Call me tomorrow and i'll explain it in detail....the 39lb. you are refering to is known as the commanded delta-p value. :good:Hermann

 

Explain it now. This is a tech thread.:harhar:

 

You injector flow rate is measured at 39psi of fuel pressure, so a 19lb/hr injector only flows 19lb/hr at a fuel pressure of 39psi of fuel, if pressure is increased, injector flow rate increases, decrease fuel pressure and decrease injector flow. Also your fuel pump DC not only increases when you go into boost, but your injector flow rate will increase also. There are a lot of other variables that will determine DC for both of them. I am sure hermann will correct me when I am wrong, I am by no means a tuner.

 

Right! It is not a requirement that the ΔP value match the size of the injector. Injectors are rated in terms of their flow rate at a given ΔP, which is specified in units of pounds per hour (lb/hr)...(not pounds per square inch, psi). Actually, you'll find it is typical to spec the flow rate at a ΔP of ~3 bar (~43.5 psi). Their flow rate can be increased with a higher ΔP, which goes up as the square root of ΔP. The amount of fuel they will squirt in is the flow rate times the pulse width, PW. Duty cycle is the PW*RPM/120, with PW in seconds.

 

slightly off topic, but how are you inserting the Delta symbol? thanks

 

Is the delta of 39 at all optimal? I see the 43.5psi delta referred to on occasion and wonder if that is for better atomization and a higher overall yield at the top end where they would otherwise be maxed out or am I wrong altogether?

 

depends on the injector manufac, some rate at 39 some at 43.5. i know most bosches are rated for 43.5. I usually take that into account when i program for delta p.

 

I have been reading this and other material. What we are dealing with is:
fuel rail pressure - manifold pressure = fuel pressure to the injector.
It seems fuel injectors are rated by fuel delivery (lbs/hr) for a given fuel pressure (psi).
So our system attempts to maintain 39psi in the fuel rails (which is also the pressure to the injectors).
So when guys replace their stock injectors with "60lb" injectors, what they are doing is installing injectors that supply 60lb/hr fuel at 39psi of fuel pressure??????

 

Not quite...that 39 psi is the drop across the injector.

And yes...60 lb/hr injectors will flow at that rate at the spec'd ΔP.

 

Yeuppers! Here's some great info on the 60# injectors we are all using (I've also attached a pdf file at the bottom of this post with drawings and dimensional specs):

Siemens # FI114961
Mototron # INJ-GAS-006
Accel # 74620L
MSD/Edge # 2030

FYI, all of these above are the same injector.

Static Flow Rate @ 43.5PSI (300kPa) w/Gas: 63.20 lb/hr = 668 cc/min = 475 g/min *
Coil Resistance: 12 Ohms / High Impedance / High-Z (No ECM driver modifications required)
Gain: 0.11ms/mg
Offset: 0.055ms
Turn on time @ 14VDC: 1.14ms
Turn off time: 0.85ms @ 600KPa
Factory Tollerance: +/-6 %
Spray Pattern: Cone
Connector: Minitimer (Bosch EV1)
Factory fitted with Viton upper and lower o-rings.

 

fjules, instead of re-typing a book, I'm just going to quote two terrific articles from SCT:

Fuel Injectors 101

Before I can go into fuel systems and losing fuel pressure, I think the first thing to do is give you some information on fuel injectors and fuel pumps. This one will cover fuel injectors and the next one will cover fuel pumps.
I have been told before that this is just math and math has nothing to do with cars (this was on another site, not here) so if this is your attitude, then please just press the back button now and stop reading, this is not for you.

Fuel injectors have a flow rate. This flow rate of the injector is rated at a certain pressure drop across the injector. Meaning the injector flow is say 30#/hr at 42.5 PSI. This means that that injector will flow 30#/hr as long as the pressure at the supply side, minus the pressure in the manifold is 42.5 psi. A fuel pressure regulator is designed to keep the fuel injectors at a certain pressure drop. In Ford systems this pressure drop is 39.5 psi, in the rest of the industry it's 42.5 PSI (don't ask, I don't know why).

This is why fuel pressure is lower under vacuum. The vacuum is a pressure less than atmospheric so it lowers the rail pressure to keep the pressure drop to 39.5 psi. At WOT; when the vacuum should be zero, the rail pressure should be pretty close to 39.5 psi. If your motor has 20" of vacuum, this is actually -10 psi (vacuum in inches of Hg, is double what the pressure is in psi). So with 20" Hg vacuum, you should have a fuel rail pressure of 39.5 psi -10 or 29.5 psi. If the engine has 10" of vacuum, that's -5 psi, so your rail pressure should be 39.5 - 5 or 34.5 psi.

