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    IGNITION: What does ignition timing really mean?

    What is ignition advance and why do you need it?

    Ignition advance is how long before a cylinder hits TDC the spark plug is fired. This is essential because gasoline does not burn instantaneously (obviously) and the flame front takes a little while to engulf the air/fuel charge.

    If the plugs were fired right at top dead center, by the time the charge was combusted (if it ever would get fully combusted) the piston would be so far down the cylinder that no torque would be produced. Generally, one should target maximum cylinder pressure for about 10-20 degrees ATDC, like in this image:

    Notice what happens with the cylinder pressure in the case of detonation. At TDC, the pressure is almost as high as the peak pressure of the high power pressure curve! Now imagine standing on top of a piston at TDC (work with me here) and jumping as hard on it as possible. It's not going anywhere, because the rod and crank are pointing straight up. Instead, BITS of the pistons and valves and other parts are going somewhere. This is why diesel engines have to be built so tough, to withstand this pressure.

    That's why igniting too early is bad. Igniting too late however, and you'll produce jack squat for torque, so it's tough. There is a definite sweet spot.

    For all the rpm range and load range, maximum efficiency (and thus power, as well as fuel economy) is made by targetting the cylinder pressure to be maximized in this range. However, different loads and rpms change when you have to start the combustion.

    At low load (meaning high vacuum) engines need more advance because at low pressure, the flame speed of gasoline is slowed down (although I'm not entirely sure by how much). At high load (meaning when the engine's running near atmospheric pressures, or in boost on turbocharged cars), the flame speed is increased, especially at pressures higher than atmospheric. This is the reason why engines need less advance at high load (and why you should/need to retard ignition when you boost).

    At low rpm (like idle), engines do not need much ignition advance, for instance at idle (low MAP/rpm) 10 degrees is generally sufficient (and is the industry standard, I believe). This again places the maximum cylinder pressure at just the right spot, and should provide enough torque to keep the engine going. If any of you have ever adjusted your distributor for more advance (as I'm sure most of you have, actually), and if you've noticed that the idle is a little smoother as well, or the idle speed has increased, even, that is because the cylinder pressure peak has been moved closer towards the sweet spot, and the engine is producing more torque at idle (mind you, without using more fuel! ignition timing is exceedingly important for making the most power and best fuel efficiency!)

    As rpm increases however, fuel does not burn any quicker, or if it does it has a relatively negligible effect. However, the engine is spinning faster, so for the combustion pressure peak to hit the sweet spot, you need more advance! Take another look at the image I posted above however. Then squash the scale's x-axis, keeping the pressure curve centered above the sweet spot still. You'll notice in your mental image "hey, now that the engine's going faster but the combustion isn't, that curve for the cylinder pressure is starting to creep up along the TDC line!". This is why at higher rpm, you really have to watch yourself with ignition timing. Too much advance and you'll blow your engine up real quick at high load and rpm. It's also easy to have too little advance, which means you're losing out on torque.

    Luckily, there has been extensive dyno tuning of BPs over the years, and there is quite a bit of information out there about optimal timing maps for our engine (and for non-BP guys, I'm sure there's information out there for you too).

    Anyway, for a small engine like the BP, 28-32 degrees of maximum advance should be fine. Any more than this is unnecessary and probably will reduce your engine's efficiency. For boosted engines, ignition timing should be watched quite carefully, but there's almost no reason to go below 20 degrees of advance (probably 16 degrees ABSOLUTE minimum, although that's pretty conservative) even with a full BAR of boost or more. If you're having trouble with detonation at this amount of advance, retarding the spark or dumping more fuel in is NOT the answer. Most likely you have a hot spot on your engine, or in the combustion chambers.


    This is the ignition timing map that I am currently running in my own car. It is based off of Flyin Miata's dyno-tuned Link setup for Miatas, with changes to the cranking areas of the map as well as setting a flat idle advance of 12 degrees. This timing map runs perfectly happily on my 12:1 compression engine. A slightly modified version of this map is being used in TheRielDeal's car (I believe) with MAP bins for boost. I think we set his car to drop to a minimum of 20 degrees advance and it's been running perfectly with no knock.
    Ask me about engine management!

    #2
    Matt, let's say the cylinder pressures are doubled above your figure of 1 BAR (so we are running 30psi). Do you know the kind of curve or equation we should use to determine what the maximum advance should be.

