Originally Posted by keavze
so am i right to say that for .21 engines, i would need 0.5mm of actual head clearance to use 25% at sea-level with airtemp 80-90F and humidity 70-80%?
From my experience:
Minimum effective clearance (piston to head) on any .12 engine is 0.4mm
Maximum effective clearance is 0.8mm
Minimum effective clearance for any.21-.32 engine 0.50-0.55mm
Maximum effective clearance 1.1-1.3mm
Rule I've always gone by, on the safe side is:
0.60 on a .21 running 25 - 30% nitro and a #7 plug
0.50 on a .12 running 16 - 20% and a #6 plug
Relative humidity on ambient.
The more humidity (more water on the air), more chamber is needed. I'll add one 0.10mm shim for each 20% it raises over 70%. I use a small weather station for measuring temp, barometric pressure and relative humidity on the track. It is a great tool to know the quality of the air your engine is breathing.
Height over sea level.
As you go higher, there is less oxygen on the air, so you have to reduce space on combustion chamber (more compression). The more you approach sea level, you need to make more space for the additional Oxygen present in the air.