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detonation on 2T

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  • detonation on 2T

    DETONATION ?

    Interesting point made:

    STOPPING THE SHOW

    Anything that contributes to lowering the temperature that the endgas reaches will make detonation less likely. Anything that slows the process of conversion from normal gasoline into a sensitive explosive, will make detonation less likely. Anything that speeds up combustion, so that is it completed before the conditions needed for detonation can develop fully, will make deto less likely.

    Therefore the following will work;

    (1) Lower intake temperature

    (2) Lower throttle position, lower volumetric efficiency, or reduced turbo boost the less mixture that enters the cylinder, the less it is heated by compression.

    (3) Lower intake pipe, crankcase, and/or cylinder, piston, or head temperatures. This year's Yamaha 250cc road race engine, for instance, has a copper cylinder head insert to conduct combustion heat away faster, resulting in a lower combustion chamber surface temperature.

    (4) Lower compression ratio. The less you squeeze it, the less it is heated.

    (5) A more breakdown resistant fuel, such as toluene or isooctane. If straight chain molecules are not present, the fuel will not be broken down so rapidly by preflame reactions.

    (6) A negative catalyst something that will either pin down active radicals or convert them into something harmless. Tetraethyl lead, MMT, or other antiknock compounds are the medicine.

    (7) Retarded timing shortens the time during which proknock reactions can take place.

    (8) Incylinder turbulence or anything else that will speed up combustion (faster burning fuel such as benzene). This works by completing combustion before the time bomb of preflame reactions cooks long enough to cause autoignition.

    (9) Higher engine rpm This simply shortens the time during which the mixture is held at high temp. In Honda experiments in the 1960's, they found that an engine's octane requirements began to decrease steadily over 12,000 rpm, and were under 60 octane up near 20,000. In a more accessible example, note that engines knock when they are "lugged" run at low rpm, wide open throttle and stop knocking promptly when you shift down a gear and let the engine rev up more. This stops deto by not allowing enough time for the reactions that cause it.

    (10) Redesigning troublesome exhaust pipes. Some pipes give great numbers on the dyno, but can't be used because they cause seizures. They either simply overcharge the engine in some narrow rpm band (pushing it into detonation just as too much turbo boost would do), or back pump mixture from the header pipe that has picked up too much heat (this is why nobody heat wraps header pipes anymore).

    (11) Avoiding excessive backpressure. Exhaust pipes always create back pressure, but the more there is, the higher the fraction of hot exhaust gas that will be unable to leave the cylinder during exhaust. Its heat, added to the fresh charge that next enters the cylinder, may push the engine over the line into detonation. Sometimes a one or two millimeter reduction in tailpipe ID will get you a couple of extra horsepower, but it may also push enough extra heat into the charge to make the engine detonate after a few seconds.

    The number of ways of playing footsie with detonation is endless, but nothing works every time. This guarantees that we will never be bored, and will never run out of seized pistons.

    So, ceramic coating on 2T pipes maybe bad?

    Sure, on a 4 stroke you're keeping the gas hot, and accelerating it out the pipe.

    On 2T, you're messing with the local speed of sound (travels faster in hot air) - thus altering the rpm range your expansion chamber is going to work at, and maybe sending hot(ter) gases back into your combustion chamber... causing more likely detonation.
    Last edited by thro; 24-08-2009, 10:28 AM.
    “Crashing is shit for you, shit for the bike, shit for the mechanics and shit for the set-up,” Checa told me a while back. “It’s a signal that you are heading in the wrong direction. You want to win but crashing is the opposite. It’s like being in France when you want to go to England and when you crash you go to Spain. That way you’ll never get to England!” -- Carlos Checa

  • #2
    The choice of material also makes a difference - Stainless, Mild steel, titanium etc..etc..

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    • #3
      Detonation is when the explosion happens before the piston is at top dead centre; the two forces meet and clash: Not good!!ever!!
      Seizure is a lack of lubrication or excess of heat (expansion of piston skirt/top.
      Fuel has a cooling function as well and keeps the ignition top temp somewhere in the range that is required.
      Fuel/air ratio together with the RAD factor.

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      • #4
        Originally posted by nath View Post
        The choice of material also makes a difference - Stainless, Mild steel, titanium etc..etc..

        No doubt.

        I guess the point I'm making though is that taking a pipe design that was made for stainless or mild steel, then ceramic coating it no doubt alters its characteristics (or undoes much of the tuning the original pipe manufacturer did - assuming they actually did any of course).

        So, that finely tuned pipe you just bought and then wrapped/coated is no longer finely tuned, and could end up sending excessively hot gases back into your cylinder (because heat that would normally be lost is still contained inside your nicely insulated pipe).

        Food for thought I guess.... at the very least you'd want to carefully re-jet...
        “Crashing is shit for you, shit for the bike, shit for the mechanics and shit for the set-up,” Checa told me a while back. “It’s a signal that you are heading in the wrong direction. You want to win but crashing is the opposite. It’s like being in France when you want to go to England and when you crash you go to Spain. That way you’ll never get to England!” -- Carlos Checa

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        • #5
          I think you would have some serious trouble trying to jet an issue like that out.

