is there some type of formula used in figuring out fp's at full vac, 0 psi, and boost that takes into account injector size? in other words, is there some sort of function that takes into account the amount of fuel needed in the cc at any given vac/boost and the size of the injectors being used, and yields the fuel pressure needed at that given vac/boost state? hope that made sense
#45017, "RE: fuel pressure formula/injector size - fp relation" In response to Reply # 0
You might find some of what you are looking for here: http://www.2gnt.com/www/corbin/chooseinj.html. I don't think this is going to solve the part about what pressure is required for x amount of cc of fuel at x amount of vac/boost, but should give a decent starting point. The thing is though, this really isnt necessary for efi cars since the duty cycle of the injectors is electronically controlled. That means that regardless of the pressure applied to the injector, the computer will still control the amount of fuel that flows through it using data from the o2, TPS, ECT, IAT, and MAP sensors. The only time that the fuel pressure is in direct corrolation to the amount of fuel being delivered is when the injector duty cycle is maxed out. I hope this helps.
Matt 1995 Eclipse GS 2.0L, S16g Turbo, 8 Injectors, 26psi
1998 Eclipse GSX 2.3L Stroker, AEM EMS Converted to Speed Density, FP3065 Turbo, 35psi , and so on...
#45019, "RE: fuel pressure formula/injector size - fp relation" In response to Reply # 1
Matt is right - the computer will regulate the injectors based on the o2 sensor and keeping the fuel trims in line is really what's important. But there are formulas to help guess at what the fp settings should be. I put this spreadsheet together to help me figure out these kind of questions. On the "injector sizing" worksheet the top section tells you how much flow is required for a particular amount of HP. The lower section has rising-rate simulations that you can play with to figure out what your static and max fp should be. Play around with the lower section. Try to get your static fp to match the N/A requirements - and try to get your max-boost fp to match yourr expectations. Pay attention to the boost-corrected HP numbers - those are the numbers you really want to match up on. http://www.users.fast.net/~metaljim/garage/TurboCalc.xls
#45023, "RE: fuel pressure formula/injector size - fp relation" In response to Reply # 2
Hey jim do you know the actual math involved? I have looked over corbins stuff, but i want to make a flash calculator and put it up so that everyone can use it right on the web. it think it would be handy! let me know
____________________________________________________ four nails four corners four riders and four horses
#45040, "RE: fuel pressure formula/injector size - fp relation" In response to Reply # 4
RC Eng gives some of the math - where they fall short is when you start boosting. I haven't found a good web based calculator for FI flow - especially rfpr applications. Forced induction throws a bit of a slant on injector flow - cause all of the sudden you have pressure in the manifold which makes the pressure in the rail less effective. The difference is easily accounted for in standard FI applications(1:1 fpr) - the flow remains constant by increasing the fp by the same amount thats in the manifold. It's a bit more difficult with rising rate fuel systems - that's where I think the flow curves on my spreadsheet are usefull.
And of course, you could rip apart my spreadsheet - the math is in there. You'll notice there's 2 HP sections on the spreadsheet - cause it's different for turbocharged and non-turbocharged applications. The left "Flow Rate" section is for N/A - and the "Boost corrected Flow Rate" section is for forced induction. I actually have a working prototype of this injector flow simulator - written in java. Right now it's an app, but hopefully someday I'll get around to making it an applet.
#45063, "RE: fuel pressure formula/injector size - fp relation" In response to Reply # 7 Mar-19-03 05:24 PM by 97eclipseRS
wow thanks a lot guys. those are some really great references, and yeah jim, if you could post the java app or code that'd be great too. i was just looking for a way to fine tune my fp for my application, and although it has been given, what fp should be at x time using x injectors, i was just wondering if there was some sort of formula behind the numbers, or if it was just trial and error over time. if the computer controls the duty cycles, then it's only controlling when and for how long the injector "opens", but the fp applied to the injector determines how much fuel actually gets in, correct? i'm not datalogging or anything, and although i can monitor egts/af/etc., i wanted to know the theory behind using the fp's that i'll be using, so like i said, i could fine tune it a bit.
#45067, "RE: fuel pressure formula/injector size - fp relation" In response to Reply # 8
Allright, here's what I have so far. Like I said it's in a very prototypical state. The injector flow curves basically work and the exit button works great. On the injector simulation screen the name for the fuel scenario and injector set don't work - but everything else pretty much does. you have to enter the BSFC and duty cycle like .525 or .85(percentage format). The curves don't automatically update - you kind of have to click and scroll on them a bit to get the whole picture...but the numbers should be good. The curve options screen isn't working yet either. Anyway - you need these two files. http://www.users.fast.net/~metaljim/garage/FuelTrimmer.jar http://www.users.fast.net/~metaljim/garage/trimmer.bat
Install them in a directory and run trimmer.bat. If you have unix or macOS - just go to the directory and run java -jar FuelTrimmer.jar
I'll keep working on it. As you can see from the prototype I want to be able to save fuel scenario and injector sets. I also want to make an injector upgrade screen - something to help with questions like..I have these injectors and if i upgrade to these injectors how should I modify the fuel pressure settings and rfpr. That'll be more of an simulation comparison screen as opposed to the single simulation.
And Ken - yeah the computer controls the injector by adjusting the duty cycle. Once you get datalogging you'll see how injector size, static fp, and rise rate are actually treated by the ECU by looking at the long term fuel trim(LFTF) and short term fuel trim(STFT). But the trick to using the math is to make the injectors "look" stock to the computer at idle AND provide enough fuel at maximum boost.
Here's how the math is put to practice - If you run stock injectors then you want to use the stock fp for static and target a pressure that'll get the little 235's where you think they should be(in horsepower) at your max boost level. What you need from the math is a rising rate.
Here's how that same math is used to calculate fp for an upgrade - If you use bigger injectors - set the static fp to something that "looks" like the stock injector hp rating - and get then get the bigg'ns to where you think they should be(in horseposer) at your max boost pressure. What you need from the math is your new static fp setting, as well as a rising rate.
#45122, "RE: fuel pressure formula/injector size - fp relation" In response to Reply # 9
the links don't work , but the way you described it it sounds really useful. now i pretty much understand what you're saying, except when you talk about making the static fp of the bigger injectors(in my case 30lb) "look" like stock hp rating, i'm not totally sure i understand what you mean. do you mean set the static fp so that the injectors are delivering the same amount of fuel, thereby the same hp, as the stock ones would be at 0psi? you then also have to adjust the idle fp to look the same also, correct?
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no need to reinvent the wheel. thanks!
, but the way you described it it sounds really useful. now i pretty much understand what you're saying, except when you talk about making the static fp of the bigger injectors(in my case 30lb) "look" like stock hp rating, i'm not totally sure i understand what you mean. do you mean set the static fp so that the injectors are delivering the same amount of fuel, thereby the same hp, as the stock ones would be at 0psi? you then also have to adjust the idle fp to look the same also, correct?