Flow rate of the L-jet injectors.
03-23-2004, 04:20 PM
#16
Also remember that as the fuel pressure rises, the lbs/hr rating will not rise equally - it will be a lesser curve, and will trail off over 4bar (14.7x4). Also, as the pump is required to push these higher pressures, there may be other inter-system changes that will affect the true output volume of the injectors.
03-23-2004, 05:22 PM
#17
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Originally posted by BrendanCampion
Also remember that as the fuel pressure rises, the lbs/hr rating will not rise equally - it will be a lesser curve, and will trail off over 4bar (14.7x4). Also, as the pump is required to push these higher pressures, there may be other inter-system changes that will affect the true output volume of the injectors.
Also remember that as the fuel pressure rises, the lbs/hr rating will not rise equally - it will be a lesser curve, and will trail off over 4bar (14.7x4). Also, as the pump is required to push these higher pressures, there may be other inter-system changes that will affect the true output volume of the injectors.
Soiunds like Tim has this dialed in rather nicely.............
03-23-2004, 05:36 PM
#18
Yep. Thats why tim has the 30lbers in there. He knows you can control those quite well with pressure and not have too fat of a fuel curve down low. Very nice indeed.
03-24-2004, 03:10 AM
#22
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Originally posted by Gretch
"The S4 cars call for higher pressures. Accoring to the WSM around 3.8 bar, or around 55 psi above manifold pressure."
With 19# injectors.
So you get the same flow capability with 30# injectors and a static pressure of 40psi or 33 at idle. Exactly the "at idle" tuning called for in Murphy's kit. Tim's setup, with a rising rate regulator sensitive to boost will then cover the aditional fueling requirements.
At full boost (8lbs in my case, 1/2 an atmosphere) I should need 50% more fuel to avoid going lean, correct?
That means the fuel pressure has to rise to 50 psi, correct? It does that easily. If the 19# injectors were sized for 80% duty rate, there should be at least that much head room in the 30# injectors, at least at 8psi of boost.
Am I all wet here or is it this basic?
"The S4 cars call for higher pressures. Accoring to the WSM around 3.8 bar, or around 55 psi above manifold pressure."
With 19# injectors.
So you get the same flow capability with 30# injectors and a static pressure of 40psi or 33 at idle. Exactly the "at idle" tuning called for in Murphy's kit. Tim's setup, with a rising rate regulator sensitive to boost will then cover the aditional fueling requirements.
At full boost (8lbs in my case, 1/2 an atmosphere) I should need 50% more fuel to avoid going lean, correct?
That means the fuel pressure has to rise to 50 psi, correct? It does that easily. If the 19# injectors were sized for 80% duty rate, there should be at least that much head room in the 30# injectors, at least at 8psi of boost.
Am I all wet here or is it this basic?
Not exactly all wet, but I think a quick jump into in orifice theory might make injector selection (and related fuel pressure) a bit more cohesive.
First, pressure rises as the square of flow through an orifice (for our purposes an injector). So to double the flow through an injector takes four times as much pressure. Don't forget that this assumes the pressure on the business end of the injector to be static - obviously it's rising in a boosted app. so that must be figured into the end equation. In other words, if there's 20 PSIG boost that the injector is spraying against, then there's that much less differential, even though you raised the fuel pressure four times, the injector is fighting to push out that fuel harder than it was at say, 1 PSIG. Which means less fuel. Not a huge deal when folks are putzin' around at 5-8 PSI boost. A big honkin' concern when folks start running 15-20+ PSIG boost.
(this is not gonna be pretty, as my scientific notation skills for email kinda suck, but here we go....)
( New Flow)
New Pressure = Old Pressure X _________ squared
( Old Flow)
In the above equation 'new flow' over 'old flow' should be contained in one set of parenthesis. The 'squared' should be to the upper right of the parenthetical equation. Sorry, toldja it was gonna be rough.
This one is even worse, so I'm just going to do it longhand and hopefully you'll see how the equation works...
If you know the flow of an injector at a given pressure, then you can calculate the flow at a new pressure with the following....
New Pressure
New Flow = Old Flow X the square root of ------------------
Old Pressure
If you don't like math (I sure don't like doing it by hand), or simply can't make heads or tails of the mess I put up there, Google 'injector calculator' or some such. There's about a million of them out there that use this formula.
Hopefully this helps you understand how the numbers work, so you can get a feel for how injectors and pressures should be modulated.
Greg
Addendum: The formulae post out even worse than they look when I typed them. Just scoot "New" and "Old Pressure" over to the right, so that they are over the "division" bar. Ugh.
03-04-2007, 09:17 PM
#23
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Sorry for digging up an old thread, but as a mod on F6 I know how annoying duplicate threads are 
A little background first: we have a US 928S with the following mods:
Euro heads, cams, pistons
Borla exhaust
And now a programmable ECU equipped with a MAP and temperature sensor.
We only just got the ECU in and programmed, and as we got some hesitation near redline we got the ECU programmer and checked the injector duty: low and behold 100%. Our search in fuel injector info led me to this thread.
What injector flowrate would you recommend? We currently run stock injectors and 2.5 bar I believe. We could bump the pressure to 3 bar but that would still put them over 90%. Given the fact that we can't even get full potential from the engine I guess that's not an option huh?
TIA
A little background first: we have a US 928S with the following mods:
Euro heads, cams, pistons
Borla exhaust
And now a programmable ECU equipped with a MAP and temperature sensor.
We only just got the ECU in and programmed, and as we got some hesitation near redline we got the ECU programmer and checked the injector duty: low and behold 100%. Our search in fuel injector info led me to this thread.
What injector flowrate would you recommend? We currently run stock injectors and 2.5 bar I believe. We could bump the pressure to 3 bar but that would still put them over 90%. Given the fact that we can't even get full potential from the engine I guess that's not an option huh?
TIA