Now, on a blower car it's the same thing. The fuel pressure regulators on the car have a gain of 1:1. This means that for every 1 psi of boost, it will raise rail pressure by 1 psi, to keep the pressure drop across the injector to 39.5 psi. So, if you are running 10 pounds of boost, your fuel pressure should be 39.5 + 10 or 49.5 psi.

This is why you should always set fuel pressure with the vacuum line off the regulator; this gives you the true pressure drop across the injector. Now, if you set your fuel pressure to 45 psi, all the above stuff is still true, but your base pressure is now 45 rather than 39.5.

Now, you can make a smaller injector flow more by raising fuel pressure, I think we all know that. There is a mathematical equation that tells you pretty much exactly how much the flow rate will change based on this new pressure drop.

To determine the new flow rate of your injectors you take the square root of the new pressure divided by the pressure at the rated flow times the rated flow. Let's go through this once. If you have an injector that flows 30#/hr at 39.5 psi, and you raise fuel pressure to 55 psi, what is the new injector flow rate? You would take 55/39.5 which is 1.392, and then take the square root of this, which is 1.18 and then multiply this by 30, which is 35.4 #/hr. So, a 30# injector at 55 psi is now a 35.4# injector.

The same thing applies for losing pressure. If you have those same 30# injectors but your fuel pressure drops to 35 psi, your new injector flow rate is 28.2#/hr.

Now you can roughly calculate how much HP a set of injectors will support with some math as well. On an engine dyno there is one value that is measured called BSFC or Brake Specific Fuel Consumption. This is the pounds of fuel that it takes to make one Hp for one hour. Typically a Naturally Aspirated 4.6L engine will have a BSFC of about .5 and a blown car will be about .6, the actual numbers may be slightly different, but these are close.

If you have 8 injectors and each injector is rated at 30#/hr, then you have a total fuel flow of 8 time 30 or 240 #/hr of fuel. If your car is blown with a BSFC of .6, then you can take this 240#/hr of fuel and divided by the BSFC of .6, then these injectors would support 400 HP at the crankshaft. There are cars out there making more than this, I have worked on some, but in those cases fuel pressure was raised up higher. In some true race only cars, you can run them a little leaner and get a little lower BSFC as well. In the above example if you raised fuel pressure to 55 psi with these 30# injectors, you would be able to support 35.4 times 8 divided by .6 to get a HP of about 472.

Now the beauty of a returnless system is this. It measures the pressure drop across the injector so it constantly adjusts the injector flow rate based on the actual pressure across the injector. In addition, you can electronically raise fuel pressure, only at higher flows, to increase injector flow rate.

Fuel Pumps 101

Like fuel injectors, fuel pumps have a rated flow at a certain PSI. For example, a 255L/hr pump is rated at 255 L/hr with a pressure drop of 40 PSI between the inlet and the outlet. Now, in most cases the inlet pressure is 0 psi, or atmospheric pressure, but in some cases it's higher. Like when you add a T-Rex inline pump. Now your stock pump pumps to the T-Rex and the T-Rex pumps to the motor. The pressure at the inlet of the T-Rex should be higher than atmospheric since you have your intake pump pushing into it.

Like fuel injectors the flow rate of a pump changes based on the pressure drop across the pump. It even follows the same equation, the rated flow times the square root of the pressure at rated the rated flow divided by actual pressure.

But, unlike a fuel injector, whose flow increases when you raise the pressure drop across it, a fuel pump loses flow when you run more pressure. An easy way to understand this is the following example;

The faucet in your sink is turned off. There is pressure at the other side, but now no water comes out, there is no flow. As you turn the faucet on slowing, the flow increases, but the pressure drops across the valve in the faucet drops. At full on, there is probably the same pressure on both sides of the valve in the faucet, meaning no pressure drop, but it's flowing a bunch of water.

