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  • SM4 trigger accuracy with more or less teeth?

    I think this was covered in the old Autronic forum. I think i remember Ray saying any more or less than 8 teeth on the crank wouldn't gain you any better timing accuracy?

    If you were to make your trigger setup from scratch for a 4 cylinder with the "best" timing accuracy in mind that the SM4 can do what would you do?

    I see some systems out there use every tooth even if on a 60-2 setup.


    Thanks Raymond.

  • #2
    If it is a normal engine and not "light mass crank" like two strokers I would go for two teeth where one is loacated your total advance + 10 degrees, normally around 50 deg BTDC. If I would push it further 4 teeth or 8 teeth can be used. I would stick with the gear tooth sensor. The 2 teeth is the simpliest setup. 4 teeth can still use pulse per cycle ic you set Trigger events cycle to 8 (if using the tacho o/p you must use 8 cyl tach here). Else you can set it up as divider which is little more comlicated.
    Robert Jansson
    MRM-Racing

    Comment


    • #3
      will i need any mods to my standard star-wheel crank trigger on a 2jz before i put the motor back together?

      Comment


      • #4
        I didn't know you could use more than two teeth on a pules per cycle setup. My old 1.02 chip doesn't have a divider setup option i don't think.

        So you could use 8 teeth on pulse per cycle and set the trigger events to 16, but the tacho would need some recalibrating?

        That seems too simple.

        Comment


        • #5
          Yes that is correct with the trigger events. I had one 60-2 6-cyl engine. That car (Volvo) has a 4-cyl tacho (even the 5-cyl 850 has 4-cyl tacho). So I ended up setting the motronic setting as per 4-cyl car and only fuel & ignition as a 6-cyl. So the tacho O/P is dependent on the trigger events.

          On the early chip with no divider settings you can do so but the trigger events maximum is 16. 4-cyl engine can have max 8 teeth at crank and 6-cyl 12 teeth.
          Robert Jansson
          MRM-Racing

          Comment


          • #6
            Originally posted by rzsupra View Post
            will i need any mods to my standard star-wheel crank trigger on a 2jz before i put the motor back together?
            No, but what chip do you have in SM4?
            Robert Jansson
            MRM-Racing

            Comment


            • #7
              not sure but ray hall supplied it 'set up' for the 2jz , i guess that means its loaded with the cal map and appropiate chip ?

              Comment


              • #8
                Trigger accuracy - more pulses than cylinders

                There are a few other things that need to be considered before everybody starts fitting multiple extra teeth to all their cranks.

                All Autronic ECUs do a Load measurement at the same time as the Cylinder Pulse that occurs at the Cylinder I/P Lead angle. When the Trigger Events / Cycle value is the same as the number of engine cylinders (a normal configuration) then a Load measurement is taken at the same relative point on every cylinder. For example if the Cylinder I/P Lead angle is 60 deg then a Load measurement will be taken at 60deg BTDC on every cylinder. This means the Load value will be most stable because manifold reversion pulses, etc will also be occuring at the same relative times on each reading.

                If you increase the number of Trigger Events / Cycle then the ECU will also be taking additional Load measurements. If you double the Trigger Events / cycle there will be two Load measurements per cylinder NOT at the same relative positions. If you have a 4 cyl engine with 60deg Cyl I/P Lead there will be alternating Load measurements at 60deg and 150deg BTDC effectively on each cylinder. If there is a significant reversion effect in the manifolding this may appear as an unstable Load reading (which some other ECU's seem to have as standard). This will also affect Manifold Rate fuel trims if the cal is using that in a Trim table.

                Extra Trigger Events / Cycle could also have a similar effect on Ignition timing. A lot of people are aware that if you set a flat ignition table and watch the timing with a timing light when you suddenly accelerate the engine with no load you will see an Autronic retard the first pulse and then be correct for the rest. This is with one pulse per cylinder. If you do the same test with other ECU's and one pulse per cylinder you will see the timing retarded the WHOLE time the engine is accelerating. If you fit three pulses / cylinder to these engines for the other ECU's the retard effect nearly goes away.

                The Autronic has a very good predictive timing algorithim that works with one pulse / cylinder. Engines do not run continuously smoothly even though they appear to. They slow down on compression strokes and accelerate on combustion strokes. Introducing extra Trigger Events / Cycle could destabilise the predictive timing in a similar way to the Load calculation. Using a camshaft driven Cylinder pulse has the same effect (seen as timing scatter). All Autronic CPU's up to SM4 v1.05 (& all SMC /SM2) do the prediction the same way.

                For RAC with v1.02, if you increase you trigger Events / Cycle but are not using all four ignition outputs you can set one of the unused outputs as a tacho pulse with the correct number of pulses

                Comment


                • #9
                  Thats some really nice info there Ian. Whats the difference from 1.06 on compaired to the 1.05 and older chips?

