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Thread: Add Highway mode to LT1

  1. #1
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    Add Highway mode to LT1

    Over the past few days I have been looking through the features and decoded options for my LT1 Firebird and L98 Corvette. While sifting through the L98 tune (7277 ecu) I found it has a highway mode. Set it and sure enough the car runs leaner as advertised. Now I looked for a way to replicate it in my LT1 ecu but its nowhere is sight and if you leave closed loop enabled, its locked to the stoich afr constant. Has anyone seen a way to run a table for closed loop afr similar to the older highway mode? I'm not sure if this is how it works but would it be possible to take the 4 points of the stoich afr table and make them seek out a value from another table? You could set the table to reference map vs rpm like a VE table.

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    Fuel Injected! steveo's Avatar
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    i started writing a highway lean cruise type thing a long time ago. i found space for the code and figured some thresholds and constants based on some other similar patch. it never got into testing as i ended up running open loop on that car.

    i might have the patch around somewhere but i bet its nowhere near working as my assembly code isn’t awesome

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    I have not tried this, but I have given this some thought before. You could tune the car in open loop to run 14.7 in every cell, then find the cell that gets hit at your intended highway cruise RPM/airflow and adjust it to be leaner. Then use Power Enrichment to add fuel when you have more load than cruise to drop AFRs down around 13.2 or so when it kicks in. This way you should have 'safe' stoich on almost all of the VE/MAF, super lean cruise for a specific range of RPM/airflow, and richer mixture for heavy throttle application. It's not perfect because you give up closed-loop O2 feedback, but it works in theory.

    A similar thing was done on an LS: https://www.youtube.com/watch?v=keGz_1BwcdQ

    Technically there is space available in the computer to put a table of some sort and some arbitrary code, but you'd have to figure out how to create the appropriate jump commands to make the PCM use it. Creating custom code for the PCM is above my pay grade, though.

    EDIT: Damn, steveo beat me to it. Super cool that you were working on a lean cruise patch. If you ever want to add that back to your pile of things to develop, sign me up to test it out! I've got some Corvettes here that would make great testbeds.

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    Thats the video that got me looking into this again.

    Steveo I am not familiar with writing patches but if you want a tester I am all yours to try it. The engine is mildly ported and cammed with a raise in Compression ratio and its bolted to an M6. I also daily the car so it would be nice to see some improvement in milage when I'm not romping on it.
    Last edited by space387; 2 Weeks Ago at 08:25 AM.

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    Another option is if we can figure out how to limit the Closed loop mode to upper limits like exceed a MAF or TPS value. Does anyone know if you remain in closed loop if you enter PE mode?

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    Fuel Injected! steveo's Avatar
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    its in closed loop but not updating trims. so it uses your stored blm data to trim fuel in pe. most people use the blm locker patch so that doesn’t happen

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    ok, today I tested out today by setting the CL constant to 15.3 and once warmed up it was defaulting to the new AFR. When I exceeded the PE threshold it dropped to 12.5:1 for a target. I'm about to run through the logs to see if it actually did this or just showed the target in EEhack.

    So further investigation its just a display number, but, the EGO sensor did show it ran richer. To be safe I will be applying the BLM locker on my next flash. Just to be clear its only during Pe it locks the BLM to 128 correct?
    Last edited by space387; 1 Week Ago at 07:58 PM.

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    Fuel Injected! steveo's Avatar
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    yeah the blm locker in EEX is only for power enrichment...

    ....but wait .. wait what are you doing?

    i don't understand.

    dont run your car at 15.3:1 all the time..... that'd be dumb.

    you have to realize that "highway lean cruise" isn't the same as "leaning it out all the time during cruise". it's supposed to be lean during very specific and non-damaging conditions. there's a reason GM never put this code in the LT1, you know.

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    Fuel Injected! steveo's Avatar
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    you also can't achieve that AFR in closed loop, it'll just 'target' it, and your CL trims will pull it back to an actual value of stoich,

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    steveo speaks the truth. You can't make your LT-1 run richer or leaner in closed loop by changing the CL afr target. The oxygen sensors will detect lean combustion and add fuel to your BLMs to correct it back to whatever stoichiometry is for the fuel you're burning. If that happens to be an ethanol blend it's probably closer to 14.3-14.4 to 1. What you're doing is equivalent to trying to herd cats.

    For a properly functioning lean cruise you have to jump out of closed loop in lower load regions, and ideally make the PCM use a separate AFR target table. Also some additional spark advance may be beneficial to compensate for lost torque that results from leaner (and slower) combustion.

    I tried summoning kur4o to this thread because this is something he could probably whip out in a couple days (a real lean cruise strategy for $ee).

    I would be very interested in a true lean cruise solution for the $ee mask. Unfortunately I have several other projects to complete before I would be able to do any testing, and the weather has taken a massive dump on my joy-riding schedule for the forseeable future.

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    Maybe look into what the australian's had in that model. PCM hack.com might have something. Convert to 411?

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    Quote Originally Posted by dfarr67 View Post
    Maybe look into what the australian's had in that model. PCM hack.com might have something. Convert to 411?
    Australia did not use the LT1 based engine. They had their own Holden 308 based engine that ran $12P coding in a 808 ECM (Basically 165')

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    Quote Originally Posted by dfarr67 View Post
    Maybe look into what the australian's had in that model. PCM hack.com might have something. Convert to 411?
    I second an 0411 swap with EFI Connection 24x

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    Swapping to another computer and ignition system for a four-figure cost seems like a cop-out answer when there is still plenty of potential left in $EE. I wouldn’t personally give up yet, not when the platform is being actively developed.

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    Quote Originally Posted by NomakeWan View Post
    Swapping to another computer and ignition system for a four-figure cost seems like a cop-out answer when there is still plenty of potential left in $EE. I wouldn’t personally give up yet, not when the platform is being actively developed.
    My 5.7 Vortec gained about 15-20 ft/lbs across the whole RPM range with the 24x ignition. The prior 0411 swap also gave it much better driveability. With the 24x swap I went to the newer 1 meg PCM from a DBC van. That added the ability to run E85 which added even more power.

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