Interesting info on B3702 Injection timing

[DrkPhx]

EFICalibrator - Thanks for the explanation. That's great information to know.
Like Mr Prick asked; is there a basic formula to use?

[eficalibrator]

It's hard to give you a single formula that will work most of the time since you guys all like to pick snowflakes for cam and head combinations. (No two are alike) To make things worse, different ECUs describe the timing with different units. Some use degrees of crankshaft rotation relative to TDC, others degrees relative to BDC, and many GM ECUs use "cam reference pulses." The cam reference pulse is still a unit of angle (which is time at a fixed RPM), so you're really tuning how much time "lead" you're giving the fuel delivery much like ignition timing. Just like ignition, there's a sweet spot that gives better results and it's possible to go too far. This is one of those grey areas that more experienced calibrators end up building a "feel" for as they do more of them.

[5.7ute]

That is a great explanation Greg, thanks.

[dfe1]

OK, it looks like the info here went sideways pretty hard...

If you see a change in AFR resulting from injection timing, it just means that you didn't get a complete burn in one case. The object is to completely evaporate and burn whatever fuel you inject, regardless of what's going on with the aircharge or port velocity. This was one of the exercises we did in my last advanced GM tuning class with a 23x+ camshaft in a GTO. Once you find clean combustion, it's a lot easier to dial in the VE (or VVE) tables, THEN you can address idle airflow targets.

Greg- What did you find in your experiment with the 230+ cam. As I see it, if you go strictly by intake valve opening point, the values in B3702 should theoretically be reduced with a long duration cam because valve opening occurs "sooner" (more degrees before TDC). The situation is similar to that of a cold engine- the injector has to fire "sooner" to provide adequate atomization time. If the intake valve opens earlier, then it would stand to reason that you also have to fire the injector sooner to promote atomization. If you delay opening (with a larger number in B3702) then the injector is likely to be firing against an open, rather than a closed valve. However, you state that you "add delay" if necessary when tuning engines with aggressive cams, but stock offset values are smaller at low temperatures than they are at high temps. I'm trying to reach a logical conclusion and things just aren't adding up, which to me indicates that overlap is a more important consideration than intake valve opening point.

[Highlander]

Greg- What did you find in your experiment with the 230+ cam. As I see it, if you go strictly by intake valve opening point, the values in B3702 should theoretically be reduced with a long duration cam because valve opening occurs "sooner" (more degrees before TDC). The situation is similar to that of a cold engine- the injector has to fire "sooner" to provide adequate atomization time. If the intake valve opens earlier, then it would stand to reason that you also have to fire the injector sooner to promote atomization. If you delay opening (with a larger number in B3702) then the injector is likely to be firing against an open, rather than a closed valve. However, you state that you "add delay" if necessary when tuning engines with aggressive cams, but stock offset values are smaller at low temperatures than they are at high temps. I'm trying to reach a logical conclusion and things just aren't adding up, which to me indicates that overlap is a more important consideration than intake valve opening point.

Often times the timing is to the degree where the intake valve closes LATER in the compression stroke, not that much sooner. If the piston is still going up in the exhaust stroke and you open the valve you will get too much reversion... As RPMs go up overlap helps fill the cylinders better, so i get why you would want to delay the injector firing because you will want to fire it where there is more air speed.

[Goldfinger911]

This is very interesting. I did a lot of experimentation with this:
http://forum.efilive.com/showthread.php?t=9288

I have a custom cam, but similar to the MS3. Here is what I came up with.... I think your results lend some merit to mine. Thanks and keep us posted.

--cut--
-1 RPM = 360deg crank rotation
-1000rpm = 360000 deg <---- typical big cam idle and to keep the math easy
-16.67 revolitions per second
-6000 degrees of rotation per sec
-0.0167 revs per ms
-6 degrees of rotation per ms
-Therefore when advancing the intake opening event by 2 deg, this means firing the injector .334 ms sooner.
--cut--

[SSpdDmon]

I wonder what this means for someone like me who is running a 60# injector in a stock engine? It seems I'm always fighting a rich condition at idle. I had assumed that was because I couldn't get the pulse width short enough. So, my "solution" was to zero out the small pulse adjust (B4005) and start pulling time out of the voltage table (B3701). Now, I'm wondering if my problem is because the bigger injector is slower to open....and therefore I need to reduce the offset in the timing table.

[5.7ute]

I wonder what this means for someone like me who is running a 60# injector in a stock engine? It seems I'm always fighting a rich condition at idle. I had assumed that was because I couldn't get the pulse width short enough. So, my "solution" was to zero out the small pulse adjust (B4005) and start pulling time out of the voltage table (B3701). Now, I'm wondering if my problem is because the bigger injector is slower to open....and therefore I need to reduce the offset in the timing table.

Jeff, I would think that you would cause more problems & not actually solve the issue you are having. Your problem is the injectors cannot supply a small enough amount of fuel to give a stoich idle. Offsetting the injectors may change your wideband reading but you will still have too much fuel for the operating conditions.

[eficalibrator]

I wonder what this means for someone like me who is running a 60# injector in a stock engine? It seems I'm always fighting a rich condition at idle. I had assumed that was because I couldn't get the pulse width short enough. So, my "solution" was to zero out the small pulse adjust (B4005) and start pulling time out of the voltage table (B3701). Now, I'm wondering if my problem is because the bigger injector is slower to open....and therefore I need to reduce the offset in the timing table.

No, the fundamental problem is more likely that you have incorrect data for both short pulse adjust and offset tables. Both of these have a very large impact upon pulsewidth calculation at idle and cruise. If you're not properly converting from desired fuel mass to actual injection time, you have little hope of getting much else right.

[SSpdDmon]

No, the fundamental problem is more likely that you have incorrect data for both short pulse adjust and offset tables. Both of these have a very large impact upon pulsewidth calculation at idle and cruise. If you're not properly converting from desired fuel mass to actual injection time, you have little hope of getting much else right.

Well Greg, it's been my experience that you can only shorten up the IPW so far by adjusting down your IFR, MAF, or VE before you hit a wall with bigger injectors. At that point, you need to pull time out of the voltage adjustment and short pulse width tables. I've searched around and found what appears to be the right voltage adjustment numbers. But even with the short pulse table zero'd out, I'm still seeing 11:x AFRs at idle, which is why I was pulling more time out of the voltage table.

If you have some insight on how to correct this, I'm interested in listening to what you have to say (as are others I'm sure). However, just coming in the thread to essentially say "you're doing it wrong" and not offering suggestions isn't really helpful. It's frustrating. I understand you need to sell books and DVD's. But, I'd honestly rather you not post at all if that's all you have to offer.