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Canadian Daves JustKDX
Ignition Timing Question (long)
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[QUOTE="Rich Rohrich, post: 95864, member: 16241"] If the timing was substantially retarded initially there is no doubt you could effect a fairly dramatic change by advancing it. Overly retarded timing will tend to raise the exhaust gas temperature and change the pipe tuning to a higher rpm causing an even more pronounced "hit" during transition and a general loss of bottom end response. Given the rather poor combustion quality of two-stroke engines at low rpm and part throttle, running too little timing can be pretty obvious. As everyone has pointed out, it's clearly a double-edged sword when you advance the timing. How far you can go advanced has a lot to do with the loads, heat, and rpm range the engine will see. An engine that sees mostly part throttle , moderate load and rpm duty can run a lot more initial timing advance than a engine that sees extended high load part throttle or WOT operation. The basic idea behind ignition advance on any conventional piston engine is to try and time the peak cylinder pressure to occur so as to apply the pressure on the descending piston in the most mechanically efficient way. This tends to be in the range of 15-20 degrees ATDC. As rpm increases and less time is available for charge burning to take place the engine will need to fire earlier (more advance) to account for the diminished time. The overall idea is to use the minimum timing necessary to obtain the best possible torque. Higher engines speeds reduce the overall burn time by virtue of additional turbulence in the combustion chamber, less time available to lose heat to the surrounding chamber surfaces, higher flame speeds, and increased mixture density. In four-stroke engines with narrow valve included angles, and efficient combustion chambers these factors can more than offset the reduced time available for combustion to take place, and retarding the timing will actually produce more torque. We are starting to see newer four-strokes take advantage of this fact and using more advanced digital ignitions that have the ability to retard the timing at the appropriate time. This is a VERY general explanation of the four-stroke side of things. Two-strokes tend to have fairly specific reasons to retard the timing at high speeds. Including the above points, we can add, more efficient scavenging at high speeds which improves mixture density and speeds combustion, plus the mixture density as a whole is much more dense than on a four-stroke because of crankcase compression. Dense mixtures burn considerably faster, so less timing is needed. Once the engine is operating in the range of the pipe tuning, the volumetric efficiency will be at it's highest which will obviously be the point where maximum cylinder filling occurs. Again this will improve the mixture density and speed things up, so less timing is needed. In a two-stroke race engine this tends occur at high rpm Two-stroke exhaust valve equipped engines will have higher dynamic compression ratios at high rpm which will also help reduce the burn time. But probably the most important reason for retarding the high speed timing on a two-stroke is to use some of the available combustion heat to manipulate the pipe wave timing and improve the scavenging/ram tuning. Less efficient combustion tends to raise exhaust temperatures, so they are effictively trading off some thermal/combustion efficiency to improve the scavenging and increase the delivery ratio of fresh charge. Like most things in engine design, it's a tradeoff based primarliy on the intended use of the engine. More radical two-stroke road race designs like the TZ and RS motors have fairly narrow power bands, so they use these effects to maximum advantage. Add to all this the basic simplicity of a two-stroke combustion chamber, which by it's very nature will provide a fairly fast burn rate and it's pretty apparent that there are probably more similarities than differences between the 2 engine types , and they have nothing to do with the physics involved. It's more a case of the differences in the basic environments. This probably seems more complicated than it really is. Ultimately if you decide to vary (advanced or retarded) from the stock spec on initial timing you'll need to keep an eye out for excessive heat as well as signs of spark knock. The sprakplug has a remarkable ability to clue you in on these things. Anything stronger than trace knock will tend to show up on the surface of the plug's nose, and excessive timing has a nice consistent habit of overheating the center electrode and ground strap. Blueing and scaling are sure signs that there is more timing than the engine really needs. IT's worth experimenting, but take small steps and it's advisable to check the stock timing marks against the manufacturer's spec for piston positon. The procedure is listed in most service manuals. Good luck. [/QUOTE]
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Canadian Daves JustKDX
Ignition Timing Question (long)
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