How lean is too lean?

Fuel and Air Systems

  1. 7milesout

    7milesout Well-Known Member

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    Guys (and gals?),

    I don't know how to ask this accurately, or any clear than I have below, so bear with me. I've searched all over the internet and I'm not finding what I am trying to find. I'm guessing it is because, it is a very complex issue.

    I've always heard that damage caused by a lean fuel mixture is only at WOT. But maybe that was all I ever heard … back in my Wankel rotary engine days. I'm going to assume that it is true for piston engines as well. But to a degree. Meaning, at cruise rpm, there's not the same cylinder pressures or heat (as WOT) so running lean at cruise rpms is not bad. But if it is too lean at cruise speeds it will just lose power (or maybe build heat also).

    Here's my condition, which leads to my question. I have my Edelbrock 1405 equipped LA360 cruising at say 45 - 60 mph in the 14.0 - 14.5 AFR range. But when I go to accelerate (not WOT, just accelerate at all), what happens when my foot pushes on the pedal? I'm not (directly) adding gas, but I am directly adding air by opening the throttle valve. So, my 14.0 - 14.5 AFR at cruise immediately gets leaner when I open the throttle. I'll hit 15.5 - 16.0 AFR trying to pick up speed. So here's the question: At this non-WOT condition, where is the threshold that XX.X AFR (lean) is OK, but YY.Y AFR is too lean and could lead to problems, even at this power mode / cruise rpm? Or … is this lean condition not an issue because I'm not at WOT?

    The way mine runs even at these high / lean AFR numbers, it runs very strong. If I didn't have an AFR gauge on it, I would not even know it was lean, as it runs great.


    7milesout
     
    Last edited: Jun 7, 2019
  2. mderoy340

    mderoy340 Well-Known Member

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    You are to lean when the leanest running cylinder starts to detonate. Dual plane intakes are the worst for uneven fuel distribution. Read each cylinder plug with a magnifying glass and stagger jetting will get you close, but a carb motor is all about compromise. Best to be slightly rich then lean. A stiffer power enrichment spring may help your situation.
    TPI is not even perfect in today's motors. 02 sensors are only reading an average of the bank so a dirty injector, low fuel pressure, etc can run a cylinder or two lean. The knock sensor then retards timing and sets an engine code.
     
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    • AJ/FormS

      AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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      This is not right.
      Anytime you move the primary throttle valve, you should have a near-instantaneous increase in fueling to match the change in cfm. It's your job to make it so.
      "Lean is mean" works at the racetrack, maybe.
      But on the street is causes too many headaches. A tad rich will respond better to quick changes in throttle inputs, with fewer hesitations, and makes you feel more connected to the power-curve. Oh and you can run a lil less pumpshot, which has a dramatic effect on city fuel-consumption.
      A lean cruise is OK, within limits. But if you go too lean, then your plugs will get hot, and when you come in off the hiway, you will get that sickening feeling that something is wrong,lol. Take a breath; as soon as the plugs cool off, throttle-response will return to normal. This sucks for passing tho, cuz you gotta wait for the plugs. I don't like it, so I don't run that lean.
       
      Last edited: Jun 7, 2019
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      • Mattax

        Mattax Just the facts, ma'am FABO Gold Member

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        Bingo. That's using your noodle. I know, same thing I told Mopar R&D, but its true. Tune to performance.
        see this post: How to properly drill carb jets?
        Leaner with more throttle: Exactly and then some. Its what is supposed to happen.
        We have to go to the books based on testing and experimentation for the automotive and aircraft industries.
        This from Larew's book on carburation:
        blob-jpg-jpg.jpg
        Notice the fuel air mix gets leaner as the load increases to about 60 % max load. Nothing magic about 60%. An engine with more efficient part throttle could be leanest at 80% max load and run better.
        More fully explained here:Need Opinions. Have nothing to compare to.

        Threshold: You'll have to experiment to discover how lean your particular combo will accelerate better for a given load. Generally if you have lean high speed cruise dialed in, the part throttle acceleration will be darn close. So, tune for steady state situations first (idle, high speed cruise, low speed cruise, WOT) then work on transient situations as needed.

        To get the high speed cruise, run steady at 60 - 65 mph with leaner jet - rod combos until it revs and tries to die. Find a safe place to do that! When it does - you must slow down. Don't punch it or anything stupid. Might even have to pull off and restart. Drive back to the shop at a slower speed and richen the jet-rod combo for part throttle and while keeping the WOT unchanged from the best at the dragstrip. The Eddy manual has a chart to help with that.

        Actually at the speeds your describing, the increased pull on the fuel is immediate with the increase in air flow. At interstate speeds change in air velocity through boosters causes an immediate change in the pressure in the main fuel passages. The air bleeds are used reduce the pull on the fuel. Its a well matched system giving an engine exactly what it asks for immediately.
         
