Timing Part 1- Theory
Ignition timing is a frequently misunderstood and misinformed topic in the moped community. A basic understanding of the physical principles occurring in the combustion process, and the system by which the spark plug motivates this controlled combustion, will allow the tuner to adjust ignition timing to the optimal point in the combustion cycle for maximum power. This article is the first of two parts, detailing the theoretical role of ignition in the power cycle, and the implications various performance modifications have on optimal ignition timing.
To begin, lets look at the role of the spark in the combustion cycle. The ignition event occurs between the compression and power strokes. After fuel and air has been forced into a small volume and raised to a higher pressure, the spark ignites that volatile mixture and the resultant heat and pressure force the piston down to provide motive force (powahhh!!). This at the outset seems like a fairly simple proposition, but in reality there is quite a bit happening in the few milliseconds during which the piston approaches top dead center, and begins its decent.
In reality, considering the spark and combustion of fuel as a finite event (happening all at once), is an oversimplification. The piston is traveling at such a high speed, if you were to slow things down to a comprehensible speed, the movement of the flame itself through the combustible mixture in the cylinder, would seem to happen very slowly. This ‘flame front’ speed is based on the pressure of the mixture of gasses, which is changing as the piston nears top dead center, it is also based on the fuel/air ratio, and the temperature of the gasses in the cylinder.
Going back to the cylinder now, the piston is coming up from bottom dead center, the cylinder is full of air and fuel mixed. As the piston nears the top, the points will close, breaking the flow of current through them, and diverting the electricity to the spark plug. The point at which the points ‘break’ or open, is the point used to determine timing using a timing strobe, a test lamp, or a cigarette paper. There can be a little bit of dwell downstream in the system also, from 1-3 degrees. A hotter plug will spark more readily, also a smaller gap will jump faster, so if you change these, your timing might change incrementally.
When the spark jumps across the electrode gap, the mixture, which by now should be at a very high pressure, will ignite. If the fuel mix is correct, and the piston is still traveling upwards, the flame will burn up the fuel and air mixture in the cylinder just as the piston is passing top dead center (the most powerful combustion will occur when the pressure in the cylinder is highest- e.g. top dead center), and maximum power will be achieved as the pressure from the burning gasses pushes down the piston.
Combustion theory and implications:
Now perhaps it is a bit clearer why ignition timing is so important. If the timing is off by even the slightest fraction of a millimeter, peak power won’t be achieved. This is why many people will say ‘the power is in the points’. In fact there is no power whatsoever ‘in the points’ unless they are set wrong for your particular application. Understanding the various effects modifications can have on the ignition timing, will facilitate better ‘guesstimating’ as to the optimum setting of the points. The key is the relationship between the fuel, exhaust, and temperature conditions and their contribution to flame propagation.
Flame propagation in the cylinder is a relatively simple concept when we go back to thinking of the combustion in super-slow motion. Chemistry tells us that the rate of any chemical reaction is proportionate to the initial energy. In the engine, that energy manifests itself as temperature and pressure. We assume no change in temp. We started with an initial volume of fuel/air mix and compressed it by the compression ratio to get our final pressure. Compression ratio has a huge effect on final pressure, and reaction time- read flame propagation time, but its not the only factor.
The initial amount of fuel/air in the cylinder is called volumetric efficiency, the amount of the cylinder displacement that is filled when the piston closes the exhaust port. A more efficient intake and transfer tract, along with a pipe, or the timing can all change the volumetric efficiency. Anything that increases the VE, will make the flame propagate faster, and will require ignition retard. When a pipe comes ‘on’ the powerband, it is pumping more effectively, and increasing VE. Port timing also can contribute to varying VE at different points in the powerband.
The other major contributor to flame propagation is cylinder bore diameter. For the fuel to all be burned the flame must cross the entire surface of the bore. It might seem small, but going to a larger bore requires more time to burn the fuel.
There is a delicate balance when adding performance parts or modifying your bike. Everything has the potential to change the timing, and we have very little recourse with the simple points magneto system most stock bikes have. The original design is for a relatively constant powerband between 1000-3000 rpm. Wild variations in the efficiency of various parts at various speeds can require dramatically different timings.
Some commonly occurring problems with timing are pretty obvious, but the small, silent loss of power at various speeds that is frequently cited by the ‘power in the points’ crowd, is something that can only be avoided by painstaking tuning. In fact, points magneto systems aren’t nearly accurate or tuneable enough to deliver peak power at all ranges, but usually a happy medium can be found that allows for good performance across a wide powerband.
Should the spark jump too soon, a condition known as detonation occurs. This is frequently referred to as ‘knocking’ or ‘pinging’ and in a highly tuned moped engine sounds a lot like a tin can full of marbles being shook. It is confusing to troubleshoot because of its sibling condition, pre-ignition, which shows up in exactly the same way. Its hard to tell the difference because they both cause the same effect, the noise you are hearing is the fuel/air mixture in the cylinder igniting before the piston reaches the top of its stroke, so the upward movement of the piston is met with a downward explosion.
In detonation conditions, the spark is jumping too soon or the flame is propagating too quickly (same thing, relatively speaking). This is why some bikes with aggressive porting or exhausts require more ignition retard than stock, or some bikes need retarded at higher speeds. As the pipe hits or the ports work more effectively, the amount of air in the cylinder (volumetric efficiency) can increase with a supercharging effect, causing the flame to travel faster throughout the cylinder.
In pre-ignition, the condition is being caused by hot surfaces inside the engine due to the wrong heat range spark plug, high cylinder temps, or too low of octane fuel for the compression. While both conditions result in the same effect, the causes are dramatically different, and the solution is as well. Due to the extreme risk of engine damage these need to be understood and correctly diagnosed.
On the other side of the coin, backfiring and power loss are telltale signs of the timing being too retarded. If the combustion occurs as the piston is already heading back down, it doesn’t take much time at all for the burning gasses to blow right out the exhaust. If this happens, the combustion only acts on the piston for a brief instant, and the piston cant harness the burning fuel. Backfiring can occur in extreme cases where the flames either blow out the exhaust, or even back down into the crankcase when the transfer ports open. Typically in cases where the timing is only very slightly retarded, the bike will have poor or no throttle response, or loose throttle response under load, but maintain it when it is on the kickstand. When there isn’t anything to push, it can rev up, but once load is applied the short amount of time the gasses push on the piston before blowing out the exhaust, isn’t enough to overcome resistance. The transfer ports can even flow backwards or stall depending on the extent of porting or the pipe.
Timing need not be a mystery, or entirely guess and check. We are just starting to see the types of programmable CDI units that would allow us to dial in perfect timing, but in the meantime, the points magneto systems are perfectly workable, reliable, and offer a very simple, direct type of tuning. Having a good idea going into it about the effects other performance parts can have can facilitate easier tuning, and a better understanding of our bikes.