                  By the sounds of that i'll leave it as it is. I would of only been doing it for fun anyway. Do you know how the Autronic deals with the early subaru trigger setup, i take it they would only use two of the 6 teeth?

                  Comment


                  • #10
                    Originally posted by Ian View Post
                    For RAC with v1.02, if you increase you trigger Events / Cycle but are not using all four ignition outputs you can set one of the unused outputs as a tacho pulse with the correct number of pulses
                    Thats really neat that. I just had a play with a costom ignition setup and never knew it would be that easy. You could use that for lots of interesting things.

                    Comment


                    • #11
                      For the purpose of this thread the v1.06 and onwards have an additional parameter called the Digital Angle Filter. When set to Fast it operates the same as earlier SM4 & SMC / SM2 CPU's. When set to Slow the prediction is averaged over more Cylinder Pulses to minimise scatter.

                      Also when using dwell the v1.06+ has a wider tolerance than v1.00-v1.05 before applying dwell extension. It is a complex scenario because the dwell pulse may be initiated several Cylinder pulses before the timing point to achieve the desired dwell time. If the engine speed increases suddenly (after initiation of the dwell pulse) and the output is triggered at the correct position the dwell pulse will be shorter than planned. Depending on the coil minimum dwell requirements the short pulse may undercharge the coil resulting in a missfire. Under certain circumstances the SM4 (and the old dwell boards) will extend the dwell to a minimum value to ensure a spark, which will be slightly retarded, rather than risk a spark missfire. In pre-v1.06 CPU's the tolerance was found to be a bit conservative so it (the tolerance) has been widened in v1.06 and later to achieve better timing accuracy when accelerating.

                      For the 6 tooth Subaru crank the Autronic uses the 65deg teeth for all timing. The Subaru uses the 10deg teeth for cranking position. When cranking it turns the dwell on with the 65deg tooth and off with the 10deg tooth. Maybe the Subaru ECU couldn't calculate the engine position accurately at cranking so it needed an extra tooth at the cranking ignition point. That then raises the question of ECU's that require cylinder pulses at the cranking position - how good are they if they need a tooth there?

                      Comment


                      • #12
                        Nice posts Ian, this is the information we like to see here!
                        Robert Jansson
                        MRM-Racing

                        Comment


                        • #13
                          Originally posted by Ian View Post
                          There are a few other things that need to be considered before everybody starts fitting multiple extra teeth to all their cranks.

                          All Autronic ECUs do a Load measurement at the same time as the Cylinder Pulse that occurs at the Cylinder I/P Lead angle. When the Trigger Events / Cycle value is the same as the number of engine cylinders (a normal configuration) then a Load measurement is taken at the same relative point on every cylinder. For example if the Cylinder I/P Lead angle is 60 deg then a Load measurement will be taken at 60deg BTDC on every cylinder. This means the Load value will be most stable because manifold reversion pulses, etc will also be occuring at the same relative times on each reading.

                          If you increase the number of Trigger Events / Cycle then the ECU will also be taking additional Load measurements. If you double the Trigger Events / cycle there will be two Load measurements per cylinder NOT at the same relative positions. If you have a 4 cyl engine with 60deg Cyl I/P Lead there will be alternating Load measurements at 60deg and 150deg BTDC effectively on each cylinder. If there is a significant reversion effect in the manifolding this may appear as an unstable Load reading (which some other ECU's seem to have as standard). This will also affect Manifold Rate fuel trims if the cal is using that in a Trim table.
                          So on my 6-cyl motronic engine with trigger events set to 4 (have no spare ign O/P since I use the mux) it will read the load 4 times each fully cycle. Meaning the measurements will not be equal to all cylinders.

                          One other customer to me with a bike was recomended 8 teeth at crank for good accuaracy, and now I understand why he has so unstable readings that he asked for a filter on the load. Now he has two teeth and works fine.
                          To solve this problems with tacho the way to go is a add scalable tacho o/p on the SM4 (if you want proper load meassurments).
                          Will ad that to my wish list.
                          Robert Jansson
                          MRM-Racing

                          Comment


                          • #14
                            I respectfully defer to me' learned colleague, as usual.

                            Chris
                            Last edited by Chris; February 9th, 2008, 04:55 PM.
                            He gets his lap dogs to do his talking for him. Buzzard is a good example.

                            Comment


                            • #15
                              Originally posted by MRMRacing View Post
                              So on my 6-cyl motronic engine with trigger events set to 4 (have no spare ign O/P since I use the mux) it will read the load 4 times each fully cycle. Meaning the measurements will not be equal to all cylinders.
                              That is correct, as you have used that 4th ignition output there is no easy fix. Apart from either recalibrating the tacho, going to waste spark or making a 3-2 pull-down circuit the only other thing I can think of is to put a 2-tooth pulse generator on the crank or a 4-tooth one on the cam.

                              Is there not a version of the Ford V10 that has 36-1 on the crank which means you have to set it to a non-matching pulse count?
                              Last edited by Ian; February 9th, 2008, 02:43 PM.

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