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Thread: Add Highway mode to LT1

  1. #46
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    Quote Originally Posted by steveo View Post
    what works for one engine won’t necessarily work for another. comparing this airplane engine to an lt1 highlights the total absurdity of your generalizations both here and elsewhere. combustion chambers flame fronts and air fuel charge dynamics are an insanely complex thing with so many inputs and outputs and that’s why tuning is required in the first place. lots of engines melt down when lean, and lots can run 25:1+. lots of factors involved.
    A V8 engine is not going to melt down making the 40-50 hp it takes to cruise. Even at 80-100 hp its not going to melt down. I don't care if you were to run it lean enough to cause misfire. You will melt the cats from misfiring before you hurt the valves or pistons. The only way you will melt it down is to run it lean at high rpm and high throttle. You are actually more likely to destroy it at long periods of stoichiometric air/fuel at WOT as the combustion temps are hottest.

    Its actually not absurd at all. It was also a proof of concept not execution. The principle does work and I have applied it to Chevy V8s. Keep in mind that engine was running at a lower rpm than most of our vehicles and also running at about 30 in/hg manifold pressure. That is WOT for a non supercharged car at sea level.

    Also having a couple of EGT probes and a gauge that monitors both has taught me a thing or two. Under sustained WOT with an air/fuel ratio of around 12.5:1 my EGTs will rise to about 1,350F. In stoich cruise at 70 mph the EGTs run around 850. In lean cruise they drop to 775. The excess air is cooling the combustion process. The oil temps also drop. With both EGT and Oil Temps lower that means the pistons are running cooler and so are the valves.
    Last edited by Fast355; 1 Week Ago at 10:42 PM.

  2. #47
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    One other observation from a carb to TBI swap years ago. I swapped a 305 with a bone stock Q-Jet to TBI. I had the VE table tuned to a BLM of near 128 across the board. I had the same exact ignition advance curve I had in the mechanical HEI. My highway fuel mileage tanked. Went from 18 mpg to 15 mpg. I had a spreadbore intake still on the thing with a big block TBI unit and a GM TBI to Q-Jet adapter plate off an early 454. I ended up pulling the TBI off, and putting the Q-jet back on. I left the TBI distributor in place. Next highway trip I was up to exactly 18 mpg. I datalogged the fuel trims with the Q-Jet in place. The BLM values were all pegged out in the 160 range. The narrowband was burried at near 0 volts. I ended up turning on lean cruise in the 7747 ECM. I put the TBI back on. I set the air/fuel ratio in lean cruise at 16.5:1. I was instantly back up to 17.5 mpg. The factory Q-Jet was running in the 17:1 range at highway speeds.

  3. #48
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    nobody is questioning that richer AFRs are necessary at high load and RPM.

    the fact a carb'd iron headed 305 ran 17:1 ratios at highway speeds from the factory isn't useful data for tuning any sort of lean cruise for an LT1, which has a totally different spark control, fuel atomization, combustion chambers, and cooling system.

    do you happen to have any parameters useful for the LT1 specifically?

    i do, at least for the aluminum headed version of it. at dead coast i ran 16:1 without issues. above that, it was never smooth enough for my taste when leaner than stoich. on the highway with some momentum it wasn't a huge problem (lean cruise patch would probably be alright there) but in slow moving traffic you could feel it didn't like it at all. fiddling the timing around to try to smooth it out was eventually successful but it just felt better, had good power, and reasonable fuel economy with a bit more gas.

  4. #49
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    Untuned individual trims for lt1 is number reason for crap running engine at light load. At idle they are relative easy to dial but off idle is totally different thing. Untill the airflow equalize at off idle you might need dynamically changed trims based on rpm, map and airflow. Real nigtmare to tune. Now if you lean the mixture enough some cylinders might get just too lean, other might be just right and the unbalanced power cylinder to cylinder will lead to weird feelings.

    Once I tried running closed loop at idle.
    Upon switching from open loop with slightly richer mixture I noticed immediate rise in map, around 5kp.
    So closed loop was running leaner but consumed more air due to rise in map, leading to more fuel needed.
    In reallity it is possible to run leaner all the time with higher map readings and poorer fuel economy.

    Considering combustion temperature, the extra duration of the burning process with lean mixture[Lean mixture is known to burn much slower] can lead to more heat being accumulated in the engine, especially the valves. You are off peak temp for sure but other factors need to be taken into account. All pcm strategies for cat overtemp are tied to lean running under certain load and airflow, You can hit cat overheat threshold under 1 min of runnig 16.5:1. Also when the cat overheat is on, the pcm commands 11:1 mixture to cool down the cat, negating any gains.
    Less fuel also mean hotter intake mixture which further reduce power. When fuel evaporates it cools the intake temperature, increasing efficiency. So you get less cooling on the intake valve and longer burning process that hits the valves.
    The temperatures produced at burning gasoline is also constant, like all other burning fluids. I suspect the different temps are from the different quantity of the fuel used at different AFRs and the overall heat dissipation the engine makes.
    So with leaner mixture the heat is distributed unevenly in the engine. Cooler cylinders but hotter valves and piston top.

