Back in the OBDI days..... GM computers had "Highway Mode", and would push mixtures as lean as possible. 17:1 was not unusual. However, it wasn't implemented from the factory. Some guys experimenting with burning their own chips discovered it, and played with it.... I was lucky enough to be able to take advantage of that, and tweaked the ECM in my 88 Firebird Formula 350, and I was able to manage 23 MPG on the freeway... Sometimes even better.

 

Interesting... sucks that the ECU will keep 14.7 afr. Even if I could get around it, the one issue with trying that here is there are no flat areas to take advantage of it. The longest flat area is approx 2 miles & the rest is much less than that or on a grade.

Next tune is installed... need a test drive on it...

 

Yeah, given those conditions, likely wouldn't help you much.

 

test drive done, definitely stronger on the bottom end cruising... 1300~1500 rpms @ lower speed on a hill was not bad. On the previous build it would want to downshift to maintain speed, seemed to hold speed better with this tune.

I may go back and bump that very bottom end another 0.5* (that is the resolution)

 

Update... Found a feature in the vcm scanner i did not know about, leveraged it to get the injectors dialed in with LT fuel trims in the +/- 10.0 range in all cells hit in the scanner. Dialed up timing and reduced the dfco enable temp.

Out for a week or more so it will be a bit before i have feedback on this tank (just filled it)

 

Observation on drive to airport ... Power is "different"... no longer has that "punch" it had prior to the tune update. It got me thinking about what was done and cause & effect. The total fuel on each injection event was enough to give it a good bump in power with the new pump & injectors. Now that it is dialed back to get the ltft closer to 0's with reduced duration that power dropped some. This has me thinking that even with the ltft dialed in prior to the tweeks, there was power to be had with the over fueling it had.

So the thought i might investigate is to find a resistor to put inline with the fuel pump to lower the pressure/volume some to act as it does now, then short it on demand for the increased fuel as needed. The ecu will keep 14.7 afr but, in doing so it appears to me it is having a bit of a lag getting there. That lag appears to show as a bump in power.

If i did not drive it and feel these changes, i would not believe it. Going to revert back to tune from last friday... ( Adjusted fuel gauge & fan temps) home friday nite to make the change.

 

What you discovered is magical 0.85 lambda (12.5:1 AFR). That's what you want to aim for, for max overall power at the sacrifice of fuel economy.

https://www.safrtool.com/SAFR-AFR-values.asp

• 6 AFR - Rich Burn Limit (engine fully warm)
• 9 AFR - Black Smoke | Low Power
• 11.5 AFR - Best Rich Torque at Wide Open Throttle (WOT)
• 12.2 AFR - Safe Best Power at Wide Open Throttle (WOT)
• 13.3 AFR - Lean Best Torque
• 14.6 AFR - Stoichimometirc Air/Fuel Ratio Value (Stoich)
• 15.5 AFR - Lean Cruise
• 16.5 AFR - Usual Best Economy
• 18 AFR - Carbureted Lean Burn Limit
• 22+ AFR - EEC / EFI Lean Burn Limit

 

Yeah but wouldn't the ECM attempt to keep 14.7?

Would the power enrich settings be tied into that somehow?

Would your expectations be that I should be able to keep both in the form of 14.7 in the low load/high vacuum low rpm range & 12.5 everywhere else? I can post a pic of the tables before & after the mods if that helps for a visual... I left everything else in higher loads and higher rpms alone so I'd expect that it would perform as it did prior to the changes. Note that I did not "test out" those higher load cells since it was my expectation it would be unchanged while driving.

 

By tightening up the fuel trims, you're telling it to adhere to 14.7 more strictly. Yes power enrich I would expect to aim closer to 12.5 .

 

And the PCM only pays attention to the O2 sensors in closed loop. Beyond certain throttle openings, it will automatically go into open loop, and run off the defined tables in the programming. I *think* a certain amount of engine load will do the same, but, not 100% on that one.