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  • The "Importance" of SFI

    wasn't sure where to put this, so OT seems acceptable for now...

    when i get bored, i tend to think a lot about things that would make normal people go "why bother?"....

    so, i started thinking about GM's various injector firing schemes over the years... came across information that is contrary to what most people are told, though i've had this verified by a few sources now:



    SFI intentionally fires the injector at a _closed_ intake valve (once for every 2 revolutions, synchronized to precede the intake event on the respective cylinder) - that's what you want. The "hot" intake valve improves vaporization of the fuel as it "waits" to be drawn into the cylinder. An open intake valve allows gas flow that interferes with the spray pattern and does not provide the best fuel vaporization
    now, i've heard of this for quite some time, and it does make sense, so i thought i'd run with it. grabbed cam specs for a 3x00(not sure where, so it may or may not be 100% accurate), and generated this in CAD:



    now, this is assuming the 3x00 cam has 262* of duration on the intake lobe. note that i have it positioned so the "graph" starts with cylinder 1 intake opening. not necessarily accurate, but will work for this demo. note the rings, cylinder 1 is the largest, then goes further in cylinder by cylinder down to #6. an 8 or 4 cylinder can be generated using the same method with more or less messyness, but we're 60V6, so it would be wrong to use something else.

    now, the red portions of the graph are when the intake valve is open, green when closed. now, we want the injectors firing when the valve is closed for any 1 cylinder, something easily done with SFI, just need to know when the valve closes and start pumping fuel. with MPFI, a compromise must be made...

    using double-fire MPFI as an example we have this:

    assuming the injectors fire at the top, cylinders 2, 3 and 4 have closed intake valves, with 5 just about to close. the injectors will fire again at cylinder #4, at that point 5, 6 and 1 are closed with 2 just about to close. with this method, all cylinders will have 1 injection event where the injectors at least start spraying against a closed valve, if not finishing, if the pulse is short enough.

    now, we look at single-fire MPFI, which i had THOUGHT was a good idea until now...

    only 1 injection event, let's assume it happens at the top of the chart. only 2, 3 and 4 will happen against a closed valve. 1, 5 and 6 will always be shooting(initially at least) with an open valve.

    so, now i'm pretty much turned off of the idea of single-fire MPFI, but now let's compare double-fire MPFI to SFI: both are shooting against closed valves in all cylinders, with rising fuel demands making the SFI "advantage" less noticable, since if enough fuel is required, both will be shooting with open valves. because of this, SFI has an advantage at lower RPMs, but how significant is it?

    grabbed a log from the MC, at a hot, closed loop idle at 600RPM, i was seeing a PW of 2.6mS(keep in mind, this is SFI). at 600RPM, 2.6mS translates to the crank revolving 9.36*(1.3% injector duty cycle) between the injector opening and closing. if we look at a new chart:



    we see a couple of new lines. there are two up top and one on the bottomside. the furthest right on the top represents the time the injectors are open in SFI mode (only showing 1 for sake of clarity, not all 6), the line to the left of it and the bottom line showing double-fire MPFI mode injector opening time(since they're fired twice per 4 stroke cycle).

    now, the SFI line is 4.68* away from the top line, not 9.36, since this chart displays 720* of crank revolution, and the double fire MPFI is half of that, 2.34*.



    make of it what you will, but from what i've just simulated, SFI has damn near no advantage over double-fire MPFI in terms of being able to shoot against a closed intake valve, since double-fire assures that at least one of the injections per 4 stroke cycle in each cylinder will be against a closed valve.



    so, half rant, half information. while it would be nice to have injectors fire against the intake valve as soon as it closes, it's just not that critical, really. and once you start getting into situations with longer pulse widths, or cams with more duration, the SFI "advantage" is completely non-exisitant. if you can get something to work better with one or the other, it's not the hardware/method, it's calibration.
    1995 Monte Carlo LS 3100, 4T60E...for now, future plans include driving it until the wheels fall off!
    Latest nAst1 files here!
    Need a wiring diagram for any GM car or truck from 82-06(and 07-08 cars)? PM me!

  • #2
    car with a big cam appear to idle better in SFi then i MPFI

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    • #3
      Is that MAF vs MAP as well for your SFI vs MPFI comparison on idle?
      Ben
      60DegreeV6.com
      WOT-Tech.com

      Comment


      • #4
        Originally posted by LX9Power View Post
        car with a big cam appear to idle better in SFi then i MPFI
        calibration.

