over many years of service that they work in a stock system.

Noobz347 · Aug 19, 2023

I dunno. I don't have the "conviction" that a lot of you guys have about fuel pumps. My setup is as follows:

347
Kenne Bell
42# injectors
stock regulator
stock fuel rails
stock fuel lines
stock pickup (something I've been meaning to address one day)
stock fuel tank
Kenne Bell Boost-A-Pump regulating pump voltage at a constant
Walbro 255L high pressure

Now, because of the extra displacement and possibly the cam, I [might] be consuming a little more fuel at idle than a stock 302 but if so, it can't be much. I've never had fuel return problems or weird fuel issues as a result.

A couple of notes that may or may not apply: I also run a Kenne Bell Boost-A-Spark so, both my fuel system and ignition system stay saturated at a constant voltage.

It makes me wonder how many legit 'fuel issues' exist in the wild that are the result of inadequate power supply to these components at different power levels. Even a 3G conversion is no assurance.

For those that have read the "Fuel Flow Forensics" article in the resource section, you will note that power fluctuation at the pump causes the fuel volume to fluctuate widely.

Noobz347 · Aug 19, 2023

One other side note...

Always run the biggest, heaviest, largest capacity battery you can fit in the battery tray. [This] is the component that does the most work at reducing power fluctuations.

AeroCoupe · Aug 19, 2023

Sounds like you are using the stock pump with the KB boost-a-pump as a lift pump to the 255 lph inline pump. Stock pump will supply 132 lph with the KB BAP at 40 psi. With a 255 lph pump pulling fuel from it you drop the pressure the stock pump has to work against so my guess is it will flow even more and obviously enough to support your go levels.

Noobz347 · Aug 19, 2023

AeroCoupe said:
Sounds like you are using the stock pump with the KB boost-a-pump as a lift pump to the 255 lph inline pump.

I'm not sure I understand this. The Boost-A-Pump is not a pump. It's an adjustable voltage regulator that powers the Walbro 255L in-tank fuel pump.

AeroCoupe · Aug 19, 2023

Are you using the KB BAP on the stock intank pump?

Fox fuel pump upgrades

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