hmx

Hey all. I've been lurking for a while, reading up on PROM tuning. I'm a graduate student on a budget, so I get to work on my car all the time. I've got an '89 Chevy Caprice which used to have a 305 in it, until one of the heads cracked. I found a friend of a friend who had (surprisingly) an original '89 L05 (350) longblock crate motor that had been sitting in his garage for the past 20 years, so I got that for $700 and swapped it out for my 305.

The car has an 8746 ECM and the chip is an APFN, but the car was knocking and had bad power sags, after the motor swap, so I found an old '89 cop car at a junkyard and snagged that computer (also an 8746) which had the ANLU chip in it. Swapping that out solved most of the problems, but there is still a small power sag when the engine is first started up, especially when the weather is cold. It surges a bit at idle, and at idle, if you rev it and let it fall back down quickly, the car stumbles and almost cuts out, then pops back up to a normal RPM, though it's a bit rough. After reading a thread on here, that sounds like the car is running rich, perhaps because the computer thinks that the car is running lean and thus trying to richen up the mix.

I have checked the mechanical problems. The O2, MAP, IAC and TPS are all new and perform correctly when looking at them on a datalog. I leak tested the system, and it holds vacuum; the intake doesn't leak either, as the engine doesn't rev when starting fluid is sprayed around it. I've replaced the exhaust manifolds, the fuel pump (twice), the fuel filter (once a year), the starter, and the radiator.

After datalogging, I see that the O2 sensor bottoms out (lean) on the highway and the BLM is up around 140 at most all RPMs/MAP readings. The fuel injectors have been cleaned, and I replaced them a while back BEFORE the car had its engine swapped, so I'm wondering if they are to blame, as I have read that the L03 injectors are ~40 lbs/hr, whereas I want injectors that are 55-65 lbs/hr for the 5.7L, and specifically 65 lbs/hr for the ANLU chip that I currently have in the car.

After reading BMmonteSS's posts regarding how to edit the BPC for your fuel injectors as a constant on TunerProRT. I loaded up the ANLU with the proper def file, but there is no BPC on the constants table. I would assume that the 8746 does not have that as a constant? The only thing I saw that was near that was the BPW, which is calculated using the BPC as a term.

This highly detailed post brings me to two questions:

1. Would fuel injectors of too small a flow rate cause the computer to think that the engine was running lean and thus force a high BLM?

2. If my .BIN file doesn't have the BPC as a constant, how does one incorporate engine displacement and fuel injector flow rate into the BPW term? There are a great deal of terms that make up the BPW (see below) and I am not sure how one would go about setting the correct BPC value from the BPW term.

Thanks for any assistance in advance.


BPW = BPC * MAP * T * A/F * VE * F33C * BLM * DFCO * DE * CLT * F77

Where: (everything labeled "term" is from a lookup table)

BPW = Base pulse width in milliseconds
BPC = Base pulse constant term (table lookup, basic engine air and FI
calibration plus EGR calibration)
MAP= Manifold absolute pressure term
T = Inverse air temperature term
A/F= Inverse air/fuel ratio term
VE= volumetric efficiency term
F33C= Battery voltage compensation (for the effect on FI open/close delay
and fuel pump delivery)
BLM= Block Learn Term (what the ECU has learned from the O2 sensor)
DFCO= Decel Fuel Cutoff Term (when to turn fuel off on decel)
DE= Decel enleanment term (when to lean instead of cutoff)
CLT= Closed loop correction term (result of O2 sensor Proportional/
integral closed loop)
F77= Turbo boost multiplier (if used)

Ronny

iTrader: (1)

The BLM can only add a finite amount of fuel. If you are globally seeing BLM 140 + or - that would suggest the BLM is not maxed out. Not sure what the stock calibration has for max but I believe it is around 160. so you are prob at stoich when in closed loop. A reading of 145 is adding about 14% more fuel(increasing PW of injs). so the smaller injs are OK for CL operation. Problem is they will not be up to task for AE-PE and I would believe your startup and cool coolant operation(OL) may be off. you need to get the larger injs OR change the BPC in .bin AND increase the fuel pressure to maybe 16 lbs. Fuel pump may not be up to task to hold 16 lbs at WOT.

this calculator will allow you to recalc the BPC: http://www.users.bigpond.net.au/ben_...ctorsizing.xls

Quote: but there is no BPC on the constants table

Should be there? Look again. I believe it is in constants?

hmx

All of this makes perfect sense; thanks for the response! I need to double check my fuel pressure, but if I recall, I had it up over 13 lbs. to begin with, around 15 I think. It's been a while since I changed the fuel pump and I, in my infinite wisdom, did not write any of that down.

