Injector Flow Rates Vs. Boost Pressure

[Coffee Commando]

While trying to find the perfect balance of fuel injector size for 330 guestimated horsepower with 6 PSI of boost I came across the below information on a Mustang Performance web site that takes into consideration of the fact that boost pressure reduces fuel pressure output from injectors which are designed to spray into vacuum. I used the calculator at Witch Hunter's web site and did some extra math to consider that I would be using a 49 PSI fuel pump and came out with 36 lbs being a perfect match until I found the below information. In conclusion, I actually need 42lb injectors for 6 PSI of boost.

Simply put, any pressure that is exerted in the manifold above atmosphere pressure (for example, 6 PSI at sea level) needs to be compensated for either in the fuel pressure at the rail (fuel pump with 6 more PSI) or the injector flow rate (36 lbs being perfect for NA means that in order to run 6 PSI of boost I would need another 6 lbs at the injector. 36 + 6 = 42 lb injectors with a stock fuel pump).

My numbers were calculated using .55 BSFC.

--------------Fuel Injection Vs. Boost Pressure-------------------

Injector Selection


As you add larger heads, cams and other power-adders, it's also
necessary to increase your fuel output. Choosing an injector for the
proper application is relatively easy, however, it's different for a naturally
aspirated engine versus a supercharged engine. To calculate the proper
injector size you need to know the Brake Specific Fuel Consumption
(BSFC). The BSFC is how much fuel is used per horsepower per hour. For
a naturally aspirated engine, that figure falls between .45 - .5 BSFC.
Supercharged engines need more fuel, so the figure is slightly higher
at .55 - .60. When you're doing your calculations, remember that you
don't want the injector to run at 100%. A safe buffer zone and recognized
standard is to let the injector run at no more than 80%. The lower the
BSFC number, the more efficient your assuming the engine is. So, unless
you've spent time on the dyno and know what your BSFC is, use the
higher BSFC number for your application and calculations to be safe.

Injector size = (HP x BSFC) / (number in injectors x duty cycle)

Example: Choose an injector for a 350 horsepower naturally aspirated V8 engine.

(350 hp x .5 BSFC) / (8 inj x .8 duty cycle) = 175 / 6.4 = 27.34

In this example, the answer was 27.34 for an injector size. If you've been researching injectors you'll
know that this falls between our 24 lb and 30 lb injector selection. The 30 lb injectors would be the better
choice, versus buying the 24's and cranking up the fuel pressure. If you already have 24 lb injectors, you
can always try increasing the fuel pressure to attempt to feed the engine. Most of us are always looking
for the next gadget to add horsepower and the 30 lb injectors would give you a little play room while
meeting the needs of the current engine. If you put in an injector that is too large for the combination,
the vehicle will have a hard time running correctly. Once you get as high as a 42 lb injector, you may find
you need a custom chip burned or an engine management system. Don't forget to match your Mass Air
sensor calibration with the proper injector.
____________________________________________________________________________________

Injector Horsepower Limits


To find the approximate horsepower limit of a particular set of injectors we can use the following formula:

Horsepower limit = (Injector size x number of cylinders x duty cycle) / BSFC)

Example: What is the approximate horsepower limit of a 24 lb injector in a supercharged V8 engine?

(24 x 8 x .8) / .6 = 153.6 / .6 = 256

So, 256 horsepower is a safe limit for the 24 lb injectors if you have a supercharged engine, assuming a
maximum 80% duty cycle. A similar naturally aspirated engine with 24 lb injectors would have a safe limit
of 307 horsepower (BSFC of .5). Remember that these figures are simple mathematical equations. What
you or the next person can get away with all depends on your engine design, camshaft, timing, etc...
Most of us will never get an exact calculation for the combination we have, but the formulas will get us
into the ball park we need to be in.




____________________________________________________________________________________

Fuel Pressures


Some people think that cranking the fuel pressure on an injector will get them the extra fuel they need.
However, it's not really that simple. For starters, the engines computer will make every effort to correct
any fuel that you add or subtract that differs from it's look-up tables while running in closed loop. When
running in closed loop, the computer is constantly monitoring the oxygen sensors (O2's) and adapting to
the O2 readings. The computer will only compensate to a certain percentage. The computer is designed
to adapt, learn, and create new tables based on what it learns during closed loop. This also allows the
computer to adjust for sensor wear over the years, small engine problems, etc.. At wide open throttle
(WOT), the computer is in open loop and the O2 readings are not part of the fuel ratio calculations. At
WOT, the computer uses the learned adaptive tables from closed loop to kind of guess at the proper air
fuel ratio. This is one reason why tuning is very important.

