Originally posted by dino.tw
Sorry,....but are you sure "add" more shims???
The density of air(or you can say oxygen) is less and less with altitude. So the compression should be less at high altitude with the same shims...that is what I knew sorry bro.
You are right Dino, but i made my comments because somebody suggested to use 30% Nitro. So lets restate all, asuming you are going to use same Nitro contents.
For high altitude you need to advance ignition timing, which can be done either with a hotter plug, or reducing combustion chamber (less shims),but you'll have to compromise or you'll loose top end or low end.
IGNITION TIMING FACTS
· You advance ignition with a hot plug, which will give you more torque or low speed
· You retard ignition with a cold plug, which will give you more rpm’s or top speed
The above is true in both cases if you have the same compression ratio (combustion chamber shimming), and use the same nitro % on fuel.
But if you change compression ratio (combustion chamber shimming, and or nitro %), you have to compromise.
· If you increment compression (smaller combustion chamber = less shims) you are advancing timing, thus incrementing torque, so you will need to put a colder plug to compensate for top end., and more generated heat, which could cause pre-ignition.
· If you reduce compression (bigger combustion chamber = more shims) you are retarding timing, thus incrementing top speed, so you will need to put a hotter plug to compensate for torque, and less generated heat.
The ideal combustion chamber for .12 engines at sea level is:
· For 16% nitro = .015 inch or 0.381 mm
· For 20% nitro = .016 inch or 0.410 mm
· For 25% nitro = .018 inch or 0.457 mm
· For 30% nitro = .021 inch or 0.533 mm
So you have to shim the engine accordingly to get those total combustion chamber sizes, but making a new chart considering higher altitude.
I would also suggest to contact any Mexican R/C web site, because they have a very developed market and are experts in running at high altitude