Fuel lines and injectors

kenewagner

Registered User
It has been brought up in diffrent threads about the smallish fuel lines, injectors that are maxed out. Just throwing these questions out there for discussion purposes.

1 At what point (Horsepower) is the small fuel lines a problem delivering flow and how much of a problem are the small lines in relation to injector duty cycle?

2 At what point (Horsepower) is the 255 pump inadquate to deliver fuel. Can the 255 pump be enough with bigger lines for big HP motors?

3 At what point does the stock fuel regulator become inadquate

4 What fuel pressure is recommend for a High HP motor 350, 400 400+

Just some things I would like to put out there for debate. It is slow lately on the boards so what think the rank and file?

Ken
 
Mathematically, the 255 HP should support 470rwhp considering 14V, a .60 BSFC and 20% drivetrain loss.

I don't think it's worth very much thought for the stock fuel lines. If you want the full output of the Walbro, you'll need bigger fuel lines. If you have restrictive fuel lines, the injectors will respond the same way as if the fuel pump was out of supply volume, and they will increase duty cycle.

Instead of asking for fuel pressure, the question should follow along the lines of what size injector do you need. The only thing fuel pressure does is alter the output of the current injector being used. All injectors are rated at 43.5PSI. That is what they are flowed at and what advertised mass flow they present it as. If you run an 83 lb/hr injector at 40PSI, you turn it into a 79.5 lb/hr injector. If you run it at 50 PSI, you turn it into an 89 lb/hr injector. Of course, you need a regulator to change that.
 
Mathematically, the 255 HP should support 470rwhp considering 14V, a .60 BSFC and 20% drivetrain loss.

I don't think it's worth very much thought for the stock fuel lines. If you want the full output of the Walbro, you'll need bigger fuel lines. If you have restrictive fuel lines, the injectors will respond the same way as if the fuel pump was out of supply volume, and they will increase duty cycle.

Instead of asking for fuel pressure, the question should follow along the lines of what size injector do you need. The only thing fuel pressure does is alter the output of the current injector being used. All injectors are rated at 43.5PSI. That is what they are flowed at and what advertised mass flow they present it as. If you run an 83 lb/hr injector at 40PSI, you turn it into a 79.5 lb/hr injector. If you run it at 50 PSI, you turn it into an 89 lb/hr injector. Of course, you need a regulator to change that.

Always can count of you to have a PHD answer:p I think your wrong on not giveing the stock lines much thought. Pretty much everyone out there except the big dogs have for years been running a 255 pump through stock fuel lines, so why tell people to upgrade to a 255 pump while running stock fuel lines if the stock fuel lines cant hand the full output of the pump. Also are you working the pump harder trying flow its capacity though lines that cant handle it. Just looking for some imput;)

Ken
 
The pump could very well have a good amount of head pressure thru the stock line and filter, if its restrictive enough to not allow the volume of fuel to flow thru it. Going to the larger feed line releases that head pressure and allows the volume of fuel to flow that the pump wants to move.
 
Why not run get a second stock fuel line, use a second 255lph pump, then run a line to each fuel rail. Keep the stock return line as is.
 
At 275rwhp, the walbro 255lph high pressure pump with 60lb/hr injectors, stock regulator and stock lines, there was no problem with the fuel line size.

At over 400rwhp with 80lb/hr injectors, same regulator and pump, same lines there was no hope of keeping up. Fuel pressure dropped significantly as engine RPM increased. I've heard of folks with similar issues and equipment just upgrading the lines to bigger supply and resolving the problem.

The Walbro 255lph HP pump flows about 51gallons (about 193lph) per hour at 60PSI.

So at what point do I think the lines become an issue? About the time you need to feed 80lb/hr injectors or are at the high Duty Cycle on 60's.

I do not have a feel for when the fuel pump isn't enough. I guess I would start taking a hard look once you are over 420rwhp and/or needing 80lb/hr injectors.

I think the stock fuel pressure regulator becomes inadequate when you start doing a custom rail or want to play with adjusting fuel pressure. Otherwise it seems to be pretty good and fits well with the tubing sizes of the fuel rail and return line diameter.

As far as what fuel pressure to use. Since our pressure is boost referenced, I don't know that it matters if you have higher than stock base pressure. if you have plenty of pump but are on the edge with injectors, there could be some value to bumping up pressure. But in the end, if you need to bump up pressure, you should probably be getting new injectors.
 
At 275rwhp, the walbro 255lph high pressure pump with 60lb/hr injectors, stock regulator and stock lines, there was no problem with the fuel line size.

At over 400rwhp with 80lb/hr injectors, same regulator and pump, same lines there was no hope of keeping up. Fuel pressure dropped significantly as engine RPM increased. I've heard of folks with similar issues and equipment just upgrading the lines to bigger supply and resolving the problem.