    Comment


      #3
      as in you are running 2 BAR of boost?

      I'm not entirely sure, as I have yet to find a graph showing the flame speed of gasoline for pressurized manifolds. I believe that the flame speed is fairly linear as you increase MAP from hard vacuum to atmospheric, and when I made therieldeal's ignition map I kept it fairly linear as well.

      I think I set it to 4 degrees retardation per BAR of boost (which is about 1 degree per 3.7 pounds of boost). This seems to work fine for him.

      As far as actual physical models? I'm not sure. I'll look into it.
      Ask me about engine management!

      Comment


        #4
        Is there any scientific reason you used 1 degree for 3.7 pounds of boost, or did you base this on the linear nature of the vacuum side of the map?

        Comment


          #5
          I just based it off of the nature of the ignition map in the vacuum portion. It seems to work properly.

          Again, I'll try and get more information tomorrow about the flame speed of gasoline in pressurized conditions. That should be able to show how much less advance you'd need.
          Ask me about engine management!

          Comment


            #6
            A great read as well

            Comment


              #7
              Also it is worth mentioning what affects timing aside from spark timing itself:

              - temperature (hotter means faster combustion)
              - pressure/density (more equals faster flame)
              - A/F ratio (the leaner or richer you get off 12.5 AFR the slower the flame will propagate)

              Comment


                #8
                Another good read:

                Comment


                  #9
                  What would the preferred timing advance for a boosted na BP. Thats a BP head, and internals. With the BPT mannys, cams, and vj20 with the BP26 ecu
                  Mx3 RS,BP-turbo
                  http://www.cardomain.com/ride/2628805

                  Comment


                    #10
                    It is difficult to pinpoint a particular ignition curve because there are so many factors to consider and this may even vary from one motor to another. The best thing to do is if you have full control of ignition timing - take it to the dyno, get a knock sensor, and up ignition until you get knock, then back it off a degree or 2.

                    Comment


                      #11
                      I'm not sure personally that that's the best way to do it, although it's a method that a lot of books advocate. In my experience though, by the time you're knocking, you're well beyond the max power point (more than 2 degrees). Also, all that knocking is a bad idea, even if it's subtle.

                      Still a good idea to get a knock sensor in case you get a bad tank of gas, but you shouldn't tune based off of it. Also, at high rpm, it can sometimes be hard to make out whether a noise was knock or not. Most knock detection circuits stop going off after about 3000 rpm because really, they can't tell either. (Knock sensors are really just microphones)

                      A tune with 30 degrees max advance, 20 degrees minimum at max boost should start you off well, then take it to a dyno and do a few runs, increasing the advance at high load by one or two degrees each time, until you see the power drop (letting the car cool down a bit in between runs to avoid heat soak). Then, drop the advance back a couple degrees. This method should do you fine.
                      Ask me about engine management!

                      Comment


                        #12
                        wow 20 degrees. I was thinking more like 16 but Ill see what she will do.
                        Mx3 RS,BP-turbo
                        http://www.cardomain.com/ride/2628805

                        Comment


                          #13
                          i've ran 16° but it likes 14° alot better under boost. so i've found.
                          16 degrees is alot smoother all around tho. mind you this is with the GTR intake manifold & VF12 hybrid not the stock BP system & VJ20. perhaps you could get away with more. idk just be careful with it.
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                          1992- project FE3..... 313 WHP @ 9.3psi




                          I pet my dash when I get into the car..."good car"
                          he actually has a mazda tree, parts grow on it

                          Comment


                            #14
                            Originally posted by Masterbanter View Post
                            wow 20 degrees. I was thinking more like 16 but Ill see what she will do.
                            he means 20 total advance under boost.

                            When you guys are adjusting timing at idle, you are bumping the entire map up. So, if your ECU is running 10 degrees at idle and 20 degrees under boost, when you turn it to 16 at idle, it will run 26 degrees under boost.

                            without a standalone you can't control ignition advance - you can only advance or retard the ENTIRE ignition map with the distributor which is just falsely telling the ECU the cam angle when you turn it.

                            So I don't think 20 degrees at idle will fly.

                            Comment


                              #15
                              Here is a low RPM timming ma off my haltech.
                              Thats all I'll give away.
                              Attached Files

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