So wondering about ignition timing.
Long back before getting the RD i was absolutely not bothered abt any such thing called ignition or its timing. I remember once my uncle took my shogun for a ride and returned saying the timing is not proper. I didnt have a clue and responded with a dumb nod. My uncle was bright enough to pick up that i had zero clue and wisely refrained from trying to drill heavy stuff in.
Well with the RD in the scene, it was a different story altogether.
Very soon the bike began to misfire and act strangely. Especially when it got wet. The mech announced that the points are screwed. I cautiosly enquired abt these 'points' and got a lengthy lecture on points-based-ignitionsystems.
And soon i got sick of those infernal points and the mech suggested changing to CDI. now what is CDI, i wondered. So out came another lecture. Cut it short to say that within the first month of getting along with the RD, i became aware of where the spark comes from and that we have points and CDI systems.
Well even then i had no clue abt ignition timing; Enlightenment dawned when later on the bike began to loose power. One fine day the mech opens the magnet cover, fiddles with a strange plate hidden beneath a rotating magnet and i take it for a ride and find that man there is great boost.
I was told that he changed the ignition timing. So it was time to read up on what it is.
There are lots and lots of nice links on the theory of ignition timing. Just google.
To understand Ignition timing, a fundamental point needs to be noted - Combustion is not an instantaneous process.
I used to think that the moment the spark fires, thecompressed mixture used to go wham and smack the piston. Not so. There is a finite amount of time taken for the combustion to happen fully and the push be delivered to the piston face. This is the base of the concept of ignition timing.
So what does this mean. The piston comes up. It compresses the mixture. The piston reached the top dead center, TDC. Now the mixture is fully compressed. Suppose the spark fires now. The compressed mixture begins to ignite. But - the piston is on its way down now. By the time the mixture combusts fully and hits the piston face, the piston would already be on its way down. This translates directly to a weaker push on the piston and a loss of power.
Have you ever punched anyone on the face. Just imagine what would happen if you are throwing a straight right with all your force and the guy jerks his head back, fast. When you land, the nose is moving backwards fast and you connect with much less effect than you hoped for.
So what would you do. You know that the guy is gonna move his head back. Yeah there is only one way. Throw the punch early. So that when the nose begins to move back your punch is on the way and is almost on it. You land nicely and break the bone. Full contact.
So its the same case out here, you need to fire the spark early. That is depending on the piston speed, you will need to fire the spark at a certain point where the piston has not yet reached TDC. So by the time piston reached TDC, combustion has already kicked off and just when the piston begins to leave TDC, it gets punched right on the nose.
The image shows the "ignition delay". Notice how the pressure in the chamber
rises when the combustion kicks off and nears completion.
But this presents problems when you put this idea on a bike. The ignition system has to be effective across the revv range, that is throughout the piston speed range. At lower revvs, the piston is moving slowly and you need to fire the spark only just before the piston reached TDC. There is enough time for the combustion to complete by the time it leaves TDC.
But if its moving much faster, this wont be enough; Fire the spark just before TDC and the piston would have reached TDC and left in a flash, before the combustion threw the punch. So it needs to be even more early.
There are two choices here - Either modify the ignition to vary the timing according to the rpm; Or choose an ignition timing that is 'optimum' - something that does not do much damage throughout the revv range.
Now when we spark the plug before the piston reached TDC, we say that the timing is Advanced. Basically moving the spark timing away from TDC is called Advancing and moving it towards TDC is called Retarding. Makes sense since a retarded timing is something thats too late and ineffective. Similar to we calling someone retarded if he is far too late in picking up stuff.
Looking at this from the RD perspective: The stock US spec RD has an ignition timing of 1.8 BTDC. BTDC stand for Before Top Dead Center. This means that the spark appears when the piston is moving towards TDC and has to travel 1.8mm more to reach there. For the Indian HT, the timing is 2mm BTDC. And for the Indian LT its 2.4mm BTDC.
So in the case of RDs, optimum static timings are chosen which ensures a smooth average performace throughout the revv band.
