Anyone have recommendations for the injector opening time setting for the FAST when using 160# Bosch injectors?

My idle VE is currently set at 27% and then it ramps very quickly up to 75% or so at low speed cruise. I was told that the idle VE is too low and is a result of the injector opening time being set too high (currently 1.4 mS).

Thought I'd ask if anyone had any suggestions before just taking shots at it.

BTW, a 1% change in VE at idle currently makes a roughly .5 change in the A:F ration (e.g. changing from 27% to 28% will change the A:F from 13.8 to 14.3)

Thanks in advance.

 

That's what it sounds like to me, that your IOT is too large. Forgive me for the following, 'cause you might know more than I do about this stuff, but I'm gonna go through it anyway:

Now, I know nothing about the FAST system, but on my Megasquirt, the IOT correction is just a constant added to whatever the computer calculates to be the proper amount of fuel. If you've got the IOT set too high, the VE table will be extremely low in the low RPM/load corner, rise rapidly, and flatten out quickly.

The IOT correction is actually the injector opening time minus the injector closing time. This is because the pintle doesn't move until the coil is either charged up or partially discharged. So you need some time to open before it sprays, plus the calculated spray time, and then there's a bit of time at the end where it keeps spraying before it shuts off.

Therefore, the correct IOT is usually quite a bit lower than the actual "opening time" of the injector, because you've got to compensate for the extra flow at the end of the injection event.

However, there are other things that alter your IOT - if you're running low impedance injectors, you must either supply them with peak-hold current, or you must add external resistors or otherwise limit the current. Peak-hold supplies full voltage and really high current to the injectors to snap them open more quickly, and then drops the current down to what will barely hold them fully open, so the current in the coil doesn't have as far to break down and the pintle can close quicker. The voltage supplied to your injectors also depends on the alternator, the wiring to and from your injectors, and any fuses/resistors in the way, as well as the peak-hold settings in your ECU.

I've set my IOT to about .6, and I get a nice flat VE table. I'm using peak-hold, with about 20% PWM after 1.3ms of full current. I'm using low impedance injectors, but they're 29 #/hr... The size of the injectors shouldn't really matter though.

Now as to your other issue - changing your IOT may not affect your sensitivity to VE changes. You may be able to get your idle VE up to 60-70%, but if the FAST system limits it's resolution to 0.1ms like the early Megasquirt system does, you'll never get a decent idle with injectors that big. I'm guessing the resolution is closer to .001ms, so once you fix your IOT issue your idle quality should improve too, with some tuning.

 

Isn't that opening time at or near the minimum? My BS3 manual said that a 60# Bosch would not open below 1.4ms, don't know what a 160# minimum is, but I would think it is right around there.

Does your AFR go leaner when the VE goes up 1% or richer? I just started tuning my BS and it went leaner when I lower the VE numbers in my fuel table.

 

I just don't have a clue about where to set it. I was told .8 was a good number, but I was also told larger injectors require a longer opening time.

So, I was hoping there was a rule of thumb or something.

Mine will get richer if I raise the VE. The computer thinks the engine is taking in more air so it adds more fuel. I lower it to lean it out.

 

Well whatever happens, you've got to tune your ECU to your car, not to what people (including me) say you should tune it to. Do what works and what looks right.

Have you tried the 0.8?

I found that 1.0 was too much for my Bosch 29#'ers, that 0.8 was probably the right opening time, but when I combined opening and closing, 0.6 worked just right. Again, that depends on all sorts of variables, so what works on my car might not work on yours. 1.4 sounds like way too much fuel, evidenced by your super-low VE values at 'round idle.

 

Can you change the number of squirts/cycle on the FAST system? The way I determined 0.6 on my car was to set the # squirts/cycle to 1 and tune the VE to get a fixed number on the wideband (14.0 gives me a really nice idle). I noted the idle vacuum and speed, and then I switched the # squirts to 2/cycle. If it went richer, there was too much IOT correction, because ideally the IOT correction should exactly compensate for every time your injector opens. Playing with the IOT value got me to be able to switch between 1, 2, and 4 squirts/cycle with minimal AFR and vacuum changes, so I knew it was working right.

The logic goes like this - if you've got 1.4ms IOT and your computer thinks you need .4ms fuel, that's 1.8ms total pulsewidth.

If your IOT is exactly correct, you should be able to divide the .4ms in half and open the injector twice, for a total of 3.2 ms total pulsewidth (1.4 IOT + .2, each squirt)

If the car goes rich, it's because some of the extra 1.4ms of pulsewidth (when squirting twice) is actually holding the injector open.

 

I completely agree about tuning my combo as my combo, but surely someone has the real amount of time it takes to open an injector... If not, I'll take a stab at it. I was just hoping to avoid stabbing multiple times.

 

the problem is that there IS no "real" injector opening time. If you let your imagination run wild, you'll see that even at different duty cycles, IOT can change. Engine compartment temperature can affect it in the same way. There are lots of variables - some mean lots like the mass of the pintle and the current to the injector, some don't mean much like the heat around the injector or the heat of the fuel, but they're all part of the equation.

Keep stabbing.