I'm going to try and keep from writing a book here so I'll skip most of the details about how I got to this point and go straight to the issue.

While datalogging at the drag strip I've found that my injector duty cycles are running real high at WOT when I near 6000 rpm. One important factor I've noticed is IAT. With an IAT of greater than 70 degrees the injector DC's reach as high as 95's at the top of first and second gear, lower injector DC's as the IAT increases. With an IAT of much below 70, like 65 degrees, I am seeing 100 and if the IAT is 60 degrees I've seen injector DC's as high as 103.5. Just keep increasing the injector DC's as the IAT continues to drop. This is happening on a basically stock LT1 with ALL STOCK computer tuning in every fuel related field. The injector constant is the OEM 23.8, long & short term fuel trims all 128 during each pass. The BPW's were also steady at 10.3x at the shift points near 6000 rpm. My plugs look excellent and fuel pressure is stable @ 47 psi during the runs. I've verified my BDBK4779 bin as stock and I'm wondering why the General would program this way.

I'm also wondering what happens when the injector duty cycle exceeds 100? Is it just a number at that point and the fuel system is at maximum fuel delivery? Does an almost dead stock 93 LT1 need bigger injectors? I have only the limited mods listed in my sig and muffler delete, that's it. The car runs very well and I don't feel like I'm having any "problems" related to this. I've read that injector DC's this high are not normal but everything else about the car is just fine.

Am I reaching a fuel delivery limit, I mean when it gets colder than around 60 degrees is my fuel system no longer able to add the required additional fuel it's asking for? Is bigger injectors and a change in the injector constant the way to correct this?

Sorry for the book (I swear I tried not to write one) and thanks for any insight here from you LT1 tuners. BTW, I have all the required hardware and software to make whatever changes need to be made. It just seems odd to me that the factory tuning would be maxing out the fuel system at such a mild ambient tempurature.

Thanks, Al.

correction: The BPW's do inversely follow the IAT. I realized after I posted that they had to be changing for the Inj DC's to be different so I rechecked the logs and found this to be correct.

 

1. Increasing PW (DC) with lower IAT:

The GM speed density programs include routines for air-density correction to the calculated fuel load. That is, as
intake air density INCREASES metered fuel goes UP to maintain a constant AFR. The air density calc includes terms
for both ambient baro, and temperature corrections for ECT and IAT. You seem to be observing the effect of the IAT
part of the air density correction. You should be able to correlate the PW increase to the absolute IAT (in either *R
or *K) in some form or other. To get GM's exact math for handling the correction, you will need to dig those terms
out of the ECM's fueling code. They are generally adaptations the ideal gas law (PV = nRT - note the inverse
relationship to temp).


2. Injector DC

The duty cycle is a comparison of calculated PW to time available for that pulse. If the calculated PW @ 6000 rpm is
greater than 10 ms, you will show a DC greater than 100% since there is only 10 ms available between pulses at that
engine speed. The WOT PW is calculated from the VE (corrected for air as above) plus PE plus AE fuel.

As a general OPINION, PWs in excess of 100% that occur for only a few fractions of a second (as in drag racing) are
probably not a major concern, for this reason: GM stock code calls for tons of excess fuel at WOT. Even if the injectors
were upset for a cycle or two, there is enough excess fuel around that it PROBABLY will not hurt anything. Road racing
or marine applications are a different story - long-term, the injectors should run in their normal range of proportional
control.

However before changing anything, prudence would suggest double checking why you are seeing the high DCs with a
stock engine. GM designs are so conservative I would not expect to see the DC go out of spec like that in normal
circumstances. Checks I'd make would include verifying that ALL the injectors are clean and flowing full rates; fuel
pump, sock & filter are up to spec; thermocouples & MAP sensor are working in range; O2 sensors on both banks are
operating properly. If all that checked out, I would get WB readings of the actual AFR in each collector at high rpm/high
load. With that information in hand, one could tell whether to adjust the fuel load tables OR if more injector capacity is
needed.

DCs that high on a stock engine seem peculiar to me, hence the suggestion to look for underlying problems before making
any physical changes.