  5. #50
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    So I am curious how are you guys ( Steveo and Kur4o) picking apart and writing patches for these ecu's. I am curious to look deeper at the Highway mode on my C4 so see what if anything I can inject to the LT tune.

    Side note: Those modeling theory on the P38 experiment remember the differences in the Allison V-1710 vs our SBC engines. The allison has a bore of 5.5 inches we are using about 4 inches, this promotes a longer burn and better controlled flame front. They also show a CR of only 6.65:1 I am at 11:1, the difference here should be self explanatory. Finally they use an outdated version of Avgas with a much higher octane rating than we have, no ethanol AND lead. It is very unlikely any of us can go as extreme in our cars with a consistently varying load as they did on an aircraft engine that is set and left for 4 or so hours at a time.

  6. #51
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    Actually with lead in the avgas they have to vary the power setting every 30 minutes or so to keep from fouling the plugs.

    Forgetting the P38, I run 16.5:1 just fine on an 11:1 4.030 x 3.75" aluminum head small block that does not have the LT1s reverse cooling in a heavy, unaerodynamic vehicle. That is also on 10% Ethanol. I would shoot for about 17:1 on non ethanol laced gas. Did I also mention I am doing this on 87 octane. Runs knock free at 52 of timing at 2,850 rpm @ 70 mph.
    Last edited by Fast355; 1 Week Ago at 11:37 AM.

  7. #52
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    Give me your tune.

    Not even joking. My stock LT1 on 91 (E10, winter formulated) knocks at only 38 degrees of timing under load. If you’re saying you have a tune that makes the LT1 knock-free on 87 at coil-near-plug advance levels, let’s see it.

  8. #53
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    I think you need a new knock sensor.

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    {delete, replied to wrong post}
    Last edited by NomakeWan; 1 Week Ago at 07:50 PM. Reason: replied to wrong post

  10. #55
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    forgot to quote Fast, he needs the new knock sensor/ module

  11. #56
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    Quote Originally Posted by NomakeWan View Post
    Knock sensors work just fine, and frankly in the ESC system it's the module that has the most say, not the sensors. I can easily replicate the exact pattern on the knock map by messing with timing step by step. Pulling the advance back to stock resolved the issue. Better fuel would too, but that's not an option here in Commiefornia unless I wanna pay $10/gallon for 100-octane, which I don't.

    Stock '95: https://i.imgur.com/llQvpOa.jpg
    Advanced 37~39 degrees: https://i.imgur.com/lVb5hRn.jpg

    Also this is still a thread about Highway Lean Cruise for the LT1 PCM. So I'm still interested in that magical tune Fast355 is talking about, or a patch to add this type of function to the PCM. Don't want to drag stuff too far off topic.
    Since you're from Californicate (I was too) I don't think you'll be able to run HLC anyway. FWIR you won't pass smog with it enabled, that's why GM disabled it in the 7747 ECM back in '87. Although I'm definitely interested in what you save and what dangers you risk with it enabled. I'm running it with my EBL Flash2 on an 88 5.7L aluminum heads, 42* w/91 Octane @16.5:1 and haven't seen a lot of benefit from it, but I don't do a lot of cruising, LOL.

  12. #57
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    Quote Originally Posted by space387 View Post
    forgot to quote Fast, he needs the new knock sensor/ module
    Knock sensors work perfectly. New Delco sensors that went on the engine 6 months ago. I know they work because I occasionally see 1-2 of retard on throttle tip-in.

  13. #58
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    Quote Originally Posted by PlayingWithTBI View Post
    Since you're from Californicate (I was too) I don't think you'll be able to run HLC anyway. FWIR you won't pass smog with it enabled, that's why GM disabled it in the 7747 ECM back in '87. Although I'm definitely interested in what you save and what dangers you risk with it enabled. I'm running it with my EBL Flash2 on an 88 5.7L aluminum heads, 42* w/91 Octane @16.5:1 and haven't seen a lot of benefit from it, but I don't do a lot of cruising, LOL.
    Easy enough to just disable it before a smog test. :P Even easier if it's a patch that allows for an external trigger pin.

    Quote Originally Posted by Fast355 View Post
    Knock sensors work perfectly. New Delco sensors that went on the engine 6 months ago. I know they work because I occasionally see 1-2 of retard on throttle tip-in.
    ...isn't that just the burst knock feature built into the PCM that works independent of the sensors and module...?

  14. #59
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    Quote Originally Posted by NomakeWan View Post
    Easy enough to just disable it before a smog test. :P Even easier if it's a patch that allows for an external trigger pin.


    ...isn't that just the burst knock feature built into the PCM that works independent of the sensors and module...?
    Burst knock retard does not show up as actual knock retard.

    You won't get more than about 40 advance into a distributor that has the base timing at 0.

    The aluminum head L98 Corvettes ran up to 48 in their timing maps. They had horrible quench as well.

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    We’ll cross that bridge if we get there. I have an ignition controller for D514A coils waiting for me to toss it together and throw it on the car.

    So, your tune, maestro?

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