The Walbro 255lph HP pump flows about 51gallons (about 193lph) per hour at 60PSI.

So at what point do I think the lines become an issue? About the time you need to feed 80lb/hr injectors or are at the high Duty Cycle on 60's.

I do not have a feel for when the fuel pump isn't enough. I guess I would start taking a hard look once you are over 420rwhp and/or needing 80lb/hr injectors.

I think the stock fuel pressure regulator becomes inadequate when you start doing a custom rail or want to play with adjusting fuel pressure. Otherwise it seems to be pretty good and fits well with the tubing sizes of the fuel rail and return line diameter.

As far as what fuel pressure to use. Since our pressure is boost referenced, I don't know that it matters if you have higher than stock base pressure. if you have plenty of pump but are on the edge with injectors, there could be some value to bumping up pressure. But in the end, if you need to bump up pressure, you should probably be getting new injectors.

You & Casey always have a logical and complete look at the whole picture. Do you know what the exact ID of a stock supply line is? Would not one be able to figure flow capacity of that diameter is? What ever happen to 60s being big enough for anything we could put out HP wise:confused: I just wonder if upgrading fuel lines all the way to the fuel rail, while running a 255 pump could solve fuel problems for a lot of guys without going to a big injector like an 80? I like to think I am a logical thinker and when I look at the sizing on a stock fuel system where the fuel rail is 3/8" and the 6 foot long or better supply line is 5/16" or smaller, it just seems like a mismatch flow wise. Seems like, for an example, you have a small restrictive exhaust system and stick a huge muffler on the end thinking it will work better. (sounds like some Hondas I have seen) Im sure Fords engineers had it all figured out. Also does creating back pressure on a 255 pump trying to flow through a small line, create problems for the pump in the long run. Just seems to me that most of the time I have been here the recommended ways to solve fuel issues was ONE add a bigger pump. TWO get big injectors. And just in the last year I have seen more upgrading of fuel lines. Just wondering if we have the order of problem solving correct.

Ken
 
The stock lines are small. Especially where the push fittings go on. They round off the end of the line so it doesn't cut an o ring. Even going to -6 would be twice the size of stock.
 
We went from stock lines to -6 @ 450rwhp and saw no difference in fuel delivery. Personally I think any argument about how much or little fuel flow a given set of components is counter productive without taking into consideration all the different things that impact your results. Focusing on the biggest rocks will always net the biggest gain for your time/effort. Identifying the biggest rocks is step one.

For example, if you know that a 255lph fuel pump is marginal for your power level, trying to gain headroom by increasing line capacity is not going to be very productive.

Then on the other hand, designing your system to work as a package and then sticking to your design goals is going to be the best use of resources. When I design a product/package I look at the intended use for the item and direct all efforts at properly supporting that goal. Stating that 60lb injectors is "all we'll ever need" is a reflection of a misapplication of this thinking. I don't know who coined the phrase with respect to fuel injectors, but if it were me I would have to say that at the time it was an appropriate comment when taken in the context of what was being worked on back then and what was expected at the time.
 
We went from stock lines to -6 @ 450rwhp and saw no difference in fuel delivery. Personally I think any argument about how much or little fuel flow a given set of components is counter productive without taking into consideration all the different things that impact your results. Focusing on the biggest rocks will always net the biggest gain for your time/effort. Identifying the biggest rocks is step one.

For example, if you know that a 255lph fuel pump is marginal for your power level, trying to gain headroom by increasing line capacity is not going to be very productive.

.

So at 450 you saw no change but changing to a -6 line must have been in your package plan as something needed. I asked for advice when planning my fuel lines and it was recommended to go with a -8 feed and -6 return. At this time it seems I went overboard on fuel lines if a stock line will not restrict 450 rwhp. I should be good for the future to any hp limit my pocket book can stand.:p

Ken
 
If you can compensate with the tune via the injectors then a falling fuel pressure is ok as long as you can compensate. This is what happens or seems to happen on SC with more then 400HP. Fuel pressure drops and you increase rpms and boost.

Now for a nitrous system that relies on a constant fuel pressure at WOT, it is important for the pressure to not drop as the car will go lean when the solenoid robs fuel from rail/lines. This is why I was dead set on upgrading my fuel system last year. I guess the same can be said with the nitrous, that as long as you can compensate with the injectors then it does not matter, but it can be unpredictable and possibly dangerous if something went wrong.

If I did not run the niitrous, then i guess larger injectors and a tune to compensate for the pressure drop, would have been more then enough.
 
If I did not run the niitrous, then i guess larger injectors and a tune to compensate for the pressure drop, would have been more then enough.

If my experience is any indication, I would not be willing to state that it is possible to compensate unless the drop is very minor at the very end of peak rpm/power. It may make it survivable, but I wouldn't want to run it like that for very long.
 
Back
Top