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Performance Tuning – part 3, Ignition

By now, your motorcycle would be having high compression, good port velocities, ideal exhaust path, and the perfect charge going in. But even air compressors do just that. Let us wean ourselves from them boring machines. Let’s have a blast!

Logically, an engine should compress the charge, and then when the piston reaches its highest position, the plug should fire and down she goes. That means exactly at TDC. But the speeds at which this happens is so fast, that the seemingly small fraction of time that it takes to have that ‘blast’, is significant as compared to the time taken for the piston to move from one extreme end to another. So spark has to fire a bit before the TDC so that by the time piston starts its downward journey, the explosion is at its peak. Therefore, we now move into the world of ‘ignition timing’.

Ignition Timing understandably means the timing of the spark. This usually is measured in BTDC (Before Top Dead Centre) in the following two ways.

Distance BTDC: It means the distance of the pistons top-land from the TDC when the spark fires, when the piston is making its way up. This is usually denoted in millimetres.

Angle BTDC: It means the degrees of rotation of the crank before the piston hits TDC.

‘Advance’ and ‘Retard’ are two commonplace terms used to describe the process of firing earlier or later. The most important factor amongst all factors determining ignition timing is rpm.

An advanced timing will help your motorcycle start very easily but will not attain high rpm, and will have a nominal drop in fuel consumption. A retarded timing will not start easily and is more suited to higher rpm. Engine will ‘knock’ at low rpm.

How do we set the ignition timing?

Old motorcycles simply had an advance/retard lever on the handlebar that allowed you to retard the ignition as you gained rpm, or advance when you hit a slope. But now we can’t have that on a performance motorcycle. So we now have two types of ignition timing:

Static Ignition timing: No matter what the rpm, the engine fires exactly at the same BTDC. What we need to do is to find the optimal compromise. If you are doing a top-speed run, then keep it a bit more retarded, or a bit more advanced for better pick-up. And when I say a bit, I mean a bit. These alterations rarely go beyond 2mm changes. Both CB-Points system and CDI systems have this type of ignition.

Dynamic Ignition Timing: This system keeps you in a convenient advance when you start the ignition and adjusts the advance as you gain rpm. Obviously, it is a better option than the static system.

The number of steps in which timing changes depends upon the ignition kit. Usually street bikes have a 3-step timing correction process. A good performance ignition kit can do wonders to your motorcycle’s abilities to make very useable and enjoyable power throughout the rpm range.

Again, both CB-Points and CDI system have a dynamic curve, the CDI system uses a little computer to take care of the whole thing, whereas the CB-Points system uses a crude but effective centrifugal advance-retard system. The rpm spins something inside that adjusts the advance the ignition real-time, not in steps. Some CDI kits are programmable so you can simply alter the ignition curve as you please on your computer.

Spark Plugs: There are numerous types of spark plugs which help the engine make better use of the charge available to burn. Some spark plugs make the engine run cooler, some help it heat up to optimum temperature faster, some increase the combustion efficiency by producing fatter/stronger sparks, some make two sparks, etc.

But performance-tuning wise all you can do is select the right plug and then set the plug gap correct, since it also contributes to ignition timing. A longer gap will retard the ignition, albeit slightly.

Spark plugs help not only in enhancing performance but once the art of ‘reading’ a plug is mastered, one can determine how and where corrections are needed in various places such as the ignition timing, fuel mixture, compression, health of piston rings, operating temperature, etc.

So now your motorcycle is firing away to glory, making a lot of power. Let’s move onto getting all those horses from the engine to the road, which we will discuss in the next part of performance tuning for motorcycles.

Performance Tuning Part 1
Performance Tuning – part 2, Fluid Flow
Performance Tuning – part 4, Power Transmission

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