While in study..i came by some useful information for determining injector size that might be useful for force induction cars...

First you must find the boost pressure ratio

Pressure Ratio = 14.7+Manifold Boost pressure / 14.7

When u figure the pressure ratio (PR), calculate the CFM

CFM = Engine Size (CID) x Max Peak Power RPM X VE (Volumetric Efficiency) x PR / 3456

Liters multiply by 61 gives you the CID
Volumetric efficiency should be a reasonable estimate. NA engine usually 80% and boosted usually higher (100%)

Once the CFM have been calculated, use the following formula to determine injector size lb/hr

Injector size lb/hr = CFM x 0.44298 / # of Cylinders

Since most people are used to seeing CC/Min, Take lb/hr x 10.5 will give you the CC/min

If anything on here is incorrect, please correct me. Hope this information comes useful to some.

 

www.rceng.com there is a fuel injector size calculator there.

Remember as your boost increases in your engine...your fuel pressure regulator increases fuel pressure 1:1 ...IE 1 psi of fuel pressure per 1 psi of boost pressure.

The reason for this is...the pressure differential between the fuel rail and your intakte manifold is held constant. If we did not increase the pressure 1:1 the Fuel pressure that creates the "spray pattern" of your injector would suffer. There are pics on the internet of injectors flowing at different pressures. I've personally seen my 1000's flow bench tested at different pressures....pretty cool!

When you put two fuel pumps in series that flow the same amount you will increase the flow potential 165% 255lph*1.65 = 420lph

Precision Injectors....are Rochesters...same bodies that FIC uses. Rochester used two different kinds of discs... one set the holes are drilled in a larger diameter cirlce....the other set the holes are drilled in a smaller diameter circle. You may ask whats the significance? The spray pattern is significantly altered from one injector to the next. I noticed this when I flowed my FIC's

RC Engineering flow tests each set of injectors....and they send you the ACTUAL birth sheet of your set of injectors. Pretty nice imo.

How do fuel pumps cool themselves off? The fuel flowing through them takes all the heat with them!

Walbro pumps fuel flow falls significantly after 80psi... A good base pressure to set is 45psi (set your base pressure by... starting your car... pull off your vacuum source to the regulator....so it sees atmospherical pressure...adjust to the setting you desire)

DO NOT USE TEFLON TAPE ON FUEL SYSTEMS!!!!

Injectors are typically fired with a low side driver...meaning they have a constant 12v soure...and the ECU grounds the otherside of the injector to open the injector.

IDC...stands for Injector Duty Cycle. Time on vs. Time off. Industry standard maximum is 85%. (I've run most of my injectors at 90% with no problems.)

An injector going "static" is when the injector stays open... IDC is soo high that the injector cannot close in time...and the injector is in essence open all the time.

i'll think of more later.

 

Ha! all from a man that installs fuels systems. When you go to put gas in half of it flows out under the car

 

one more thing...check for leaks.

 

IMO, the only reliable way to calculate fuel needs is to know what your mass flow of air is (typically lbs/min or g/sec), target AFR, and specific gravity of the fuel in use (which can be found online). This way there is no guesswork like all the other calcs that have you guess at BSFC, and have no way to accomodate the different types of fuel we use. I can calculate my IDC to within .1 or .2 percent. There is some discussion on this page of my site where I was just starting to figure this stuff out, due to yet another toasted motor.

 

I always tune for 11.5-11.7 AFR on pump gas.

Your calculations will only get you soo close. Nothing is "exact" with amount of variables given in a car.

Your pump may not flow exactly 255lph. Not only that...as your boost pressure increases so does your fuel pressure...making your fuel pump flow less. @ 100psi your fuel flow is nearly cut in half. The amount injected may not be the amount actually in the combustion chamber...cam overlap...puddling on the port walls...etc.

So when you say that I'm guessing on my BSFC... your system is no different. Guessing the flow rate of air into your engine with your "target" afr. doesnt account for what is burnt in the turbo manifold, turbine housings, and exhaust before the o2 sensor. The point is... not to undersize your injector.

I use a .58 BSFC...one that I REVERSE calculated from rc engineering's site after I got my results from the dyno jet...logging my idc's....logging my afr. So for my situation it works very well. I could use 880's on my car.... but I have 1000cc injectors. These injector calcs are made so that you do not buy too LITTLE of injector.

btw if you run 12.5-12.7:1 on pump gas you WILL burn another motor up.

 

I would never run that lean on pump, and I'm not sure why you would think that I would. I tune for 11:1 since I do 30k+ miles per year. I run 12-12.5 on 117 leaded. 12 flat on the nitrous. I find that the power gain to risk increase is hardly worth running very lean, and there is a lot more to be found in other areas of tuning.

You may have arrived at the BSFC you use properly, I'm not going to pretend I know what your capabilities are. But 99.9% of the people using such calcs are guessing at it. I'm not estimating anything when I calculate my fuel flow. Since I log airflow and boost, it's easy to know where the pump is. They may vary from pump to pump or over time, but I have never seen it, even on 3-4 year old walbros. The airflow number on my car is always accurate, I validate that several ways and constantly. In fact, the calculations I do are so accurate they can point out when something in the system changes instantly, like a boost leak, or fuel pressure change. The fact remains that when doing the math with the proper inputs the results have always been incredibly accurate, for dozens (if not hundreds) of people using a spreadsheet I put together a couple years back. The crap in, crap out concept applies... Now, the result you get from any calc is only to show you the minimum injector you should run. If you still undersize it after aquiring all of the necessary information, well, you're a tool.

With DSMlink, standalone, or any system that allows you to do injector compensation without lying to the ECU about airflow, you should go big just because you can. I always recomend 950s at the least, and that number is convenient just because I like FICs. And they cover anything up to a 35R with plenty to spare, and enough for something like a T67 if you don't mind running high 90s IDC. When you have to worry about bollixing the ignition timing, it can really pay to get just enough injector for what you need, and I always recomend sizing it to the max airflow the installed turbo is capable of, in case boost ever gets away from you. All it takes is something simple like a melted BC line.

For pumps, I like to go big and run an AFPR, but back in the day when I found out the 255LP wasn't enough for what I was doing, everyone was telling me that the LP is good for 10s on so and so's car, it's good for 10s on my car. Hence the reason I no longer listen to fuel system advice from people that believe in such a strategy. With a full weight car, not making every last HP it could from a given airflow, etc, fuel flow requirements go up. I now prefer to run the numbers, install the parts, measure the results, and compare to my initial calcs. I've been right ever since that hard earned lesson, and I hope other people can benefit from my many years of success and misfortunes. If not, well, it won't affect my life.

 

<-- Big fan of single in tank walbro...feeding a surge tank in which i have two walbro's feeding a dual feed rail.

lots of feeding. I use a liter reservoir which gives me approx 12-15 seconds of 100% IDC action with 1000cc injectors with NO extra fuel being pumped from intank pump to the surge tank.