High BLMs - 6870 ECM Fueling Calibrations ($1F updated mask)
Greetings, Gearhead EFI Tuning Experts – In this installment of my ‘6870-ECM’d 350ci project, I’m soliciting the assistance of other early Gen3 and early C4 Corvette owners (original or otherwise) who have attempted to optimize the tune of their vehicles (original power plants or swaps) with the 6870 ECM.
While I’m fully aware of the 6870’s limitations and the limited ‘6870-based tuning resources on Gearhead EFI, I do plan to move to a 7165-based ECM/compatible calibration PROM. In the meantime, I’d like to eke out max performance from the current power plant with the existing ECM/PROM combo (the PROM is a Hypertech155332 – yeah, yeah, I know). As part of this task, I’m requesting assistance from the various Gearhead EFI experts in this forum to help me understand the significance of various fueling calibrations specific to the 6870 ECM (and its corresponding $1F_updated mask).
Taken directly from a HLK bin (’85 F-body/700R4/5.0L), I’ve listed here the fueling calibration details:
Scalars:
a) Stoichiometric Air/Fuel Ratio = 14.73
b) Minimum allowable BLM = 105
c) Maximum allowable BLM = 160
d) Minimum Closed-loop Integrator Value = 65
e) Maximum Closed-loop Integrator Value = 160
f) AE Enable Minimum ΔLV8 Threshold = 255 counts
g) Power Enrichment Minimum ΔLV8 Threshold = 130 counts
h) AE ΔTPSPW Scale Factor = 0.31 Multiplier
i) AE Enable Positive ΔTPS Threshold = 1.95 %TPS
j) AE Disable Negative ΔTPS Threshold = 1.95 %TPS
Tables:
a) Injector PW vs LV8
b) TPS Threshold vs RPM to Enable WOT
c) %Change to Fuel/Air Ratio at WOT vs Coolant Temperature
d) %Change to Fuel/Air Ratio at WOT vs RPM
e) %Change to Fuel/Air Ratio vs Coolant Temperature (open loop)
f) %Change to Fuel/Air Ratio vs LV8 (open loop)
g) IAC Warm Park Position vs Coolant Temperature
h) AE ΔTPS Pulse Multiplier vs AE Pulse Number
i) AE ΔTPS Pulse Multiplier vs Coolant Temperature
j) Injector Offset vs Battery Voltage
k) Injector Low PW Correction vs Injector PW
l) ΔTPS AE Injector Pulses vs Coolant Temperature
m) AE ΔLV8 Injector Pulse Multiplier vs ΔLV8
n) AE ΔLV8 Injector Pulse Multiplier vs Coolant Temperature
o) AE ΔLV8 Decay Rate vs Coolant Temperature
p) Crank Fuel Injector PW vs Coolant Temperature
q) Crank Fuel Injector PW Multiplier vs %TPS
After pouring over multiple threads on this and other fora as well as reading Banish’s two books on EFI and Tuning, here’s my understanding of the various calibrations and where applied:
1) At crank and idle: Scalars [a-e] and Tables [a, e, f, g, j, k, p, and q]
2) At part-throttle and cruise: Scalars [a-e, f, h-j] and Tables [a, b, c, d, f, h, i,j, k, l, m, n, o]
3) WOT or “Power Enhancement”: Scalars [a-j] and Tables [a, b, d, e, f, h, i, j, k, l,m, n, o]
Interestingly, nowhere in either the HLK (305ci Camaro) or the HLH (350ci Corvette) bins is there a scalar explicitly listed for “injector constant.” Gen3 V8’s of the era ran 19 lb/hr injectors in the 5.0Ls and 22 lb/hr injectors in the 5.7Ls.
In my particular case, the engine builder swapped out the locked-up, stock 5.0L for a FT-cam’d 5.7L GM crate engine and re-used the 5.0L’s stock MAFS/TPI intake, stock FPR, and 19 lb/hr fuel injectors (Rochester 5235047). The MAFS/TPI intake and FI combo is attached to an L98 intake manifold. Corvette ZZ4 aluminum heads, Hedman headers, and a 3” Flowmaster cat-back exhaust system complete the package.
As a result of this configuration, I’ve been operating and have datalogged a “perceived lean” operating V8 at both idle and part-throttle/high-speed cruise with BLMs as high as 160.
First and foremost in attacking the high BLM (“perceived lean”) conditions are to:
- …track down and fix any air-meter, vacuum, and/or exhaust leaks;
- …replace the tired, stock FPR with a Holley AFPR (or equivalent);
- …replace the old 19 lb/hr Rochester injectors with 22 lb/hr Bosch III (or better) injectors.
Thanks in advance and I look forward to any/all feedback.
