Please Explain Bigger Fuel Pump Logic!!

R

Raginstang

New Member
Sep 28, 2003
166
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SANTA CRUZ CA
#1
All right, I know this is a dumb question im sure.. but I think my fuel pump just went out and I have a question..

I do have a fairly modified engine (see Sig below) and am running the stock pump 88lph? my question is; whats the difference in running the stock 88lph pump with 24lb injectors at lets say 38 psi of pressure to running a bigger pump maybe 255 lph with 24lb injectors and the same 38 psi of pressure. will I gain anything by doing this? I don't really understand how if just by adding a bigger pump but not dramatically increasing the FP I will get gains or have a better running car? right now my car seems to be running rich. or maybe its really lean and the car is trying to fatten it up by holding the injectors open longer? anyhow.. someone please tell me why I need a bigger pump!
thanks--Mike
 
#2
You named it. The stock pump was created for a stockish motor. WHen guys get the power up a good ways (like a good HCI), it's nice to have the extra capacity. And these days the price difference for a 255 vs something smaller is relatively little, so why not?

You can use the calcuators out there to get an idea of how much pump you need, but I tend to fudge those numbers up a little bit (go a step beyond what it says, for intangibles).

My two cents. Good luck.
 
#3
yeah , i realize that the price for a stock pump and a 255lph pump is almost the same.. im just asking why do i need a bigger pump? when im not running something like 60 psi of pressure or something.. i mean i do have HCI bigger injectors,TB etc.. so im assuming i would need one.. just wondering how it makes a difference?? LOL
 
#4
Well, with the improved breathing (HCI), you move more air. So to maintain stiochiometry, you need more fuel.

It is a little analogous to why you run the 24's. Why didnt you keep the 19s? Because they might not be able to flow enough fuel for the air volume you are flowing now, right?

Maybe I dont understand what you are asking. If so, let me know and I will shut up. :D

Good luck with figuring this out.
 
#5
no, your right on HIssin.. I appreciate everyones replies.. I understand why I need to replace the injectors... but if my fuel pressure guage is reading at stock pressure or a little higher.. why would I need a bigger one?? I guess im just wanting to know how it will get the more fuel there if I keep the pressure the same?
 
#6
A 255 pump wont hurt anything.
Any fuel not used by the injectors will be returned to the fuel tank.

Also, is your pressure good through the RPM range?
Once the revs come up the engine needs more fuel and a stock pump might drop pressure. (sounds logical..)
 
#7
yeah, I was just wondering why I would need a bigger pump if I don't up the pressure? I mean if im using 24
s with a stock pump.. what would the advanage of a bigger pump be? also.. right now im at like 40 psi with the vacuum connected.. when I floor it the pressure drops to like 38 then comes up... seems okay?
 
#8
U need a bigger pump since a more milder engine like yours compared to stock will consume alot more gas than a regular stock 302 like Ford made it for...if the motor is drinking more gas up (like at WOT and especially with bigger injectors) you will need fuel to replenish the fuel since it is using up so fast and especially at higher rpms and higher fuel pressures the stock pump wont be able to keep up with the engines needs to replenish with what its using up so a bigger pump is used so there always a nice constant pressure on the injectors....dont want to blow a nice motor from running lean for a second just cuz u never got a bigger pump, that would suck......hope this helped
 
#9
wow, your not being understood verry well huh? lol
I dont know much about this but this is how i would think about it.
1.)The lines between your regulator and injectors need to maintain the pressure you set it at, to deliver the amount of fuel your injectors are asking for.
2.)if your injectors are big enough, but your pump is not the pressure is gonna drop and you will lose flow.
3.)when you have 38-40psi and your injetors open, you want the pump to push so much fuel back into those lines so the psi never drops.right??????

but if you had 60lb injectors, those dump a ton more fuel out of the line between the regulator/injectors.
if your regulator is set at 38psi and you have the biggest pump in the world the 60lb injectors are gonna drain the line after the AFPR, and since the regulator will not allow enough pressure to refill the fuel being taken out in time, thats when you need to increase pressure. its all gotta work together.