Thanks again! That's a handy little tool to have.

I'll check again, though I haven't seen it. I'll try and take a screen shot or print out what I can see from the constants table and figure out how to post it here.

Thanks!

RFmaster

See attached file for screen shot

//RF

hmx

Correct, that's exactly what mine looks like. It does not have a BPC, only a BPW constant.

Thus, I assume that I must have to replace the BPW constant in order to account for the fuel injectors, which seems to be the objective of the excel spreadsheet Ronnie posted. I've been playing with that, and I had a few questions with it.

First, the BSFC is the Brake Specific Fuel Consumption, right? And doesn't that have to be done on a dyno? I haven't dynoed my car, so I don't have this for my engine, and frankly, since it is out of whack with the injectors right now anyway, I'm not sure that the value would be useful. I've read that .45 is about peak performance for a 350 Chevy, though, which is what the spreadsheet has, so I left that the way it was.

The second is the injector duty cycle %, set at 90%. I assume one leaves this as is, too, as manufacturers don't recommend running above 90% for extended periods?

Third question: the spreadsheet says it's for the 7747 ECM. Does this still hold true for the 8746 ECM as well?

And, finally (sorry for all the questions), I used this spreadsheet to calculate what my BPW should be for my 350 motor with my current 40 lb/hr injectors, and it comes up with 214.3. Does this sound even in the ballpark? My ANLU chip gives me around 135 stock, just as is in the PDF posted above, and this seems like a radical departure from what I'm currently running.

Thanks again for all the help. This stuff is actually kinda fun to learn about once you get into it.

Ronny

iTrader: (1)

BSFC is a # that is assumed safe for engine operation. Yes it is often determined on dyno. A while back I used to run EGT( on 2 cycle) and that too was a determiner of safe fuel consumption. The optimal # for that can vary from engine to engine. I believe .45 is considered correct.

BPC-BPW are often interchanged. A larger BPC is typical of smaller injs. By increasing the BPC from the calc value you are fooling the ECU and it will deliver more fuel. that is often done when you run out of room in VE tables(>100).

right on the 90%. If they run 100% I believe they could fail. Mine are 85-90% last I checked at WOT.

7747-8746 are common TBI ECU's As far as I know the 8746 adds IAT control. Any other diff ???? So spreadsheet works for all ECU's I believe.

I put in 40 lb inj (2) at 16 lbs FP with 350 cid and I see BPC 193. supports only 178 HP.
25 lbs FP support 222 HP with 154 BPC.

hmx

Thanks for all your help! This is all making a lot more sense now. It looks like that 214 that I saw for 40 lb/hr injectors with 13 psi is actually right.

Now I'm in need of a little advice. Since I don't intend on hotrodding this car (at least at the moment), what's better: changing the BPW on a new chip or replacing the injectors with larger ones? I'm kind of leaning toward the larger injectors, as I can see the possibility of the injectors getting pushed to 100% or the mix leaning out too badly on me if I have smaller ones, but maybe I'm not seeing something that those of you with more experience can see? I'm assuming the 65# injectors will consume more fuel, but other than that, I don't see any problems with them. Perhaps I should ask this in the TBI board.

Thanks again to those of you who have helped me here!

Ronny

iTrader: (1)

The larger inks should not consume more fuel unless the tune is incorrect.

small injectors at high FP= large injs at low fuel pressure

Too small of inj will cause the engine to run lean at WOT. I believe the small inj can be run as high as 35 lbs. 1995-1996 TBI runs stock tune 32-36 I recall with 40 lbs inj. May be a diff part #? I would check with Fast355 as he has run them at high FP w/o failure. In the datalogs one can witness the DC%. If it gets 95+% you need to address that. Too large an injector will cause issues with idle that can be overcome in tune as well as running a VAFPR. I woukld keep the ones you have an up FP and check datalogs. 65's may be available in JY on cop cars. FICHT INJS can pro clean and test them. they may also sell rebuilt.