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by John Hildreth

The mystery of getting the burn to start in an engine's combustion chamber may be more challenging than you realize. From the first time you crank your bike for the day to the last time, the requirements for an efficient burn change with every situation.

The basics of combustion are pretty simple: For emissions and mileage, you need more spark advance and a leaner air/fuel ratio. For performance and acceleration, you need less advance and more fuel. After that, things really start to get complicated.

Cold starts require more fuel and a hotter spark. Once the engine comes up to operating temperature, fuel and the spark intensity needs are reduced. As the engine temperature increases, there is a decreased resistance for electricity travel, and it is easier for the spark to jump the gap; therefore, you can reduce spark intensity. Also, with increased temperature, the air and fuel becomes better atomized (similar to boiling water). Add to this the fact that higher RPMs benefit from heat retention in the spark plug, and the air/fuel movement in the cylinder increases the particles of fuel suspended in the air. All of these things make it easier to light the load, even though the time period of the combustion cycle is smaller.

Low RPM performance is one of the most challenging areas of efficient combustion. There are many plus and minuses to trying to get efficient combustion at low RPM. Low RPM performance requires increased spark and spark gap. This produces a larger flame kernel, which improves the first 10% of the burn. But low RPMs also require higher voltage which is much more corrosive to the spark plug electrode life. This makes spark plug replacement necessary much more frequently. In addition, sustaining the higher voltage is much harder to do at high RPMs. Finally, at low RPMs, you do not have the benefit of a lot of air/fuel movement in the combustion chamber. By increasing the air/fuel ratio, you get a better chance that a larger quantity of fuel will remain suspended in air, thereby helping ignition. The increased fuel requirements may not be of any concern if you're going down the drag strip where plugs are inspected and replaced after every run and fuel is replenished before each trip down the track. But if you're going to Sturgis, and plug fouling and the lack of mileage affects every tank of gas, this becomes more than just an irritation -- you may not even make it to the next gas station. So, an air/fuel ratio that delivers reasonable mileage and long plug life will also end up resulting in a loss of low-end performance.

A carbureted street bike with a correctly set air/fuel ratio should act a little cold-blooded when starting. It should misbehave somewhat when the choke is disengaged early in the warm-up stages. If it doesn't, you know your low speed ratio is too rich.

Achieving your performance goals require good choices based on the power band you desire. There are exceptions to everything in life, and the science of combustion probably has more exceptions than anything else. Your motor has to deal with hundreds of variations and still perform its assigned task regularly and efficiently. While most of us only deal with gasoline (or maybe gasohol), just having these choices adds an additional number of factors that affect the combustion process and the type of spark plug you need. Other important conditions to consider are engine and spark plug temperature; a spark plug's ability to dissipate temperature; the air temperature and humidity; fuel temperature; the distance fuel is exposed to the intake track; type of fuel, etc.

The mystery of combustion has been a source of intense study ever since the internal combustion engine came to life, and this study will probably continue until mankind runs out of fossil fuel.

If you have specific questions about choosing a spark plug, or any other performance questions, you can reach me via email at jhfxr@aol.com.

© 2004 Hildreth Performance

Last Updated on 06/14/2004