        Originally posted by SappySE107 View Post
        Is that MAF vs MAP as well for your SFI vs MPFI comparison on idle?
        also a good question.



        i may have to impliment a temporary patch for the 95 MC to switch it between double-fire MPFI and SFI via a rocker switch... could be interesting.
        1995 Monte Carlo LS 3100, 4T60E...for now, future plans include driving it until the wheels fall off!
        Latest nAst1 files here!
        Need a wiring diagram for any GM car or truck from 82-06(and 07-08 cars)? PM me!

        Comment


        • #5
          calibration??

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          • #6
            tuning.
            Ben
            60DegreeV6.com
            WOT-Tech.com

            Comment


            • #7
              I meant take two car that are tuned and the car with SFI will feel like having a 'Tamer' Idle

              Comment


              • #8
                Originally posted by LX9Power View Post
                I meant take two car that are tuned and the car with SFI will feel like having a 'Tamer' Idle
                why? if both are delivering the same amount of fuel and same degree of advance and otherwise the calibrations are identical, then they should run exactly the same.
                1995 Monte Carlo LS 3100, 4T60E...for now, future plans include driving it until the wheels fall off!
                Latest nAst1 files here!
                Need a wiring diagram for any GM car or truck from 82-06(and 07-08 cars)? PM me!

                Comment


                • #9
                  I cant go into details because I don't know why but basing myself of two (1991 cavalier 3400 and a 1999GaGT both cammed) it felt like the GA was tamer, could be other things too but both car idle were a little different.( cam were close to the same that's what i remember)

                  Comment


                  • #10
                    the grand am is MAF, and MAF is much easier for the computer to control a cammed engine than MAP.
                    Ben
                    60DegreeV6.com
                    WOT-Tech.com

                    Comment


                    • #11
                      Ah i didn't knew that.

                      Comment


                      • #12
                        Interesting; I'll probably make note of this in my Sequential Fuel Injection article.

                        About the atomization on the hot valves: Doesn't this advantage get reduced once the valve is covered in carbon deposits? It seems like it would "hold on" to the fuel for a longer time, so one intake event may pull on more fuel than another (because some fuel is retained, evaporates after the valve is closed, and drawn in with the next intake event). Granted, this effect would be minimal, but it could cause a slight drop in performance.
                        - Stephen Brand

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                        • #13
                          actually.... good point. Ford released a TSB for their 2.3 engines that had stalling issues due to the intake valves that were retaining fuel due to "dry deposits", essentially making the engine run considerably lean until the deposits were fully saturated with fuel. with todays fuels/filtering, that shouldn't be as much of an issue, but still worth thinking about.

                          also consider that not all of the fuel injected even has a chance to hit the intake valve and atomize further. to do so, the valves would need to have gigantic backside surface area and fuel flow minimal.

                          IMO: much better to have injectors that have a good spray pattern/atmoization than to rely on a specific injection method to make up for it.
                          1995 Monte Carlo LS 3100, 4T60E...for now, future plans include driving it until the wheels fall off!
                          Latest nAst1 files here!
                          Need a wiring diagram for any GM car or truck from 82-06(and 07-08 cars)? PM me!

                          Comment


                          • #14
                            Newer computer take into consideration the temperature of the intake ports, and how much fuel is going to hit them and stick initially, and then eventually get sucked in. EGR and PCV systems attribute the most to carbon deposits on the back of the intake valves, so a catch can for the PCV is highly recommended.

                            You have lots of fuel delivery options, with direct injection being the best. No valves to worry about.
                            Ben
                            60DegreeV6.com
                            WOT-Tech.com

                            Comment


                            • #15
                              newer? as far back as $88 at least, an intake runner temp is simulated, possibly even further back as well. as for the amount of fuel that sticks around or doesn't stick around.... i've taken a look at some of the LS1 stuff, WAY beyond anything i've seen in anything OBD1 as far as that's concerned. they even model the amount of fuel left in the nozzle in the injector after a firing.

                              and yes, direct injection is quite an awesome concept.... if only it were more easily controlled. if a non-direct injection PCM were used without any modifications, there is a pretty good chance that the injectors would be firing when the exhaust valve is open. so with port injection you have to worry about intake valve timing, with direct, exhaust valve timing.
                              1995 Monte Carlo LS 3100, 4T60E...for now, future plans include driving it until the wheels fall off!
                              Latest nAst1 files here!
                              Need a wiring diagram for any GM car or truck from 82-06(and 07-08 cars)? PM me!

                              Comment

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