Now, remember how the fuel pressure regulator keeps the pressure drop across the injector constant to 39.5 psi? (Even in a returnless car the same thing happens, it's just done electronically). Well, if you have 10# of boost in the manifold, and a 40-psi pressure drop across the injector the rail pressure is 40+10 or 50 PSI. This is where rail pressure is important. That means the fuel pump is now pumping into 50 PSI rather than 40 psi. And since the pressure rise across the pump is greater, it's flow rate drops. Follow the math;

You have a 255 L/hr pump that's rated at 40 psi. You have a blower with 10# of boost so rail pressure is 50 psi. What is the new fuel pump flow rate? You take the pressure at which the flow is rated, 40 psi; divide this by your base fuel pressure, 39.5 psi, plus the amount of boost you are running, 10#. This is 40/(39.5+10) or value of .81. You then take the square root of this and multiply it by the rated flow. So the square root of .81 is .9 times 255 for a new fuel pump flow of 255 times .9 or 229 L/hr. So, with 10# of boost your 255 L/hr pump is really only a 229 L/hr pump.

Let's go a step further, remember in the fuel injector example about running 30# injectors up to 55 psi to make them 35.4 # injectors? Let's say that car is running 10# of boost also. Now you take 40/(55+10) which equals .615, now take the square root of this to get .78 and then multiply this by 255 to get a new fuel pump flow rate of 200 L/hr.

Now lets translate this fuel pump flow into HP. To get from L/hr to # of fuel per hour, assuming a density of gasoline of .76g/cc, you would multiply L/hr by 1.2733 to get #/hr of fuel. While I could show the math as to how I got the 1.2733, I don't know if anyone is interested.

So, the same equation of fuel flow to HP is true as in the fuel injector example. You take #/hr of fuel delivered and divide it by the BSFC (Brake Specific Fuel Consumption) of the engine.

Going to the first example of a 255 L/hr pump blowing into 10# of boost reducing it's flow to 229 L/hr. How much HP will this pump support? You'd take the 229 times the 1.273 to get a total fuel delivered of 292#/hr. You take this number and divide it by the BSFC of .6 to get a HP of 486 HP that this setup will support.

Here is another example; Let's say you have a '03 Cobra running 20 psi of boost. The factory pumps are rated at 155 L/hr each and there are two of them. So, total pump flow of 310 L/hr. If you take 40/(39.5+20) you get .6722, then take the square root of this to get .82 and multiply this by pump flow rate of 310 L/hr to get a new pump flow rate of 254 L/hr. This is about 324#/hr of fuel and with a BSFC of .6 it will support a HP of 539 at the crankshaft. I will post data in a post late tonight or tomorrow, (data, not opinion, or my cousins neighbors car did this) that show a stock '03 Cobra pump and tank (no boost a pump) losing fuel pressure above 500 RWHP. I will also show this same car with a boost a pump and how that impacts fuel pressure.

Next example. You have a T-Rex inline pump. This pump is actually only a 190L/hr pump. And you have your stock intake pump of 110 L/hr. With 10# of boost how much flow is in your fuel system…

For the most part, the answer is, you don't know unless you measure the pressure between the two fuel pumps. I have measured this pressure, and you'll have to trust me on what that pressure is.

In a 350 RWHP car, with a stock intake pump and a T-Rex, there was 20 psi between the two pumps. So, now you take 40/(20+10) to get 1.333. Now I used 20 instead of 39.5 psi because the actual pressure drop across the pump is 20 psi, not 39.5. The square root of 1.333 is 1.155 and then this multiplied by the flow of 190L/hr is 219 L/hr. This is a total of 279#/hr of fuel delivered and with a BSFC of .6, it would support a HP of about 466 at the crankshaft.

This info is important to know. You need to understand that the more pressure you run, the lower the fuel pump output is.

 

interesting. So what is the lb/hr of the stock injectors?

 

39 to 42 depending on who is answering...lol! :beer:

 

Wow, great info here, learning a lot!
So:
Stock injectors are good for (39 x 8) /.6 = 520 HP.
60# injectors are good for (60 x 8) / .6 = 800 HP

Hermann -> So thats why you sold me those 60# injectors!:thumb2:

 

Exactly!!!:salut: Remember this Frank, your 6xx RWHP cobra is making 700 plus H.P. at the engine/crank!!:headspin:

 

ok, I think I have basic understanding of the injector side. Since we all seem to have insomnia tonight, I have a fuel pump question. How do you know if your pumps are adaquete?
We have pumps rated at X L/hr and we have eight injectors supplying 60#/hr. I guess I would first figure out how to convert #/hr to L/hr and then determine my pumps flow rate at the desired fuel rail pressure? I am guessing that pump voltage creeps into this because many of us use BAPs.

 

How do you know if your pumps are adaquete?