HTH

 

Thanks for the response Doctor J. I'll take the things you suggested in order:

1) Injectors - I haven't had them out and cleaned/checked but as the car runs well, get's excellent mileage city and highway and all the plugs read the same and good I believe this is a fair indication that none of the injectors are way off. Also, the fuel system holds pressure as it should after a shutdown. I know this isn't the last word in determining if the injectors are good so I'll withhold final judgement on this for now.

2) Fuel pump and filter - filter is new and pump supplies steady pressure (based on vacuum signal) at all times, even with the nitrous system on, verified with my in-car electric fuel pressure gauge. No problems here.

3) Thermocouples, Map & O2 sensors - As far as I can tell via the datalog's all sensors are providing good feedback. Map readings seem normal for any given rpm/load situation and both O2 sensors seem to be "switching" rapidly the way they are supposed to. I know what "looks" right on a datalog doesn't mean it's right but for now that's all I have to go by. Couple this with the results of the WB dyno time I've had (see item 4 below) and I get the feeling these normal indications can't be far off.

4) WB Dyno verification - I had my Vette on a dyno w/ WB not so long ago and the AFR @ WOT was 12.0 at the beginning of the run (3500 rpm) and fell to 11.5 at the peak rpm of around 5500. I see that the table for PE % AFR vs. RPM is setup to do just this (getting half a point richer between 3500 and 5500 rpm) so it appears the engine responded correctly to the commanded AFR at any given rpm in PE mode. Unfortunately the dyno time was prior to me having all this datalogging/tuning/chip burning hardware/software so I don't know what the Inj. DC's were, I can only assume they were no different than they currently are, the car runs the same and I've made no changes in the tuning as of yet.

I'm not saying there's nothing wrong with my car causing these high DC's at peak rpm but all current indications lead me to believe that nothing is drastically wrong. I'm not really sure which items I should investigate further or in which order, I suppose the injectors themselves are the next item I'll look at verifying via removal and testing/cleaning. I know that O2 sensors are a replacement item as they could be "switching" ok but still be out of the proper range but it's my understanding that the O2's have no effect on PE mode anyway. The exception being that if the L-term's are higher than 128 when the computer enters PE mode they will not drop to 128, GM's way of insuring the fuel system doesn't go lean in PE mode. My L-term's normally run a little under 128 so this situation doesn't happen to me.

Based on this info does anyone have any specific ideas beyond what has been talked about here? Thanks again to you Doctor J and please jump in here with any more responses based on this info.

Al.

 

Al,

Looking at your sig I notice the TB bypass and relocated IAT. Those possibly explain the higher DCs (without doing the math
to verify it) since the original GM design was for preheated combustion air. Considering a remedy then for the 100%+ DCs:

Sounds like you have taken all the basics into account - never hurts to be sure of what is already in place before modifying it.
Equal injector flows ensure even fuel distibution to all the cyls. I personally don't trust units more than 10 years old without
checking their flows... a lot of wierd components end up in the gasoline pool these days, and they are not all healthy for
precision metering devices. With all that said:


The best way to address your DC situation is perhaps not with bigger injectors, but rather to examine the excess fuel dumped in
at WOT. I would look at the actual collector AFR (via a WB) at high rpm in 3rd gear. If I observed the AFR to be 12:1 or more I
would trim my PE fuel at those rpm's to something more lean - USING GOOD JUDGEMENT - to bring the PW's and DC's down into
a more reasonable range. This works because GM was so conservative in their fuel design. I've found 12.5 or 13:1 are liveable
ratios for my port FI system, and yield a little more power output. YMMV. The one caveat to that suggestion is your mention of
nitrous - if are you relying on the main inj's for extra fuel, ignore this paragraph.


The other way to address the situation would be to crank a little more fuel pressure into the rail. Injector flow will go up by the
SQRT of the delta fuel pressure ratio. Remember to use ABSOLUTE fuel pressures in the ratio. After you jack up the FP, raise
the inj constant by a similar %. That should bring all the PWs down, without changing any of the underlying fuel maps. As
always, exercise due dilligence when altering ECM code. A back-of-the-envelope calc says that if your present rail is at 47 psig,
then bumping it up to ~53 psig nets approx 5% more flow. I have run SVOs at 52 psi on my gage with no ill effects. Again,
YMMV.

HTH