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        • 7milesout

          7milesout Well-Known Member

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          AJ - I can see I didn't convey my thought clearly. When I am trying to convey is that MY FOOT is controlling air flow, and that my carb set up is controlling the fuel flow. Before messing with this carb, I would have thought pushing the gas pedal was "adding gas." But, it's adding air … and you're right, my job is to set it up to where when my foot adds air, the setting I have installed adds the required fuel. I'm not quite there as it is, because it goes lean quickly.

          So, what I'm going to go home and do is go to the next stiffer metering rod springs. As I recall, I think I'm on the middle springs. What should happen is, as my foot opens the butterfly more, and air can flow in easier, the more open butterfly (and additional air flow) reduces the vacuum, which is what the metering rod springs are "fighting" against. So with the next stiffer spring, those springs should tend to "win the fight more" against the reduction in vacuum. And when the springs win, they pull the metering rods more out of the jets, allowing more fuel to flow to match the increased air.

          So that's my next step tonight. Unfortunately, as I type, the thunder rolls outside and I won't get to drive it, maybe not even tomorrow either, lots of rain over this weekend.


          7milesout
           
        • Mattax

          Mattax Just the facts, ma'am FABO Gold Member

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          It should go lean instantaneously. That's what we're telling you.
          It's different than when working from the idle circuit and going WOT.

          If you use the the throttle to crowd a vacuum, the leaner AFR will stay fairly constant.

          Here's a log that shows proper response opening the throttle from cruise but hits a dead spot. This past winter I took this carb off the shelf and fixed the calibration issue. Some point soon I'll put on the car and find out if I've solved it.

          First markers shows slight acceleration with increase in throttle from cruising to light acceleration. Vacuum drops from 19"Hg to 16" Hg.
          Second to third marker shows more acceleration by opening the throttle to maintain 14" Hg.
          Gently adding more throttle to drop the vacuum as low as 11.5 " should have resulted in more acceleration. That's the 'dead' spot.
          If this was a higher speed test, the AFRs would have been leaner.
          313_74bb70c202a064d32aa3e42ca1b707be_t.png
           
        • yellow rose

          yellow rose Doctor of Thinkology.

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          What did you use to develop that log?
           
        • Mattax

          Mattax Just the facts, ma'am FABO Gold Member

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          Innovate's LM1 wideband logger with their Auxillery box.
          The aux box has a RPM channel, MAP sensor, and forward/back and a side to side accelerometers built in.
          RPM in my case I tapped into the tach wire from the MSD 6T. But any frequency source can be used.
          For MAP I have a tee into the vacuum gage line and then use a flexible hard vacuum line through a firewall grommet to the unit which sits on the passenger floor.
          Throttle position sensor can be wired to it. It's on my to do list, just not been a priority.

          The software is Innovate's Logworks which can be downloaded for free. It's pretty good once you figure it out.
           
          Last edited: Jun 8, 2019
        • yellow rose

          yellow rose Doctor of Thinkology.

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          Can you even buy that anymore? Last time I looked I couldn't figure out what to buy to get where you are.
           
        • '63GT

          '63GT Well-Known Member

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          You're on the right track... Carter used to offer strip kits for this reason, but since Edelbrock took over, they've cornered/took over the market on those. seems The Carb Shop has filled the void since then.. I used to work with a lot of carter stuff, back in the day when you could still find them in salvage yards. I've moved on to Holley's since.
          The strip kits had several different springs in the kit, and even had a selection of shims to 'fine tune' if needed. (if I remember correctly..)
          The Thermoquads had a small screw in the middle of the metering rod bar, that rode on a eccentric in the middle of the throttle shaft, to adjust metering rod 'ride height'. This made all the difference in the world in such conditions you're describing. unfortunately, the afb/avs doesn't have that...
           
        • Mattax

          Mattax Just the facts, ma'am FABO Gold Member

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          The LM-1 has been replaced by the LM-2. I have one of those but have't tried it out yet.
          Yea it looks like the auxbox (LMA-3) and ST-12 hae been dropped. SSI-4 may be the closest substitute currently made. :(
          Tuner has a pretty cool setup. Since you know him, you may recognize one of the example logs that at least used to come with the Logworks download. (No its not my car - its a real road race car)
           
        • 7milesout

          7milesout Well-Known Member

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          I have 2 different "tuning kits" from Edelbrock. One for the 1407 and one for the 1405. It has a variety of jets, springs, and metering rods. I checked over this weekend, I already have the stiffest springs in the kit installed. But there are no shims in these kits. I think if I could figure out a way to shim these springs, I'd be in bidness. And a ride height adjustment to the metering rods would be awesome as well. I wonder if I can find shims from a Carter … and if those shims would fit correctly in the Edelbrock?
           
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