Interesting info on B3702 Injection timing

[mvvette97]

ok so you add or subtract from table B3702? I would like to play with this as well. I looked at my stock tune for my vette and then the trans am and both of them are different from each other. I figured they would be the same but they aren't

[DrkPhx]

School me, I must be looking at your log incorrectly, I dont see the "way lean" condition. If I take your entire log over 190 degree, I show the average stft to be .06 and .08

Lee

The LTFT's are way high.. +16-18% on average for most of the log. Look at commanded AFR vs actual in frame 3726. Commanded is 13.09 and actual is 13.81. Before the timing adjustment, the WB was lower than the commanded at 12.8-12.9 at the same spot. So it's definitely leaner. In addition the car sputtered at part throttle, especially from a stop up to speed.

[LS1_Dragster]

I wonder why mine responded so well, remember mine went rich not lean. My 454 in my firebird also responded well, no track time but just idling and cruising was noticeable.

There are 2 main differences from both my cars: 1) the intake and throttle body on both are using the edelbrock vic jr. and the holley TB 2) The dragster has open headers.

Maybe something in those mods are making the difference?

[LS1_Dragster]

DrkPhx,

What size cam and what's your altitude there?

Also, I thought about something else, the dragster never sees temps over 160 and is usually 145 at the starting line and the firebird never sees over 165, even on hot days. Maybe the cooler temps need more time to atomize the fuel?

[LS1_Dragster]

I have so many questions!

Why does the LTFT go so high when the STFT don't reflect that? I figured the LTFT are an average of the STFT. I can only log STFT because I'm semi-closed loop because I want to use table B3647 for my commanded AFR. So if my STFT are looking OK like yours is it really OK?? Or or am I lean like yours?

[dfe1]

I have so many questions!

Why does the LTFT go so high when the STFT don't reflect that? I figured the LTFT are an average of the STFT. I can only log STFT because I'm semi-closed loop because I want to use table B3647 for my commanded AFR. So if my STFT are looking OK like yours is it really OK?? Or or am I lean like yours?

In closed loop, the short term trims ratchet the long term trims up or down as required. Essentially, STFT adds or subtracts fuel as required. If it goes to maximum value and AFR still isn't correct, LTFT bumps and the process repeats until STFT can maintain proper AFR while staying within a specific range. So if a calibration is way off, you'll usually see a fairly large LTFT percentage number and a small STFT percentage number.

[eficalibrator]

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

Before adjusting injection timing, it's important to understand why it's there. Most OEM applications are timed to inject fuel against a closed intake valve. This does two things; it cools the valve and aids in evaporation. When the engine is cold, fuel takes longer to evaporate, so it must be injected sooner to get enough of the liquid evaporated to enable clean combustion.

Clean combustion is the key here. If you're burning everything you injected and you had decen charge mixing, it shouldn't matter as much when you injected it. On the other hand, if you're either not burning liquid fuel in the chamber or not mixing completely, it can skew what you think got injected into the cylinder. This is why the OE calibration injects earlier on a cold engine to allow time for fuel to evaporate and mix prior to combustion. (Look at the table and you'll see the trend!)

In the aftermarket, we have the nasty habit of plugging huge cams into engines that open the door for a couple problems. One is that the lower port velocity reduces charge mixing at low engine speeds. The other is that having both valve open simultaneously (during overlap) opens the door for "short circuiting" where vaporized fuel may escape out the exhaust valve without ever being burned if there isn't sufficient port velocity to push it down into the cylinder. This can usually be addressed by delaying the injection event slightly so that the vapor isn't there when the exhaust valve is open. The potential downside is that large quantities of liquid fuel being injected into a cylinder at low port velocity may lead to bore wash at extended idle.

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.

[mr.prick]

So, is there a method to change the injection timing or is this something that
must be done before the car starts like the IFR and pulse adjust tables?

[eficalibrator]

Yes, you can change injection timing during the tuning process. You are correct that IFR, offset, and short pulse adjust are fixed prior to operation and injection timing is adjustable. I typically leave it alone unless I have trouble getting a clean burn (usually at idle on bigger cams). If it must be adjusted, I'll add injection delay as necessary to get a clean lambda signal from my (high quality) wideband and a smoother idle. The trick is to only add just enough delay to clear things up while still allowing for the maximum amount of evaporation and mixing time.

[mr.prick]

Just X-amount-% gradually?