Injector sizing is a bit of an art and a lot of science ...
This is gonna be long so here is the BLUF
(bottom line up front - US military speak)
1. the online calculator work really - they match the long form math as long as you use the right one and put in the right info, the best one is by far https://www.rcfuelinjection.com/Technical Russ Collins has the math right behind the formulas
2. The brand of injector and type of injector matter - not just to the ECM and the injector drivers, but to the VE of the engine
3. Running injectors above 80% duty cycle works, BUT the dead time numbers the ECM software uses to calculate the Lbs/hr to build the fuel table will be off the fuel tables will be inaccurate and if the ECM runs in closed loop the compensation values will be off and in extreme circumstances where you run over 90% for a long time the injectors may shut off (hysteresis issue) the same as driving your car amp into shutdown.
4. Trust the vendor that built or tunes your car and recommends an Injector brand and NEVER buy off ebay or anyone you don't trust - they will be knock offs - I flow EVERY set of injectors before I use them - some are perfect - alot are fake and suck
NOW the Details ....
So Let me start with the stuff the "sizing charts" don't
Beyond the basic can your ECM handle low or high impedance - which will Always determine how well the ECM can "control" the injections when the Pulse width changes (idle characteristics on big injectors). The Injector design is also critical 1 hole, 4 holes, 8 holes, all determine how well the spray pattern works when you Boost reference the imnjector.
Most injectors are rated for 43.5Lbs pressure at the cap, but add 10Lbs for boost and now the injector is at 53.5Lbs - MOST good injectors even High impedance will have a good pattern there, in-fact good up to 60Lbs ,,, But if you start with 60LBs and go to 70 or 80, now the spray pattern-starts to fall off, especially as the injectors hit their duty cycle limit ...
A bit of science here --The other issue is that injectors depending on their size will have better or worse control over the pulse width (that makes the duty cycle - how many times it pulses in sec determines how long it stays open and and for how long) ALL injectors experience something called Hysteris - that is resistance to change of states of the winding in the coil of the injector. when the injector opens the coil is energized and when the voltage is pulled, the coil energy collapses (like an electro magnet) if the field collapses too slow the Injector cannot have a fast pulse width . and the Bigger the injector the slower the filed collapses - thats why injectors have DEAD time - this is the chart you fill in to the SW when you buy an injector and it is really important - you will never have to worry if you size your injectors where they Never go over 80% BUT are not under 70% at WOT
Here is where it also gets tricky. With Gas be it Race or pump, the injector can be sized to meet the engine's VE - the volumetric efficiency. There is a formula I have posted previously to get there - the calculators you use online use that equation in the macro when they ask you things such as what is the BSFC or brake specific fuel consumption - thats the basic efficiency How many Lbs of fuel are you using per HP per hour. Its a pretty smart calculation that takes in whether you are Turbo or SC or NA. So in this way those online calculations are near spot on. I know because I have done the long hand math and used the calculator and guess what they are close enough where you will never tell. You have to be very careful not to but too big of injectors especially if they are High impedance . the collapse time on the coil (we call it the falling edge) is never as well controlled by the injector drivers with high impedance injectors and its why for many years Performance injectors were only Low Impedence. So if you have to go high impedance be more careful in selection. Low impedance injectors where you have more range will idle better and have a better spray pattern across an wider Voltage/ Current range meaning if you are running say 40Lbs of boost on an LS and need 2500 CC (260Lbs) injector - you should go low impedance or the car will idle like crap.
Injector selector is one of the most important choices you can make for horsepower and drivability . I have added 100HP in the midrange and and made a car purr at idle . from being beast with a horrible idle and only top end power just by swappiung injectors and touching up the tune (it was an LS)
I could write a book - but I think that covers the basics...
