I haven't found anything like this, so I think I'm original here. If this belongs (or would be better appreciated) in another forum, let me know.

Fuel injector sizing with RPM and fuel pressure compensation

I created the default just for TBI.

You basically enter your horsepower (or expected horsepower) peak, the rpm at which you'll make that, BSFC (leave at 0.5 if not sure), the number of injectors (2 for TBI), desired final max duty cycle (80% seems good to shoot for). I am still not sure about the injector delay, but I'll leave it at 2ms for now. You can tweak it around and see the effect on all the numbers. You can also input a pulse width and it'll tell you the duty cycle for the rpm you entered. This is just for informational purposes only and doesn't effect the calcs. Finally, the bottom part is neat. You feed it your current injector ratings, at what psi they were rated (I use 11 for TBI since the spec is 9-13), and it tells you what pressure you will have to run at to meet all the goals specified above.

I'm looking for comments here. Things like accuracy, applicability, neat-factor, etc.

 

Definitely handy, sure beats doing a bunch of iterations by hand. Need to correct the Max time to fire, Max inj PW, and PW @ RPM calculations. At 6,000 RPM, 2 injectors, V8, there is 5 msec for each injector to fire (100% DC).

For a default injector delay can make it about 400-600 usec's. This depends upon the injector and fuel pressure. The larger the injector or the higher the pressure, the longer the opening/closing time is.

Nifty

RBob.

 

See that's what I wasn't sure about.

I tried to reason it all out...ok so TBI injectors fire once per ignition cycle but the time is less, I guess because instead of having one injector per cylinder (which would allow it to have 720° worth of time) it's got 4 cylinders and thus the time it has is now divided by 4. Is that how you get 5ms?

I will try to work that in.

EDIT: Ok, I think I compensated.

Injector utility

The delay you mention - that looks like what GM put in the calibration. Is that where you got it from? P&H injectors are faster than saturated, correct? If I wanted to roughly compensate for the fuel pressure delay, how might I do that? Do you suppose that at really high fuel pressures the delay becomes so bad that you essentially chase yourself trying to get enough fuel into the engine at high rpm? Or could I, say, slow the injector down by the square root of the pressure increase? I also presume that injectors open slower than they close.

EDIT 2: Also, what happens at 100% DC? Are the injectors really static or does the ECM still cut the pulsewidth short by trying to close them and reopen them for the next cycle? In other words, at 6000rpm, there's a 5ms window to fire. Subtract the delay and maybe we end up with a 4.5ms window. My question is, at "100% DC" does 4.5ms or 5ms worth of fuel come out?

 

For time between injector firings, at 6,000 RPM that is 100 rev's per second. With the 2 injectors alternating between spark plug firings, that has a single injector opening 2 twice per revolution.

Gives 200 injector openings per second. Inverse that for time is 5 msec. For duty cycle, a commanded 5 msec PW at 6,000 RPM is 100% DC. The injector will be held open as the injector driver will never turn back off.

Yes, PnH TBI injectors open more quickly then port saturated injectors. My numbers come from actual measurements of injectors under various voltages and pressures. I never tried to put an equation to it, so I don't know what it would be.

I haven't found that. The increase in flow is greater then the lost opening time. Of course, this is with engines that get shifted at 6,000-6,200 RPM. With higher RPM operating points the opening time will be more critical. Would need to run some numbers to see where it becomes an issue. And if it does, there are methods to reduce the opening time while increasing the fuel flow.

The first pulse would be 5 ms minus the opening time worth of fuel. After that the injector would just stay open for a constant flow.

RBob.

 

Looks like my DRP/s calc is off, too. I think I need to divide by 2 because these are 4-stroke engines, not 2-strokes.

Ok, so at 100% DC, you kind of get a lot more fuel, proportionally, than you may have received at, say, 95% DC, assuming the RPMs are sufficiently high. Going further with this, maybe running P&H injectors static at the HP peak could be a way to improve idle, as you could run a little lower fuel pressure. Only problem is, the fuel delivery would probably become less predictable and non-linear as you go from <100% DC to static, so the AFR would get all messed up. So maybe it's not the best idea, but on a particular engine it might be the difference between a stable idle and an unstable one. A good reason to use a vAFPR, which I hear the EBL has built-in compensation for. Brilliant!

 

http://www.users.bigpond.net.au/ben_...ctorsizing.xls

 

it confimed nicely that my FP is too low !

assumed 325 HP. added my inj at 80 lbs.

calc my FP at 23.9.

i am at 17.5 FP and show 13.3 WOT A/F on WB 11/2006.

started 13 last summer after cam swap and moved 2 lbs a time and stopped 17.5 as idle got a bit funky.

car loves fuel !

 

OK. ran it a second time. made corrections per RRob comments.

i am at 17.5 FP and lean. for 325 HP it states 18.1 FP needed.

corrected HP to 350 it states i need 21 lbs FP.

again consistent with my WB readings.

 

I like mine better!