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Should you make EFI super-lean at cruise?

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  • #16
    Originally posted by CDMBill View Post
    So the original question of 'should you make EFI super-lean...' inevitably becomes one of to accomplish what? under what circumastances? and with what trade offs? No black and white answer here.
    So true, so true.
    I'm still learning

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    • #17
      Originally posted by dieselgeek View Post
      I definitely learned to use my ears and intuition over the AFRs working on Doc and Jeff's programs, especially for anything outside of WOT use.

      BKB, I have to give you props if you have your BS3 showing good throttle response - that's one of the hardest things (for me) to tune on the BS3/FAST/DFI boxes. Way to go!
      Thanks Scott!
      Ive got about 6 years of fooling around with the things in two different daily driver cars to get the tune under control. My biggest (boggest) problem was over-fueling after a couple quick hits of the throttle. That took a few brain cells to fix... Im still discovering tweaks to improve mileage without affecting fundamental driveability and performance, but overall I cant say anything bad about the BS3 other than its a bear to learn to tune...throttle response is very crisp though...The six pack system helps as well. Ive got a MEFI set-up on a Chev 350 that was a pain until I got the "full key" to crack into more of the transient controls. They had a 14 deg idle spark control "locked in" as default, and it was killing idle quality. Heading to MATS on Thursday in LV with the two BS3 cars to do the drag racing and slalom thing all weekend. Got a couple of extra tix. Tunes can always use some outside review...
      BKB
      www.FBthrottlebodies.com
      Bruce K Bridges

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      • #18
        Originally posted by CDMBill View Post
        So the original question of 'should you make EFI super-lean...' inevitably becomes one of to accomplish what? under what circumastances? and with what trade offs? No black and white answer here.

        Well, initially I was thinking of a sort-of "displacement on demand", only managed with fuel mapping.

        The test subject would be a BBC truck motor with TBI (initially)... on decel, coast, or light cruise, I wanna know what happens if you cut way back on fuel. If it were port-injected, I am intrigued by the notion of what might happen if you just shut off fuel to a couple of cylinders completely. Then when back on the throttle, back to normal air/fuel ratios.

        Would that kill driveability?

        Or would it allow for some decent mileage out of a pig that might get 12-14 mpg at best with a TBI or carb? (more like 9mpg with the carb).
        Last edited by Caveman Tony; April 9, 2012, 08:36 PM.
        Yes, I'm a CarJunkie... How many times would YOU rebuild the same engine before getting a crate motor?




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        • #19
          Originally posted by Caveman Tony View Post
          Well, initially I was thinking of a sort-of "displacement on demand", only managed with fuel mapping.

          If it were port-injected, I am intrigued by the notion of what might happen if you just shut off fuel to a couple of cylinders completely. Then when back on the throttle, back to normal air/fuel ratios.

          Or would it allow for some decent mileage out of a pig that might get 12-14 mpg at best with a TBI or carb? (more like 9mpg with the carb).
          What you describe is "deceleration fuel cut" and it can be done on the MS really easily. I've had guys report as much as a 15% fuel economy increase when implementing this. It shuts fuel off to ALL cylinders on deceleration, it takes a little effort to tune but it's not hard. It won't kill drivability once tuned. It's not DOD but still very effective.
          www.realtuners.com - catch the RealTuners Radio Podcast on Youtube, Facebook, iTunes, and anywhere else podcasts are distributed!

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          • #20
            As a carb guy still learning to be comfortable enough to do EFI on my own - the decel fuel cut off is one of the most interesting proposals that EFI offers that carbs simply can't match.
            There's always something new to learn.

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            • #21
              My car shuts fuel off completely when coasting, at least until around 1200rpm. Works flawlessly. I had a 2 second delay on it, and I could feel it shut off (felt like a torque converter locking up), but since I switched to a .5 second delay it happens so quickly I don't notice it cut fuel at all.

              On the MegaSquirt there are two fuel injector channels (unless you go sequential), and I wired my injectors up like the Cadillac 4-6-8, so that if I wanted to turn off 4 injectors it would turn two off on each bank, the same two that GM did on the Cadillac engine. I've never actually tried it because I haven't figured out the logic on how to make it shut them down when I wanted (namely only at highway speeds), but I have had a couple times where the 12volt feed to one of the injector banks came loose (didn't hook it into the relay board all the way, my fault) and turn off four injectors. The car was very drivable on 4 cylinders, though mind you this is a 462 cid engine, so 4 cylinders is still 231 cid. Considering the tune, it didn't really want to idle on 4 cylinders, and did better if I kept the rpm's up around 1200 at idle, but driving and such I just had to give it more throttle for the extra load. Ran this way until I got home from across town. Popped the hood, saw the wire hanging, plugged it back in and all was good.

              On the tuning side of this, the AFR gauge went full lean as the non-fueled cylinders were still pumping a ton of air. So tuning would have to be done either on a cylinder by cylinder basis or by ear. Of course you don't have this issue when true DOD systems close the valves, the AFR's will continue to read normal. One of the main reasons I want to build a 472/500 Cadillac.