The Mustang injectors are a high-impedance (13.5-19 ohm) saturation type injector that produces a
cone-shaped spray pattern. Increasing the fuel pressure will slow the injector's response time and alter
its calibration to the mass air meter.

Manifold pressures also effect the nozzle outlet pressure of the injector. (Nozzle outlet pressure is also
referred to as the Delta Pressure). The outlet pressure of the injector must remain constant. This
includes while your idling with 18" vacuum or under 12 lbs of boost from your supercharger. These forces
are constantly pushing and pulling at the injector. So under vacuum, that vacuum pull is actually helping
pull fuel from the injector. Under boost, the force is trying to prevent the fuel from being released from
the injector. Most of Fords injectors are rated at approximately 39 psi operating pressure. If you've ever
hooked up a fuel pressure regulator to the rail of a stock Mustang you should see approximately 30 - 32
psi.

Nozzle outlet pressure = fuel rail pressure - manifold pressure

To calculate nozzle outlet pressures with vacuum you need convert your vacuum reading to a
pressure (psi) figure. To do this we use the following conversion:

1 psi of atmospheric pressure = 2.036 inches of mercury (HG)

Naturally aspirated example: How do you get 39 psi at the injector if you're only seeing 30 psi at the
rail? Naturally aspirated engine with roughly 18" of vacuum at idle.

First, convert the 18" of vacuum to a pressure reading (psi).

18 / 2.036 = 8.84 psi

However, this is vacuum, so your 8.84 psi is a negative draw from the injector which is pulling more
fuel from the injector. Your figure should be: - 8.84 psi

30 psi at the fuel rail - -8.84 manifold pressure = 30 psi + 8.84 psi

= 38.84 psi (39 psi) at the injector


To calculate nozzle outlet pressures while under boost conditions, the same formula applies. However,
since you're not under vacuum, there is no need to change the manifold pressure to a pressure reading.
The assumption is made that your fuel pressure regulator will increase fuel at a rate of 1 psi per 1psi of
manifold pressure.

Supercharged example: You have a supercharger pushing 12 psi of boost into the manifold. What
fuel pressure do you need to overcome the boost pressure pushing against the injector? Your
injector is rated at 39 psi.

To keep the 39 psi rating of the injector we plug the numbers into the calculation:

Nozzle outlet pressure (39psi) = X (fuel rail pressure) - 12 (psi boost)

X = 39 + 12

X = 51 psi at the fuel rail

So, to keep the nozzle outlet pressure constant (Delta pressure), your fuel rail setting should be set at 51
psi while under 12 psi of boost.
___________________________________________________________________________

Fuel Pressures and How the Fuel Injector Ratings Change


The last factor in this is how increasing the fuel pressure effects the theoretical size of the injector. For
instance, if we have a 24 lb injector, rated at 39 psi, and we increase the fuel pressure to 50 psi, we can
determine the approximate size injector our 24 lb injector is now simulating.

_________________________________________________
New injector flow rate = _/ new pressure / pressure at the rated flow x rated flow

_______________
= _/ 50 / 39 x 24
__________
= _/ 1.28 x 24

= 1.13 x 24

= 27

Our 24 lb injector, at 50 psi, is now acting like a 27 lb injector. The same math applies if you drop the fuel
pressure from the rated flow of 39 psi. Your injector would act like a smaller injector.

[CobraMarty]

Your thinking and math is all correct. Other cars have the FPR hooked to the manifold so that a idle with vac there is lower FP and at WOT N/A there is a higher base FP and then when boost is added it raises the FP by the amount of the boost and that will compensate for the manifold pressure that the injectors have to push against. My '98 has a fixed FPR in the tank and no manifold reference.

Doesn't the avenger S/C system have a 2nd FP and FPR at the engine? You are right on the edge in figuring this injector size out. With a slightly lower BSFC figure the 36lbs injecctors will work fine. You have the alumn head and intercooler and only plan 6 pounds boost so I would try the 36s first. They will be easier to control at idle and for mileage. If you plan to turn up the boost 7-8 pounds the the 42s will probably be needed. Buy and run and tune the 36s and if they are just a little small, you can sell them and get the 42s.

My vote is the 36s to start and control them with either a custom tune or stand alone ecu or AEM FIC and the stock computer.

[Coffee Commando]

The low engine speed and idle is exactly why I wanted to run 36's. 42lbs is 100% more than stock meaning it's not good for idle which is something I wanted to avoid.

The Avenger only makes up to 5.5 PSI from my understanding and have just been using 6 PSI as a safe bet.

I guess I'll give the 36's a shot.