i dont think it has anything to do with getting more fuel into the combustion chamber,but keeping the injectors supplied with all the fuel they need.
your car may be fine with 24lb injectors and 38psi
but if those injectors want more and you give it more pressure will your pump keep those lines at that pressure when you floor it? maybe, i couldnt tell you.
I COULD BE 100% WRONG THOUGH
if i am someone correct me
think about if niagra falls only had a stream flowing to it, instead of a river.
:shrug: :shrug: :shrug: hope that helps
if you have a big pump, the proper pressure, and 24lb injectors.
if you are still running lean, then you need bigger injectors in order to actually get more fuel into the combustion chamber. i would think anyway.
 
#12
87stangdiddle said:
The only True way is by using a Wideband but yes you should upgrade the pump for ur current combo
Click to expand...

I agree, you cant tell for sure unless you know your air fuel ratio. looking at your combo I would say yes. and you might be in need of larger injectors as well. im not sure though. you said you cant tell if its rich or lean.
pull your plugs, if they are white(not tan) but WHITE. your lean.
try putting new plugs before you check and only drive it for a few min, bring it up to full throttle for as long as you can without doing 100mph in a 35mph zone then pull over and check one of them.
 
#14
Does TF have a recommendation for the fuel pump? I had thought they recommended something along the lines of a 190 LPH, but I could be completely wrong.
 
#15
For a H/I/C setup a 190 will be just fine and give you some room to go unless you go forced induction. You will need a larger volume fuel pump to keep up with the engine's fuel needs at higher RPM. At WOT your computer will not regulate your fuel pressure at 39... it just allows as much fuel as it can take.
Kevin
 
#16
We know what Kevin is saying, though it is a hair off. The computer doesnt reg FP at all - it controls fuel volume into the chambers via varying the injector pulsewidth. And that is where the extra FP from an AFPR can help at WOT (foxes, not 94-95's).

A 255 costs like 10 bucks more than a 190, so weigh that into the choice.
 
#18
On a fox, O2's are indeed disregarded at WOT.

But the computer doesnt regulate fuel pressure at all - it's a simple vacuum operated regulator.

When I mentioned that injector pulsewidth varies, that was in regard to that being the computer's general-means of stiochiometric adjustment (as opposed to FP). But I wasnt speaking about WOT. When I said,
"And that is where the extra FP from an AFPR can help at WOT (foxes, not 94-95's)," that was the differentiation (though I should have explicitly stated so). Because injector pulsewidth is not adapted at WOT on a fox, the AFPR is beneficial, just like you said. :nice: . (On 94-95's, there is WOT adaptation, which was the reason for the distinction - AFPR's are useless on those unless one sees a limit code otherwise).

I am tired and was splitting hairs - my bad. :cheers:
 
#19
A larger pump will not give you more "pressure"... Instead it's able to provide more volume in order to meet the demands of engine combo at stock fuel pressures (which is ideal). The only downside to having a pump that is oversized, is unessesarily heating the fuel. This time of year, that's not a huge issue. During summer months, it's not much of an issue for an N/A car and on RARE occations it may show some uggliness in summer heat for boosted apps. if you're running right on the ragged edge of your tune. The bottom line is that the differences in heat between the stock pump and a 255 low press pump in terms of downsides, is pretty minimal. The upside, is that you don't have to worry about being able to supply adequate fuel for combos mild to appraoching wild. A 190 for yor app will be more than adequate and may prevent you from having to screw around with elevated fuel pressures in order to provide ample fuel volume.
 
#20
if the pump does not supply pressure, what does?
when you turn a AFPR up to say?....60lbs, what makes the pressure increase?

say you have 19lb injectors, & a stock pump.
with this setup, at WOT (since the pulse width does not adapt) how much can you turn up the psi before the fuel volume becomes inadequate?
am I confused?
 
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