              Hey Scott, on the MegaSquirt 3, when I go full sequential, with the VSS sensor reading, do you think I could program 4 of the cylinders to stop fueling when certain conditions are met? Namely high vacuum, greater than X rpm and signal from the VSS? On the MegaSquirt II, the logic I could program in would cause half the cylinders to drop out at lower speeds in different gears, and I figured it would get annoying in 1st and 2nd gear at low load, and letting it rev to ~3,000rpm. The vacuum and rpm would be high enough to trigger the fuel table map switch in each gear. Where as with the VSS I could say don't do it unless vacuum and VSS speed is greater than. Plus with the sequential there would be less cylinder to cylinder fuel scavenging I'd think.
              Escaped on a technicality.

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              • #22
                If you used a spare output to switch a relay that powers 4 of the injectors, the MS3 (and MS2-Extra I think?) let you use two conditions to control an output. So I *think* you could do it with VSS, but not sure.

                However, does shutting off the injectors really save you anything since you now encounter huge pumping losses? Don't you need to disable the valvetrain on those cylinders as well?
                www.realtuners.com - catch the RealTuners Radio Podcast on Youtube, Facebook, iTunes, and anywhere else podcasts are distributed!

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                • #23
                  I think pumping losses would be minimal, as the airflow is still going through with normal valvetrain actuation. Just creating hot air on compression stroke, really...

                  If you disabled valvetrain, wouldn't you then have a large vacuum to pull against with both valves closed on intake stroke? (conversely, pushing against on exhaust stroke)... unless by 'disable' you mean hang those valves open slightly on deactivated cylinders... then I think pumping losses would be nil, just frictional losses left.
                  Yes, I'm a CarJunkie... How many times would YOU rebuild the same engine before getting a crate motor?




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                  • #24
                    Pumping losses are still big with the valves going, especially with big displacement.

                    This is the angle I'm shooting for. I've gone as lean as I can to support combustion in the cylinders, so if I cut half the cylinders out of fuel, it forces the remaining four to work harder and thus require more fuel, and dropping the AFR's, which opens the possibility of using less fuel over all. Basically the limiting factor in reducing fuel into my engine is to support combustion because running the cylinders even leaner will result in mis-fires, now if I take all the fuel currently being used but distribute it to only 4 cylinders, effectively doubling the fuel in the cylinders that are getting fuel (for cruising), my AFR's at that point can then be leaned out some amount again because those 4 cylinders will likely be running at 13-something to 1 AFR's. If I can cruise highway speeds, on something leaner than 13-something to 1 AFR then it could net some gains. Shutting the valves off is definitely the most ideal and would likely net big gains, 25%?, where as this will likely be 5%(?) or so. So definitely splitting hairs at this point, and I think is really only practical on large engines that have so much volume per cylinder there is a higher minimum fuel requirement per cylinder just to support combustion, not to contribute HP. Remember, four of my cylinders is 3.8L, or 231cid, more than adequate to support highway cruising, and yeah the pumping losses are going to eat at least 75%+ of the gains according to my gut feeling.
                    Escaped on a technicality.

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                    • #25
                      I think the hot set up,mileage wise,would be to sequentaily drop defferent cylinders every cycle. I think the OEMs do this now. I quess it would be a controled fuel shut off miss. Cut every other cylinder,or every 3rd. That way you wouldn't have oiling problems in the DEAD cylinders.

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                      • #26
                        They do that when they over heat, but I'm pretty sure they don't do that on the DOD systems.
                        Escaped on a technicality.

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                        • #27
                          OK I was just wondering if this might be a better(on the engine) way to drop cylinders.

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                          • #28
                            I'd like to see some old GM engineering reports on the oiling and such of the cylinders on the Cadillac's 4-6-8, and how long it could "safely" run on less than 8 cylinders. I'm kinda interested on the LS engines too, but there are a lot of engineering design changes on the pistons, rods, rings, etc.
                            Escaped on a technicality.

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                            • #29
                              Cyclone03 may be on to something, as the original 4-6-8 was a flop. It would be interesting to understand what they do now. I don't think I'll be signing up for the engine shut off at stop light deal.
                              Drag Week 2006 & 2012 - Winner Street Race Big Block Naturally Aspirated - R/U 2007 Broke DW '05 and Drag Weekend '15 Coincidence?

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                              • #30
                                Bill said something I had wondered about with DOD - I have concerns about running a cylinder dry, I think I would rather it alternate per stroke?

                                Randal mentioned something I hadn't really thought about - AFR going wonky because of air only cylinders. The valvetrain stuff jacks with me even more. How is a mechanical cam supposed to do this with conventional spring setup? I could see it if you had actuators on the valves (electric or something) but I don't see how you would handle the valvetrain otherwise. I don't want to contaminate the intake air, would you just hold the exhaust valve open?
                                Last edited by Beagle; April 15, 2012, 07:24 AM.
                                Flying south, with a flock of bird dogs.

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