Fiddle around with the timing and the effects are interesting to observe. Start with advance; Take an LT RD and advance it a bit beyond 2.4mm. say 2.6mm. Now kickstart the bike and immediately one thing comes to notice - You need much a much more fuller kick than before; A light tap on the kicker wont do. You have to kick the thing all the way down and get the piston up to a much higher speed than ever before it starts. But makes sense. At those low revvs, the piston moves so slowly - Yet you are pushing in the spark much before the piston reaches TDC. In effect even before the piston reaches TDC - maybe just just before it - it gets whacked and sent back. So the whole system is ineffective unless you get the piston to move much faster by kicking it full and hard so that it beats the spark to the TDC and receives the slap when its either at TDC or on its way down.
Once the bike starts up, notice the idling. It would be very erratic and untidy. Take a ride and you can see that the low end has become totally unpleasant. There seems to be a total lack of bite and co-ordination at low rpms. But get on the throttle and get those revvs up and then everything seems to match up and the bike gets going.
Advance it a little more from here and when you kick it you can feel the kicker bouncing back - why is this. Here the spark is much too early and the combustion happens and is on the piston much before it has reached TDC - In other words the piston has to move upwards against negative pressure and may very well fail at it and quit and start moving backwards. Which sends the crank rotating in the reverse direction which is felt as a back-thrust by the kicker.
Similar troubles can be observed with the other extreme case - Heavy retard. Make the spark too late and then the piston might be on its way down when the combustion completes. The flame front shoots down chasing the piston deparately - the piston may beat it to the exhaust port and the combustion jumps out of the exhaust port window instead of slamming into the piston face. Else it might catch the piston but succeed only in delivering a weak blow.
From my experiences with the RD, i feel that a slight, ever so slight, retard helps the low end a lot - The bike seems to get some real meat at low revvs. I found it especially useful during the runin period. But this is kind of controversial since most of the people tended to disagree with this approach.
But with a slight retard, i notice that the kicker is much more friendly; Just a small tap and the engine fires to life. Maybe a direct effect of the improved low end torque.
Extreme advance and Extreme retard, both are superbly effective techniques for screwing up an engine.
There is a point abt wornout engines. By worn out i mean an engine which has lost compression a lot. Now a heavily compressed mixture combusts faster; That is, flame propagates much quicker through a highly compressed mixture, as compared to a less compressed one. So in a worn out engine one can advance the timing a bit more - since the mixture that gets trapped is weakly compressed as compared to those good days. So one can afford more advance than stock as the weak mixture combusts slowly and can use the extra time.
This happened to me; My LT was slowly decaying and was losing power steadily. There came a time when it used to struggle to 110. Went to the mech and advanced the timing to a bit more than 2.4mm maybe 2.6mm. There was a wonderful difference in topend. Began to see those 130s easily, once more. But then i had no clue abt what was actually happening. So later when i rebored the engine i insisted on an advanced timing - i thought advancing the timing was the solution for more power. But the fresh engine just didnt take that sort of advance. And i was left pondering what the heck happened. Reverted to the old timing and it began to work beautifully once more...
So what does it appear to come out: That retard is good for low end, and once the revvs start going up it should be advanced more and more? Well this is a bit tricky, and there are apparently different factors at play here. This is the impression i had much earlier - that more you advance, the better its at higher revvs. But after talking with experts like ron, my whole idea has changed.
My idea now is that - Retard is good for low end, yes. Then advance is good for midrange. And beyond that you need to go less advanced. Ie, the timing curve should be like:
* initially retard (close to TDC, but still BTDC) - say till 3000rpm
* then it goes to advanced (further away from TDC) - say till 6000rpm
* once its past the midrage - after 6000, it drops to retard (comes back closer to TDC)
One phenomenon which supports retard at high revvs is : More of retard does the job of transferring more of heat to the pipes; This speeds up the gas velocity in the pipes, effectively reducing the tuned length - this goes along very well with high rpms.
Generally people tend to associate advanced timing with more power; I made the same mistake. But its not 100% false since theoretically there should be a slight increase of torque towards midrange-high rpm and mathematically this should result in a slightly higher maximum bhp value.
One thing i used to notice about retard is that it used to make the bike much more smoother and gentle; It seemed quite the opposite with advanced - always felt that the bike was very rough erratic and difficult to control.
Living more with your bike, you experiment with different timings and just settle
down to that which works best for you. For me i always had a softcorner for
2.2mm BTDC for my LT - though Yamaha recommend 2.4mm BTDC.