I know this is an open invitation for flaming but I know how much time I have spent looking. I have done a search, I have read a hundred or more threads and I have stared at my Tuner Cat program until I see double and I still cannot find where or how to do my injector constant setup. I know you guys may think it's simple but I cannot get there from here. I have a 1227747 ECM. If it's as simple as lookinkg at a correct area of Tunercat and plugging in the right numbers someone please tell me where and how. I have an older Holley TBI #502-6, the one that supposedly have the 85lb injectors. Is that what I plug in when I find the right area? Then do I burn a chip and go to the next step like gathering data and making chip adjustmets for the BLM"s This may sound desperate- your right!!

 

Don't feel so bad. You are closer than you think Right now you are thinking that there is no injector constant because you can't find it. Guess what? You are right. The 747 doesn't have an injector constant. It's an older type C3 ECM. Most of the C3's don't have injector constant. I think the closest thing that you will find is something called the Base Pulse Width constant. The value specified there does NOT equal the injector constant but changing that value should have a similar effect (albeit not identical). Give it a try. I hardly ever work with the 747 so I can't help much more than that.

Tim

 

Trax is right. In TunerCat it's either called BPW constant or BPC. The stock ASDU bin has a BPC of 135, if that helps you narrow it down.

 

Thanks for the confirmation that I'm not all the way lost. I have found the field that has the BPW and yes it is at a number of 135.
Now what, is that number to be changed because I have different injectors? Do I use some formula based upon the old GM numbers and the new Holley numbers? I would like to follow Grumpy's theory of setting the injector constant first and then going to the VE fields but I just can't get there from here it seems. Thanks for the replies and hopefully some one had used the Holley TBI with a 7747 before.

 

I can tell you from experience that the BPC formula usually gets you to with +-10 from actual number you need. You'll probably end up making a few chips before you get it right.

 

Well, I'm not 100% sure, but I think the "135" value has the units of microseconds. IF that's the case, then a 55lb/hr injector will deliver the same amout of fuel in 135 usec that an 85lb/hr injector will deliver in 87.37 usec. Hope that helps......

 

For a 2 bbl TBI:

BPC = 1461.5 * (VOL/RATE)

VOL = Vol of 1 Cylinder in liters
RATE = Injector flow in gms/sec

RBob.

 

RBob,
I know this is not an exact formula for calculating the value to plug in for BPW constant in the '7747, but can you comment a little more on this. If my math is correct (I cipher purdy good) then the above formula for BPC spits out 150 for a 5.7L and 55lb/hr injectors and the stock 5.7 bin has 135 as the value for BPW I'm just trying to get a better understanding for this. I'm using the '7747 on a 5.6L engine with 55lb/hr injectors and I"m trying to figure out a good starting point. Since it's so close to the same displacement, maybe I should just leave it at 135.

 

So, If I get this right I should plug in lets say 87 in the box that now reads 135. Then I should start tuning BLM's. I know that's sounds simplified but that's how I need to start somewhere.

 

I don't know.... I need someone else to verify or backup my assumptions I made. I wouldn't follow my guess until somebody can substantiate it.

 

There has been and still is much contention/confusion/madness over the real injector flow rates of the GM units. Supposedly the 350 truck engine 350's (5.7L) injectors flowed 61.2#/hr. Some of the flow rates out there don't seem to be high enough to support the HP being made by those engines (40#/hr on a 305 is only good for about 136 HP). So, I dunno (I'm being honest).

Try the 135 and see how the VE table works out. Then adjust accordingly.

RBob.

 

I didn't see your engine displacement so: a 350 would be a BPC of 98, a 305 would be a BPC of 85 (if I did the math OK) with the 85#/hr injectors.

I'm using a BPC of 84 on a 331ci & BBC injectors with the fuel pressure bumped.

RBob.

 

Yes it's a 350 and thanks for a good place to start. I hope this gets me on a path that will allow me run a TBI setup again. It was on a carb for two years. Now all I need to do is figure out how to turn off the EGR and the VSS on this beast and I am going to try and burn my first chip.

 

Set $1B3 to zero (0). This sets the EGR enable temperature to 151 deg C. You'll need a VSS, no way around it. Too much 'correctness' depends upon it.

RBob.

 

I am sorry that I have used the wrong initials here when using VSS. I am again showing my inexperience to this game. I meant to say that I believe that I need to shut off a switch that runs the vacuum canister. I think I have read some posts saying this needs to be done. If not true please let me know, and if it is necessary which way do I go from here? Thanks again for all the patience here, maybe it will get to be fun sometime here shortly,I smell progress.

 

No CCP support on the '747.

RBob.

 

I'll add my experience here regarding the BPW in the 1227747. I am running the 4-barrel Holley TBI, and have four 85# injectors. The calculation with my 454 engine (460 since it's bored .030 over) comes out to 63 decimal. However, when I run that number it is lean everywhere. I kept having to raise the VE1 table after every run, and it finally got up to over 100% between VE1 and VE2. So, I raised my BPW up to 75, and now things are pretty stable. Four 85# injectors is like having 170# injectors on a 2 barrell. There is also a constant at location hex 2db that is a BPW offset. You can also raise that to richen things up in some areas, but I don't know exactly how it works.

Does this help?

 

Yellow, It's not all coming together yet but I know that it will and it starts with burning that first chip and judging those reactions. I needed to have a place to start and I am going to plug in 98 for the BPW like Rrob suggested , correct the EGR setting and give it a try. Hopefully I will be able to say that it's making more sense tomorrow. I am not working tomorrow and will spend some time trying to get this beast running better. I'll let ya know how it goes. Thanks fella's