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Tuning injector PWM & PW

Posted: Tue Mar 08, 2011 11:06 am
by KLattig
Hi, I'm working on my first install, and don't think I'm setting the PWM current limit for the injectors correctly. Details:
- MSII PCB ver 3.0
- SBC 350 with TBI & 55lb low-impedance injectors on top of a Edelbrock Performer dual-plane manifold. RV cam.
- Fuel only (for now)

The engine starts right up & idles well with the 'stock' cranking phasewidths - no problems there. But when tuning the PWM parameters, I'm able to go clear down to 0 PWM Current Limit %, and ~0.8mS PWM Time Threshold (any lower & the engine dies). At 0% current / 1.0 TIme, the engine idles fine, and doesn't seem to stumble when I stab the throttle (although AFR goes way high - but I think I deal with that later). But the 0 PWM current seems crazy low! The idling PW at these settings is ~1.3mS (which also seems low).

I have the Injectors running 8 squirts per cycle (alternating), and the Req. Fuel is 8.3 (not exactly sure what that means). I've tried other settings (variations of fewer squirts & simultaneous / alternating), using the 'stock' PWM settings (30% Current, 1.0 Time), but the engine idles much better with 8 squirts.
(MSQ & datalog attached)

So:
1) Given that engine idles well, should I just move on to load tuning, ignoring the idea that PW & current limit seems really low?
2) Anybody have any advice on # squirts & alternating vs simultaneous for GM TBI units? Where are these parameters in the MSQ? (I have the MSQ from the DIYAutoTune '77 Nova project, but can't find the # of squirts settings).
3) (separate question) I only have 11psi fuel at the TBI inlet. Should I consider a rebuild of the pressure regulator to get closer to 15psi? (I know that this will force me to re-do all my tuning again).

Thanks!
- Kelly
datalog20110308a4.msl
megasquirt20110308a.msq

Re: Tuning injector PWM & PW

Posted: Tue Mar 08, 2011 4:57 pm
by KLattig
Some follow-up info on this...
The engine idles well at 4 squirts simultaneous or 8 squirts alternating. 8 / simultaneous also works, but the idle is less smooth. Any less & it cycles erratically, almost dieing each cycle, until it finally conks out.

I spent some time this afternoon looking at AFRs at idle (I have a WB LC-1), and playing with the VE Analyser on the MegaLogViewer. AFRs were quite low (11-12), and VE Analyser suggested a pretty good cut to the VE table in the idling area (to the mid-teens). I implemented the changes, but found that the AFRs didn't change. VE Analyser now suggests entries in the 8-10 range in the idle cells. This seems ridiculous, so I didn't try it.

I don't have my mind completely around this, but considering all the data, I'm thinking that somehow my injectors have a lower limit as to how little fuel they are capable of spraying. These are 'field tested' injectors, so maybe they're worn out? What happens to injectors as they get old?

Anyone's insights on this would be appreciated!
Thanks,
- Kelly