          How could you possibly re-jet for a bad scavenging charge without adverse affect ?

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          • #6
            ^^ you probably won't effectively, but you'll maybe(?) keep the motor together.

            I was thinking more trying to keep the cylinder temp down (via richer a/f ratio) to compensate for the hotter charge getting blown back from your expansion chamber...


            No idea if you could effectively deal with it... just that keeping the pipes hot on 2t probably isn't such a smart move when using expansion chambers - in contrast with a 4 stroke, where you don't have the expansion chamber issues to deal with....
            Last edited by thro; 24-08-2009, 03:47 PM.
            “Crashing is shit for you, shit for the bike, shit for the mechanics and shit for the set-up,” Checa told me a while back. “It’s a signal that you are heading in the wrong direction. You want to win but crashing is the opposite. It’s like being in France when you want to go to England and when you crash you go to Spain. That way you’ll never get to England!” -- Carlos Checa

            Comment


            • #7
              more on detonation, squish gaps and overboring

              Ch 1 - Yamaha RD350 Tuning Manual

              The mixture in the squish gap will not burn at all in the initial combustion. Not only is it too far from the spark, but the flame will not travel along the squish gap because the metal of the piston and cylinder head is cool enough to extinguish the flame. When the piston descends and the squish gap widens, most of the mixture in it will still not burn because the piston is racing away from the burning mixture much faster than the flame can travel. Furthermore, it is too late in the combustion cycle for any combustion of this mixture to add much of its power to the cycle.
              With a stock squish gap of 1.9mm, about 18% of the mixture is in the squish gap rather than the combustion chamber, so 18% of the mixture is removed from the initial combustion process.
              We can get much of that 18% of the mixture back working for us by making the squish gap as narrow as possible. "As narrow as possible" does not mean zero ! There are two things which limit how far we can go. First, the gap gets narrower when the engine revs higher because the con-rods stretch a little and there is a bit of play in the bearings. So, if we set the gap to zero in a stationary engine, the piston would hit the head when we revved it. Secondly, if the gap is too narrow, the mixture cannot squish out into the combustion chamber quickly enough when the gap closes. This creates very high pressure, which causes harmful detonation - the explosion of the mixture at once. So, the optimum gap is a balancing act between these things. It is found by trial-and-error to be 0.75 to 0.9mm for the 350cc twins. At 0.9mm, around 11% of the mixture is in the squish gap. That's returned 7% of the mixture to the combustion process compared to the stock 1.9mm gap.
              Having said all this, that is not the whole story. Reducing the squish gap below the magic 1mm mark seems to perk up the engine disproportionately, for reasons which are not well understood. It also makes the engine less susceptible to detonation than increasing the compression ratio by other means. That just illustrates how little we really know about what happens inside the engine...
              So how does this cool the piston ? When the cool, unburnt, mixture is squeezed out of the squish gap by the rising piston, it plays over the piston surface and cools it. Not only does this improve reliability, but reducing the heat flowing through the piston also increases power: Heat flow through the piston increases the pressure of the incoming mixture under the piston, which reduces the amount of mixture that is drawn in.
              Reducing the squish gap also reduces the compressed volume, which increases compression. This actually optimises the compression ratio for the stock YPVS because Yamaha have left a good bit of slack here, too. There are two types of cylinder head on the YPVS models, marked Y1 and Y2. They are clearly marked on the top. The Y2 is the higher-compression head. With a squish gap of 1.9 mm even the 'high compression' Y2 head gives a lowly compression ratio of 6.0:1 (measured from the top of the exhaust port). With a squish gap of 0.9mm the compression ratio rises to 7.1:1, which is actually a pretty good ratio for road use with ordinary petrol. As a rule of thumb, you can push the compression ratio to perhaps 7.7:1 on this engine with ordinary petrol, and retarded ignition, before detonation happens. However, 7.7:1 is not a hard and fast rule, and 7.5:1 is probably a better safe maximum.
              The increase in compression can get a bit hairy when the engine has been overbored, which increases the compression ratio. For instance, with a 1 mm overbore and taking the squish gap right down to 0.75 mm, the compression ratio will rise to 7.5:1, which some would consider to be right on the limit for use with normal petrol. If you fall into this area, you might want to consider raising the exhaust port to 26mm from the stock 27.5 mm purely to lower the compression, rather than for other performance considerations. That would lower the compression from 7.5:1 to 7.2:1 in the above examnple, back into the safe area.
              Originally posted by Bendito
              If we get to a stop and we are missing a dozen bikes and you are last, it was your fault. Don't be that guy. No one likes that guy.

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              • #8
                One needs unburned fuel to excist in the expelled gasses due to the design of how it works.(scaveging effect, the return of unburned fuel into the combustion chamber and thereby cooling the exhaust crown part of the piston and overall mixture stopping predetonation and running a lean mixture)
                This is all part of the much sought after knowledge part called "the black art of tuning"

                *Jan Thiel was DA King together with some very knowledgable faceless Orientals and the famous spy deflector Hans Deegner from Easern block countries*

                fuel is a cooling agent and the correct temprature and gas mixture has to be achieved.
                this is another reason to warm up your stroker propperly before screeming off

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