Originally Posted by dino.tw
How much shim is based on the density of air. And the density of air is base on altitude;barometric pressure;weather. More close to sea level you need more shims. When atmospheric depression coming you may need to take some shims off. Sometime you may run 0.4mm in the day but 0.5mm in the night. Air temperature also has effect on the air density....etc. There is no final answer. You need to try different setting yourself.
When the shim setting is too many(low compression),the engine will get bogged down in low end. When the shim setting is too less(high compression),the plug's coil will be pushed in then burnt.
GPP Model Racing
AFM
Weather Makes The Biggest Difference!
The atmospheric pressure, temperature and humidity all affect the density of the air. On a hot day, or at high altitude, or on a moist day, the air is less dense. A reduction in air density reduces the amount of oxygen available for combustion and therefore reduces the engine’s horsepower and torque. For tweaking the fuel/air mixture and compression ratio, the air density is the most important consideration.
The Air Temperature should ideally be the temperature of the air that is going into the intake of the engine.
The Absolute Pressure (also called actual pressure or station pressure) is the ambient air pressure.
Relative Humidity is a measure of how much moisture is in the air compared to the amount of moisture that the air could hold at saturation. Relative humidity is a function of temperature and therefore changes as the temperature changes, even if the amount of moisture in the air remains constant.
The air density is the actual weight of a given volume of air. This is a key parameter for engine tuning.
Resuming:
When the air density increases, you will need to richen the air-fuel mixture to compensate. When the air density decreases, you will need lean-out the air-fuel mixture to compensate.
Use the following as a guide to correcting your setting when the weather changes:
Air temperature: When the air temperature increases, the air density becomes lower. This will make the air-fuel mixture richer. You must lean the mixture to compensate for the lower air density. When the barometric pressure decreases, the opposite effect occurs.
Humidity: When the percentage of humidity in the air increases, the engine draws in a lower percentage of oxygen during each revolution because the water molecules (humidity) take the place of oxygen molecules in a given volume of air. High humidity will make the air-fuel mixture richer, so you should lean the mixture.
Altitude: In general, the higher the altitude the lower the air density. When driving at racetracks that are at high altitude, you should lean the mixture and increase the engine's compression ratio to compensate for the lower air density.