Originally Posted by GREGORY!

in the real world (bikes) i give advance and i have torque but lacks rpm.
i give retard and i have rpm but lacks torque.
i don't talk about huge difference in degrees.and of course there is not any detonation.

in the first occasion the ignition is created sooner and you have great performance in the low rpm.but when the rpm increase the dynamic compression is increased and in interrelation with the advanced ignition you have an engine that actually it 'tightens' in her own.

so still can't understand why here is the opposite.

Originally Posted by afm

Although you can't measure or definitively set when ignition takes place inside a nitro engine, it helps to be able to visualize what's happening when you experiment with different plugs. Let's say, for example, that the fuel mixture is ignited precisely at the moment the piston reaches the very top of the cylinder. This would effectively mean the ignition timing is taking place at zero degrees of crankshaft rotation. Installing a hotter plug in the same engine makes the fuel ignite sooner because less compression is needed to heat the plug's element to the point that the fuel will ignite. Let's say that now, ignition occurs 10 degrees before the piston reaches TDC. In ignition-engine-language, that would mean that the timing is set to 10 degrees advanced, or 10 degrees BTDC (before top dead center). What does all this mean? Simply knowing that plug temperature will affect when combustion takes place and will, hopefully, help you understand why choosing the proper plug will improve performance. Generally, it's best to try to advance the timing or flash point of the fuel—in the case of nitro engines, as much as possible without going too far. If the mixture is ignited too early, then performance is lost and pre-ignition and detonation may occur., because the more nitro you run, the more you advance the ignition point. What's that mean? In an ideal situation the glow plug will ignite the air fuel mixture when the piston is at top dead center (TDC), which will force the piston down and back up for another compression stroke. When you run a higher nitro percentage (and don't go to a colder plug) you'll advance your ignition point (the point at which the glow plug ignites the air/fuel mixture), which will result is less than optimum performance since the piston is still on it's compression stroke when the air/fuel is ignited and not at TDC.

AFM

Originally Posted by NitroWD

16% gives more rpm than 25%; by how much?
25% gives more torque; by how much?

anyone?

Originally Posted by lil-bump

I would not worry about changing Nitro % based on weather conditions. If anything I would maybe change the heat range of the plug. I only do this if the weather takes a big change from qualifing day to main day. On very humid days I might go to a slightly hotter plug. I normally use cold plugs also with 16% fuel I normally use a Med heat plug just to give me a little better bottom end.


Pass you soon...

Originally Posted by afm

When humidity comes on the scene past 70%, it is best to move to a hot body glow plug. For example in your first case move to a C7TC, and I would not increase nitro. I think the C8TF is way to cold for a .12 engine and is best used with 30 to 40% nitro on a very hoy day.We should also state that without touching head shimming, and or cahnging nitro contents, a hotter plug will make the engine rev harder and a colder plug will make more torque.

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.
Inputs:
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.
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.


Hope this helps
AFM

Originally Posted by afm

Generally, it's best to try to advance the timing or flash point of the fuel—in the case of nitro engines, as much as possible without going too far. If the mixture is ignited too early, then performance is lost and pre-ignition and detonation may occur., because the more nitro you run, the more you advance the ignition point. What's that mean? In an ideal situation the glow plug will ignite the air fuel mixture when the piston is at top dead center (TDC), which will force the piston down and back up for another compression stroke. When you run a higher nitro percentage (and don't go to a colder plug) you'll advance your ignition point (the point at which the glow plug ignites the air/fuel mixture), which will result is less than optimum performance since the piston is still on it's compression stroke when the air/fuel is ignited and not at TDC.

AFM

Originally Posted by keavze

more nitro and hotter plug will advance the ignition point till maybe too early and detonation occur, can i increase the deck height (reduce compression) to 'compensate'?

by the way, how to tell if you've retarded the ignition too much?
i know if you've advanced the ignition too much, detonation will occur and you'll get pitting on the piston and button.

Originally Posted by Big Lee

AFM you are the bomb. Great wealth of knowledge.
MY question is : I race offroad buggy with a .21 GRP tuned mill and have been running 20% Nitro ( Oils - 10% Synthetic, 5% Castor ) and an O'Donnel 97T (medium) Plug, with good results. Was lacking bottom end so went lookig for answers and found RCTECH and the wealth of knowledge to go with it.

Since reading the GRP thread have learned that the usual tuning meathod of tuning the hsn first then the lsn was not a good way to tune these motors, that they loved having a rich top end and very lean bottom. WOW what a difference. Still working on the tune.

Thinking of uping the Nitro content to 25% as used by the majority of racers at the local club. Was lead to beleave early in my rc life that past 16% was only decreasing the life of the motor, so was always hesitant to go any higher than the 20% originally recomended by my lhs when I started out.

BUT these motors come head shimmed for 30%. Alot of racers also use a P3 ( hot ) glow plug.

Go for the 25% on the medium plug ???????????

How different will I find the tune, power, temps, ease of tune ???????????