[CobraMarty]

42s and 100% more than stock means roughly 190x2= 380HP. That is making some assumptions and ignoring some things but it is a rough figure. My bet is for the 36s. Interesting that 36x6injectors = 216pounds of fuel. Which at .55 and 80% is good for 316HP, right where you want to be. I have stock 21lbx6=126 + 50lb 7th injector= 176pounds fuel good for 256HP and my sprintex says it makes 275HP, so just a little leaner BSFC and a bit more DC and it will all work.

[Coffee Commando]

Where is that 7th injector at? I can only assume a very central portion of the manifold.

[CobraMarty]

It is before the S/C. The fuel gets sprayed there, goes thru the S/C and the air and fuel gets 'homogenized' all together and then into the manifold and to each intake runner and cylinder. Pictures on www.boostecus.com

[CobraMarty]

here


Don't laugh, my Volant CAI would not fit, too tall, so this is the temp filter until the Spectre and silicone elbows arrive.


Here the intake manifold and short intake runners

[Coffee Commando]

here


Don't laugh, my Volant CAI would not fit, too tall, so this is the temp filter until the Spectre and silicone elbows arrive.


Here the intake manifold and short intake runners

So, is that 7th injector controlled the same as the others? Meaning the pulse width. And when does it spray? Everytime any injector sprays? is it the same flow rate as the others?

[CobraMarty]

The 7th injector is controlled by the AEM FIC. The FIC controls the ignition timing- it can retard the timing with boost and it controls the 7th injector however you program it to. The stock ecu controls the stock injectors as usual and under boost the AEM controls the 7th.

[Coffee Commando]

Here comes the flood...

What size are the first 6 injectors? The 7th? How do you have the AEM set to use the 7th injector under boost? Does it open and stay open until there is no longer any boost? Does your ECU operate in closed-loop mode and use the 02's or are you intercepting those?

How easy and long did it take you to get the AEM setup? Are you using the UEGO Wideband 02 with it as well?

So under boost the first 6 operate as normal and just the 7th turns on? Is there a special fuel rail just for that setup? Is that how Sprintex is designed to work or is this your own creation?

[CobraMarty]

All stock 6 injectors, I did change then to 703s, controlled by stock ecu and O2 sensors. Up until boost it runs and operates like stock. Under boost the AEM turns on the 7th injector (50lbs) and retards the timing both proportional to boost. So roughly, 1lb=20%duty cycle on 7th injector and 1* timing retard, 6lbs=80%duty cycle and 6*retard. The 7th injector is taped into the fuel line before the fuel rail. So the AEM only operates under boost and only controls the 7th injector and timing retard. The AEM is about 10 wires into the ecu harness, not too bad. This is how Sprintex designed it.

[Coffee Commando]

That sounds like the way to go to maintain bottom end efficiency. I like the way it sounds but I don't see how I can hook it up to mine that way since the Avenger kit is designed to up all of the fuel injectors.

Reading the manual for the Avenger it looks like you just install the kit with the larger injectors and let the ECU do the rest. Even the stock MAP sensor is used although it is relocated and attached to a different part of the manifold. Is that safe for a supercharger to do with only 6 PSI of boost?

So when you're under boost and the AEM takes over, is it cycling the injection pulse width based off of Crank/Cam sensors? Why do you have to retard the timing? Are you running a higher amount of boost than 6 PSI?

How do you do on gas mileage with that setup?

[CobraMarty]

You really should and must likely always need to retard the timing with boost. Usually 1* per pound of boost. This is to help prevent detonation.
The AEM pulses the 7th injector taking it's signal from #1 injector and modifying that pulse width depending on how it/AEM is programmed.
The AEM can also be wire to also alter all the stock 6 injectors and timing. That is what I would do with you. If you have larger injectors, then at idle and cruise and around town you should be running rich. With the AEM you can alter the pulse width to take out some fuel even when not in boost.
What size injectors do you have with the avenger kit? Does it have an auxillary fuel pump and fuel pressure regulator?

[Coffee Commando]

Good question. I'm in Afghanistan. My kit is back home. I'll have to review the email traffic to see what's in there.

The internet here is extremely slow so that will take a while since there are .bmp's attached to the email. Sigh~

I think the avenger came with 30 or 32lb injectors. I don't remember anything about FPR's or a fuel pump.

I'm not familiar with FPRs. Do they lower pressure to the fuel rail unless the injectors open up more (under larger duty cycle due to more pedal to metal) so that in idle conditions you aren't running rich?

[SilverXJ]

Reading the manual for the Avenger it looks like you just install the kit with the larger injectors and let the ECU do the rest. Even the stock MAP sensor is used although it is relocated and attached to a different part of the manifold. Is that safe for a supercharger to do with only 6 PSI of boost?

That will not work. Stock Map sensor doesn't see boost, nor does a PCM in stock tune know what to do with it.