AAG

Greetings,TGO 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 TGO/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 TGO 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) StoichiometricAir/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) TPSThreshold 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.

AAG

After 7 days and with all the tuning experts in this forum on TGO, no one has any comments or suggestions - honestly?

I know the 6870 ECM is a pariah, but jeez there were a few F- and Y-body cars sold in 1985. I can't believe they've all disappeared or were scrapped.

InTech

It has been a very very long time since I looked at the 870 stuff. From memory I want to say you want to replace those 19lb injectors with whatever injector you need(high impedance) to match the HP level you feel you will be. Then adjust the LV8 table to match along with injector offset vs battery voltage(for the new injectors). Pretty archaic, it is 33+ years old though

I only have a HLH file here, if you can compare the 305 and 350 file, look at ALL the fuel parameters between the two files and I want to bet the LV8 is what will stand out as far as a major change.

Interesting though, if you still have the stock MAF and stock injectors it should run OK up until it runs out of injector. It seems you might have a failing MAF, ouch

AAG

Thanks for your reply, InTech - yeah, while the 6870 is an archaic ECM, it's the one I have. The punch list is long even though the car runs well with snappy throttle response. Since the bottom end is a 350ci, one item fairly high on the punch list is replacing the 19 lb/hr injectors with 22 lb/hr versions. I think it's considered good practice to replace the O2 sensor when the injectors are replaced. Because the #3 and #4 spark plugs are a bit sooty, I figure the same for the O2 sensor. I'm leaning towards a 3-wire Denso to replace the stock one-wire GM O2 sensor.

First though, I've to address some probable MAFS gremlins. One, for example, is confirming whether the engine builder retained the stock Bosch burnoff module or fashioned an alternative. I'd hate to think there's no burnoff performed on the MAFS with engine OFF. In the case with the Bosch burnoff module, I don't recall at the moment whether the ECM throws a code if the burnoff module ain't there or isn't working properly. Another is checking (as you suggested) whether the MAFS is on its way out (using the broomhandle tool).

AAG

As I've the HLH and the HLK bins, I've made a side-by-side comparison - the MAFS tables overlay each other and a few of the fueling cals (e.g., in the injector PW vs LV8 cal), I attribute the differences to the 19 lb/hr injectors in the F-body to the 22 lb/hr injectors in the Y-body.

Wrt the base SA vs RPM vs LV8 cals, there are some larger differences in SA with more SA on the 350 vs the 305. I find that curious in that both vehicles in 1985 were CI blocks with CI heads - perhaps the 305 CI heads in the F-body weren't as good as the Corvette's CI heads.

Over the past holiday weekend, I managed to take the Carmaro out for an hour-long, 60-mile run and it ran GREAT! Once in the garage, I grabbed a shortened wooden broom handle and rapped the MAFS housing while the engine was running - no stumbles, no change in RPMs. I figure the MAFS remains in good working order - on to find the other gremlims.

RBob

iTrader: (1)

The Camaro in '85 had a 305, while the Corvette received the 350 engine. Which is the main reason for the injector flow difference. The Corvette most likely had a better cam profile.

The difference in SA is likely due to the smaller bore of the 305, doesn't need as much lead to get peak chamber pressure at 15° ATDC.

With the ZZ4 heads a WOT SA in the area of 28° at RPM works.

RBob.

AAG

Got it, RBob - thanks.

Wrt the 85-era Corvette, would my 350's FT cam specs (Intake 194°/0.383" lift, Exhaust 202°/0.401" lift, and 112° lobe separation, a GM pn 12364051) be considered "milder" or "equivalent"?

In crafting my base SA table wrt my current setup, I'll set WOT SA at 28 deg for LV8 > 190 and RPMs > 4000 and datalog. Does that make sense?

RBob

iTrader: (1)

By today's standard that is a mild cam. Compared to what I don't know.

Take a look at AXCN. That is a '91 stick shift L98 '113 headed y-body calibration. Note how quickly the SA comes in. Then the dip at peak torque, then ramps back up. This is due to teh long tube runners.

Also look at ANHT, the '90 y-body calibration with a 700R4.

RBob.

AAG

Got it RBob - thanks and will do.

I've read in multiple places that, because the runners are long, the top end flattens out. It makes sense to back off SA when the engine can't breathe any harder. Now I understand why base SA tables with stock runners are what they are. I suppose then that SA tables tailored for tunnel-ram intakes don't exhibit SA dips at peak torque, yes?