The easy way is to datalog fuel pump duty cycle. You could calculate it too, but with so many other variables it can get tricky. (They dont publish flow rates thru the fuel lines)

 

OK but isn't the 60 lb. rating at 39.5psi. At 43.5psi they are 63.2 lb. and if commanded at 45psi for example then they would be 64.1 lb. Then 64X8/.6=855. What is a useful limit or limiting factor for increasing the commanded delta.

 

This is a common trick to increase the injector output, and can work quite well. The fuel pump would be the limiting factor. You need enough pump in order to support the increase in delta, in order to maintain pressure.

 

I looked back at James' 101 doc. Here is what it looks like for 20# boost:
For 8 injectors supplying 60#/hr => (8 x 60)/1.2733 = 377 L/hr of fuel for DeltaP of 39psi. To maintain 39psi with 20lbs of boost the fuel pressure in the rail needs to be 59psi. So I would need to find out what our pumps L/hr is at 59psi @ 12V. If insufficient then look at the same numbers but at 17V pump voltage to decide if a BAP will get me there?

 

Ask and you will receive!!:friends:http://www.kennebell.net/techinfo/general-info/BAPsvtFocus_kens.pdf GOODNIGHT to all!!

 

This is a great thread! Lots of good info!

For those that may be interested, I wrote a calculator to size the injectors using the method I previously described. I prefer this method because it takes the guesswork out of having to figure out what value to use for BSFC, (which is a function of AFR, compression ratio, and engine and blower efficiencies, etc.)

It is posted as a Word document, so you need to rename it to change the extension to turn it back into and Excel file. So after you download it, change the "doc" after the period to "xls". Then it should open up just fine in Excel. (This handy posting tip thanks to eschaider!)

Edit ~9PM: Reposting directly as an Excel file. Hope it works! (Otherwise, it's the same file as before. So if you already downloaded it, you're all set.)

How well does it work? Seems to come out pretty close. See an example case here:
Click => 60lb Injectors enough for 22-23psi?

 

It is posted as a Word document, so you need to rename it to change the extension to turn it back into and Excel file. So after you download it, change the "doc" after the period to "xls". Then it should open up just find in Excel. (This handy posting tip thanks to esaider!)

Eric, guess what, you can now upload csv and xls files. I had Mr. Brooks add them in.

Valid file extensions: bmp csv doc gif jpe jpeg jpg mp3 pdf png psd txt xls zip

 

This is a good thread. Nice work Eric with the calculator. Looks like in cooler weather the 60's aren't going to cut it for my setup. I wonder if I should get bigger injectors or just raise the fuel pressure? Thoughts? My pump duty cyle is around 85 currently. I really don't won't to splice in an injector driver if raising the pressure can work as well, to a limit of course. Hope I didn't get to far off track.


ps Is the "injector rated at delta P" and the "actual delta P" backward Eric?

 

Rod - Yeah, I had it upside down there! Good catch, and thanks! I'm reposting the revision.

 

Rod - Yeah, I had it upside down there! Good catch, and thanks! I'm reposting the revision.

:good: No thank you! Good tool.

 

Pump info (from here => http://www.modularfords.com/f17/ford-gt-pump-info-look-inside-49292.html ):

 

good info

 

Been away for a few days and found this thread when I came back. Truly awesome!!

This is the straight skinny on how to make horsepower in our engines and why just putting the big pulley on the bottom and little pulley on the top doesn't always produce the result you might expect.

It's not a very big stretch if you think about more boost to see why some dyno curves lay down when you might intuitively expect them not to. Our fuel systems have simply run out of the reserve capacity necessary to feed the engine! The 'little' Cobra pumps have no more to give. Hello GT pumps.

No intention to Hi-Jack the thread but an excellent complement to this information might be what happens to our engine's appetite for fuel and its ability to produce power as we start to put 18/20/24 or more psi in the intake manifold. Eric's injector sizing spreadsheet teases us by turning on the complimentary light about power production in the process of calculating injector size. We should see some interesting data about heat, blower efficiency and fuel requirements. I suspect the mental gymnastics are well worth the effort for anyone thinking about going there.

James, your Killer Chiller adds real value in terms of lowering IAT2 temperatures and in the process adding significant reliability to the engine. A few words about that would have real substance. All things being equal none of us like blowing up pieces.

I just love this Modular Fords site.:ilovemf: Thanks to all who contributed to this thread.

Ed

 

And if I'm reading this correctly the mild increase in boost from the cold winter air can push you over the range of safety if your injectors and FPDC are operating close to max in the summer? Is this correct?

 

Yes sir, that is absolutely correct, and yet another reason why many need to careful evaluate their setups.

Regards,

James