Fuel rail test

[Redfox0099]

Can we replace the stock fuel line with a bigger one from the tank up to the front....somewhat easily and not horribly costly?

[lafrad]

A returnless fuel system like we have in the LX/Jeep/Mopar world today relies on a fuel pressure regulator in the fuel tank. It is a fixed regulator with no control to change fuel pressures.

The properties of fluid flowing through tubing, with restrictions like "injectors" limiting fuel flow *guarentee* that the fuel pressure at the fuel injectors WILL BE LESS than the fuel pressure at the outlet of the regulator. This is due to the fact that fuel flowing THROUGH a tube encounters resistance... which is generally influenced by the size & length of the tubing. The more fuel flow you demand, the more the pressure drops. In Rob's case, 6 PSI does NOT seem like a huge problem. In Hemi31's case, the pressure drop is so significant that the injectors can no longer efficiently inject fuel into the boosted pressure of the manifold.

The PCM has tables within that estimate the fuel pressure drop of the system, and adjust injector pulsewidths accordingly. The R/T and SRT have slightly different calculations, for which I have not followed up to figure out "why".

Also availble in the tuning, is the ability to adjust the fuel injector timing. The systems available to the LX computers sequentially fire the injectors, with each injector timed to the cylinder it is injecting fuel for. There is 720 degrees of control, so it is possible to adjust the timing of the injector to be anywhere within the 4-stroke engine's cycle. The CMR software available can only adjust the "base" injector timing... each injector has specific "offsets" from that "base" timing that are not available to be changed in the CMR.

keep in mind, that, by definition, an injector "injecting" at 80% duty cycle is open 80% of the time. This means, that @ 80% duty cycle, out of 8 injectors, AT LEAST 6 injectors are open at any point in time, and at certian points, 7 will be open. And with a firing order like our hemi's have, its very probable that 4 injectors per bank will be open for significant amounts of time.

If you have a system where there is restriction in the fuel rails, Injectors that are "closer" to the source of the fuel will "bleed off" pressure from the fuel that is going to the next injector in the system. If you are running close to the limits of the capacity of the fuel system, that pressure gradient in the fuel rails could be significant.

[lafrad]

Can we replace the stock fuel line with a bigger one from the tank up to the front....yes somewhat easily and not horribly costly? only if there is a group by for pre-bent pre-fitted hardline

So, get on that, would ya?

[lafrad]

I like the fuel rail upgrade because I feel it balances out the pressure to all the injectors Nathan......there is constant pressure to all.I think anyone running larger than stock injectors should do it IMO.But the boosted guys with cams,heads and 8psi or over need to log pulse width and then consider pump and line upgrades if need be.

The "Constant pressure for all" is the most significant part of this. if you can make the "restriction" int he fuel rails essentially zero, that means all cyls will be equally fueled, with little chance that "as you go lean, #1 & 2 go REALLY lean".

Funny part about all of this: is that an oxygen sensor reads an entire "bank" of cyls. So the computer has no idea that the front cyls are leaning out and the back are fattening up... and its happy to run the engine like that until it grenades.

[Redfox0099]

Um...me?

How about i look at it this weekend.

[lafrad]

RobAGD if you see 6 PSI drop on the "upgraded" setup I wonder what your drop in pressure was on the stock rails?

I bet he still saw "6psi drop" with stock rails. The major restriction in our fuel systems is the fuel line and fittings between the tank and each metal tube on each bank.

[lafrad]

Um...me?

How about i look at it this weekend.

yea you! ;-)

I'm just jokin.

Any shop with good tube bending skillz can get you a 3/8 or 7/16 line bent and installed on your car with "not too significant" effort. Problem is getting the 7/16 line properly attached to the factory slip-fitting on the fuel tank (do they sell those things in a form that can attach to hard line?)
On the front, you might as well use a flair fitting to the upgraded fuel rails... that is the least restrictive that you can get.

[InferAl]

Well on my set up now, I see a 6 psi drop at WOT

5.7 Hell fire, Street/Strip Heads, AEM, Cats, Zoomers, AH

Ford 39#'s

W/ BT Rails, and even rear and front cross overs.

Just an little fyi

-R

I bet he still saw "6psi drop" with stock rails. The major restriction in our fuel systems is the fuel line and fittings between the tank and each metal tube on each bank.

He sees the drop even with hte new setup

[lafrad]

He sees the drop even with hte new setup

Right,
I'm suggesting that I would expect there to be the SAME pressure drop at the front of the driver-side rail, with or without upgraded rails.

[InferAl]

I was under the car the other day looking where a -8 line could be run right to the fuel tank, I think it is a 3/8 quick connect fitting on the pump. The line seems to run up from the pass side over the tank, don't know if the tank would have to be dropped or pop up the pump from the back seat and run it down

[Hemi31]

Right,
I'm suggesting that I would expect there to be the SAME pressure drop at the front of the driver-side rail, with or without upgraded rails.

my point is that drop on 1 and 2 cylinder would be much less with the crossover in the front.

[Johnparts]

The drop is caused by the stock lines not the rails, that and the demand from the pump and regulator assembly it's like sucking though a straw the little straw you get in a cocktail compared to the giant straw you get with a big gulp. This all has been figured out on other cars guys it simple physics and fluid dynamics the stock feed in the tank is 5/16 so even if you open it up after that point you still have the 5/16th restriction even if the line is 1/2 inch

[Johnparts]

Let me break it to you like this. I built a 1965 Mustang a few years ago with a modern fuel injection system. When the car came to me the motor and trans were in the car running but running poorly. The stock pick-up was in the tank with an external pump pulling through the 5/16 pick-up in the tank. The only upgrade I made to the fuel system besides bigger fuel lines was a 3/8th pick-up in the tank and I increased fuel flow by 30% and the car began running better and became more tunable.

[lafrad]

The drop is caused by the stock lines not the rails, that and the demand from the pump and regulator assembly it's like sucking though a straw the little straw you get in a cocktail compared to the giant straw you get with a big gulp. This all has been figured out on other cars guys it simple physics and fluid dynamics the stock feed in the tank is 5/16 so even if you open it up after that point you still have the 5/16th restriction even if the line is 1/2 inch

Any restrictions BEFORE the fuel pressure regulator only come into play here if your pump cannot properly supply the pressure needed to keep the Pressure regulator happy.

Also, pressure drop is based on line size AND diameter (well, area).

I would expect a 3/8 or 7/16 line from back to front, after the regulator, to help maintain fuel pressure at *the same fuel flow rate*, in a significant way, with the SAME fuel pump.

I still think a regulator at the rails with a return system is the *absolute best* way to go.

[HEMI~C~]

Updates to the fuelish thinking thread that may help inform on this subject. Bottom line.... I upgraded the entire fuel line to -8 from the fuel pump to the rail....boost referenced external FPR and filter and still saw a significant fp drop at WOT with my combination....with the Boost A pump at max. No doubt the the fuel line/ext fpr helped improve the situation but the pump became the limiting factor. A second pump was needed.

Chris